Jump to content

Wikipedia:Reference desk/Mathematics: Difference between revisions

From Wikipedia, the free encyclopedia
Content deleted Content added
 
Line 1: Line 1:
<noinclude>{{Wikipedia:Reference desk/header|WP:RD/MA}}
<noinclude>{{Wikipedia:Reference desk/header|WP:RD/MA}}
[[Category:Non-talk pages that are automatically signed]]
[[Category:Non-talk pages that are automatically signed]]
[[Category:Pages automatically checked for accidental language links]]
[[Category:Pages automatically checked for incorrect links]]
[[Category:Wikipedia resources for researchers]]
[[Category:Wikipedia resources for researchers]]
[[Category:Wikipedia help forums]]
[[Category:Wikipedia help forums]]
[[Category:Wikipedia reference desk|Mathematics]]
</noinclude>
[[Category:Wikipedia help pages with dated sections]]</noinclude>


{{Wikipedia:Reference_desk/Archives/Mathematics/2011 August 20}}


= December 4 =
{{Wikipedia:Reference_desk/Archives/Mathematics/2011 August 21}}


== How much is this ==
{{Wikipedia:Reference_desk/Archives/Mathematics/2011 August 22}}


: <math>\sum_{k=0}^{n} (-1)^k</math>
= August 23 =
[[Special:Contributions/37.162.46.235|37.162.46.235]] ([[User talk:37.162.46.235|talk]]) 11:00, 4 December 2024 (UTC)
::1 for ''n'' even, 0 for ''n'' odd. For <math>n \to \infty</math> see [[Grandi's series]]. --[[User:Wrongfilter|Wrongfilter]] ([[User talk:Wrongfilter|talk]]) 11:18, 4 December 2024 (UTC)
:I hope this is not homework. Note that (for finite <math>n</math>) this is a [[Geometric series|finite geometric series]]. &nbsp;--[[User talk:Lambiam#top|Lambiam]] 21:51, 4 December 2024 (UTC)


== Multiplying a Factorial by Another Factorial ==


= December 6 =
When it comes to dividing a factorial by another factorial, you can "cancel" them out. But when multiplying a factorial by another factorial, is there a shortcut there too, or is it just figure out what factorials are, then multiply them? --[[User:Thebackofmymind|Thebackofmymind]] ([[User talk:Thebackofmymind|talk]]) 10:10, 23 August 2011 (UTC)


== Is there anything that would prevent peforming Weil Descent on binary curves of large characteristics ? ==
:Your question is like the following: When it comes to dividing an X by another X, you can "cancel" them out. But when multiplying an X by another X, is there a shortcut there too, or is it just figure out what X-s are, then multiply them?
:Hope this helps.
:[[User:HOOTmag|HOOTmag]] ([[User talk:HOOTmag|talk]]) 10:25, 23 August 2011 (UTC)


The ghs attack involve creating an hyperlliptic curve cover for a given binary curve. The reason the attack fails most of the time is the resulting genus grows exponentially relative to the curve’s degree.
:The only identity I can think of, for m>n, is <math>n!m!=(n!)^2(m \cdot (m-1) \cdot ... \cdot (n+1))</math>, that may or may not make the mental calculation easier. [[User:Widener|Widener]] ([[User talk:Widener|talk]]) 11:20, 23 August 2011 (UTC)


We don’t hear about the attack on finite fields of large characteristics since such curves are already secure by being prime. '''However, I notice a few protocol relies on the discrete logarithm security on curves with 400/500 bits modulus resulting from extension fields of characteristics that are 200/245bits long'''.
::So, for example, 9! × 11! = (9!)² × 10 × 11. [[User:StuRat|StuRat]] ([[User talk:StuRat|talk]]) 09:19, 24 August 2011 (UTC)


Since the degree is most of the time equal to 3 or 2, is there anything that would prevent creating suitable hyperelliptic cover for such curves in practice ? [[Special:Contributions/2A01:E0A:401:A7C0:28FE:E0C4:2F97:8E08|2A01:E0A:401:A7C0:28FE:E0C4:2F97:8E08]] ([[User talk:2A01:E0A:401:A7C0:28FE:E0C4:2F97:8E08|talk]]) 12:09, 6 December 2024 (UTC)
== Factorization ==


= December 7 =
Help me factorize X squared + 4x + 3 + mx + 3m <small><span class="autosigned">— Preceding [[Wikipedia:Signatures|unsigned]] comment added by [[User:Agdesi|Agdesi]] ([[User talk:Agdesi|talk]] • [[Special:Contributions/Agdesi|contribs]]) 15:02, 23 August 2011 (UTC)</span></small><!-- Template:Unsigned --> <!--Autosigned by SineBot-->


== Mathematical operation navigation templates ==
:Here is a hint: gather terms in the same power of ''x'' together and rewrite the expression as
::<math>x^2 + (m+4)x + 3(m+1)</math>
:[[User:Gandalf61|Gandalf61]] ([[User talk:Gandalf61|talk]]) 15:05, 23 August 2011 (UTC)


{{hat|collapse=no|reason=RDBury is right, this discussion belongs at [[Wikipedia talk:WikiProject Mathematics#Navigation templates for mathematical operations|Wikipedia talk:WikiProject Mathematics]]}}
::Also, try to group the "unknowns" into a similar format, as in <math>x^2 + ((m+1) + 3)x + 3(m+1)</math>. Now, "m+1" is in both terms containing m. -- [[User:Kainaw|<font color='#ff0000'>k</font><font color='#cc0033'>a</font><font color='#990066'>i</font><font color='#660099'>n</font><font color='#3300cc'>a</font><font color='#0000ff'>w</font>]][[User talk:Kainaw|&trade;]] 16:15, 23 August 2011 (UTC)
If anyone with some mathematical expertise is interested, I'd appreciate some additional input at [[Talk:Exponentiation#funny table at end]]. The question is whether our articles on various mathematical operations could use a [[Wikipedia:Navigation template|navigational template]] (aka "{{tl|Navbox}}"). Our [[Exponentiation]] article tried to use {{tl|Mathematical expressions}} for this purpose, but it doesn't really work. I've created {{tl|Mathematical operations}} as a potential alternative, but the categorization and presentation I've created is probably na&iuml;ve. (The whole effort may or not be worth it at all.) —[[User:scs|scs]] ([[User talk:scs|talk]]) 00:36, 7 December 2024 (UTC)
:[[Wikipedia talk:WikiProject Mathematics]] is a better forum for this kind of thing, since it's focused on Wikipedia's mathematical articles. --[[User:RDBury|RDBury]] ([[User talk:RDBury|talk]]) 04:07, 7 December 2024 (UTC)
:: {{Re|RDBury}} Excellent point. Thanks. —[[User:scs|scs]] ([[User talk:scs|talk]]) 13:49, 7 December 2024 (UTC)
{{hab}}


= December 8 =
== Why the long tail is just a 2 variables graph and not a 3 variables graph?? ==


== For each positive integer <math>n</math>, which primes <math>p</math> are still primes in the ring <math>Z[e^{\frac{\pi i}{n}}]</math>? ==
You have the amount or % of the different products being sold.
The amount of % of population.
And the amount or % of products sold.
So you have 3 variables, but use a 2d graph to explain long tail.[[Special:Contributions/201.78.194.18|201.78.194.18]] ([[User talk:201.78.194.18|talk]]) 20:54, 23 August 2011 (UTC)


For each positive integer <math>n</math>, which primes <math>p</math> are still primes in the ring <math>Z[e^{\frac{\pi i}{n}}]</math>? When <math>n=1</math>, <math>Z[e^{\frac{\pi i}{n}}]</math> is the original integer ring, when <math>n=2</math>, <math>Z[e^{\frac{\pi i}{n}}]</math> is the ring of [[Gaussian integer]]s, when <math>n=3</math>, <math>Z[e^{\frac{\pi i}{n}}]</math> is the ring of [[Eisenstein integer]]s, and the primes in the [[Gaussian integer]]s are the primes <math>p \equiv 3 \mod 4</math>, and the primes in the [[Eisenstein integer]]s are the primes <math>p \equiv 2 \mod 3</math>, but how about larger <math>n</math>? [[Special:Contributions/218.187.66.163|218.187.66.163]] ([[User talk:218.187.66.163|talk]]) 04:50, 8 December 2024 (UTC)
:Have you looked at the [[Long Tail|long tail article]]? It's all explained there. <span style="white-space:nowrap;">— [[User:Fly by Night|<span style="font-family:Segoe print">'''''Fly by Night'''''</span>]] <font color="#000000">([[User talk:Fly by Night|<span style="font-family:Segoe print">talk</span>]])</font></span> 21:51, 23 August 2011 (UTC)


:A minuscule contribution: for <math>n=4,</math> the natural Gaussian primes <math>3</math> and <math>7</math> are composite:
= August 24 =
:*<math>3=(1-\omega-\omega^2)(1+\omega^2+\omega^3).</math>
:*<math>7=(2-\omega+2\omega^2)(2-2\omega^2-\omega^3).</math>
:So <math>11</math> is the least remaining candidate. &nbsp;--[[User talk:Lambiam#top|Lambiam]] 09:00, 8 December 2024 (UTC)
::It is actually easy to see that <math>7</math> is composite, since <math>2</math> is a perfect square:
:::<math>2=i(1-i)^2=\omega^2(1-\omega^2)^2.</math>
::Hence, writing <math>\sqrt2</math> by abuse of notation for <math>\omega(1-\omega^2),</math> we have:
:::<math>\begin{alignat}{2}7&=(3+\sqrt2)(3-\sqrt2)\\&=(2\sqrt2+1)(2\sqrt2-1)\\&=(5+3\sqrt2)(5-3\sqrt2)\\&=(4\sqrt2+5)(4\sqrt2-5).\end{alignat}</math>
::More in general, any natural number that can be written in the form <math>|2a^2-b^2|,a,b\in\mathbb N,</math> is not prime in <math>\mathbb Z[e^{\pi i/4}].</math> This also rules out the Gaussian primes <math>23,</math> <math>31,</math> <math>47,</math> <math>71</math> and <math>79.</math> &nbsp;--[[User talk:Lambiam#top|Lambiam]] 11:50, 8 December 2024 (UTC)
:::So which primes <math>p</math> are still primes in the ring <math>Z[e^{\frac{\pi i}{4}}]</math>? How about <math>Z[e^{\frac{\pi i}{5}}]</math> and <math>Z[e^{\frac{\pi i}{6}}]</math>? [[Special:Contributions/220.132.216.52|220.132.216.52]] ([[User talk:220.132.216.52|talk]]) 06:32, 9 December 2024 (UTC)
::::As I wrote, this is only a minuscule contribution. We do not do research on command; in fact, we are actually not supposed to do any original research here. &nbsp;--[[User talk:Lambiam#top|Lambiam]] 09:23, 9 December 2024 (UTC)
::Moreover, <math>-2</math> is also a perfect square. (As in the Gaussian integers, the additive inverse of a square is again a square.) So natural numbers of the form <math>2a^2+b^2</math> are also composite. This further rules out <math>11,</math> <math>19,</math> <math>43,</math> <math>59,</math> <math>67</math> and <math>83.</math> A direct proof that, e.g., <math>11</math> is composite: <math>11=(1+\omega^2+3\omega^3)(1-3\omega-\omega^2).</math> There are no remaining candidates below <math>100</math> and I can in fact not find any larger ones either. This raises the conjecture:
:::
:::''Every prime number can be written in one of the three forms <math>a^2+b^2,</math> <math>2a^2+b^2</math> and <math>|2a^2-b^2|.</math>''
:::
::Is this a known theorem? If true, no number in <math>\mathbb Z[e^{\pi i/4}]</math> is a natural prime. (Note that countless composite numbers cannot be written in any of these forms; to mention just a few: <math>15, 1155, 2491.</math>) &nbsp;--[[User talk:Lambiam#top|Lambiam]] 11:46, 9 December 2024 (UTC)


: I'll state things a little more generally, in the cyclotomic field <math>\mathbb Q[e^{2\pi i /n}]</math>. (Your n is twice mine.) A prime q factors as <math>q = (q_1\cdots q_r)^{e_r}</math>, where each <math>q_i</math> is a prime ideal of the same degree <math>f</math>, which is the least positive integer such that <math>q^f \equiv 1\pmod n</math>. (We have assumed that q does not divide n, because if it did, then it would ramify and not be prime. Also note that we have to use ideals, because the cyclotomic ring is not a UFD.) In particular, <math>q</math> stays prime if and only if <math>q</math> generates the group of units modulo <math>n</math>. When n is a power of two times an odd composite, the group of units is not cyclic, and so the answer is ''never''. When n is a prime or twice a prime, the answer is when q is a primitive root mod n. If n is 4 times a power of two times a prime, the answer is ''never''. [[User:Tito Omburo|Tito Omburo]] ([[User talk:Tito Omburo|talk]]) 11:08, 8 December 2024 (UTC)
== [[real number|Real]] [[cube root]]s of the [[identity matrix]] ==
::For your <math>n</math>, <math>n=3</math> and <math>n=6</math> are the same, as well as <math>n=5</math> and <math>n=10</math>, this is why I use <math>e^{\frac{\pi i}{n}}</math> instead of <math>e^{\frac{2 \pi i}{n}}</math>. [[Special:Contributions/61.229.100.34|61.229.100.34]] ([[User talk:61.229.100.34|talk]]) 20:58, 8 December 2024 (UTC)
::Also, what is the [[class number (number theory)|class number]] of the [[cyclotomic field]] <math>Q[e^{\frac{\pi i}{n}}]</math>? Let <math>\gamma(n)</math> be the [[class number (number theory)|class number]] of the [[cyclotomic field]] <math>Q[e^{\frac{\pi i}{n}}]</math>, I only know that:
::* <math>\gamma(n)=1</math> for <math>n=1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 24, 25, 27, 30, 33, 35, 42, 45</math> (is there any other such <math>n</math>)?
::* If <math>m</math> divides <math>n</math>, then <math>\gamma(m)</math> also divides <math>\gamma(n)</math>, thus we can let <math>\varphi(n)=\prod_{d|n}\gamma(n)^{\mu(n/d)}</math>
::* For prime <math>p</math>, <math>p</math> divides <math>\varphi(p)</math> if and only if <math>p</math> is [[irregular prime|Bernoulli irregular prime]]
::* For prime <math>p</math>, <math>p</math> divides <math>\varphi(2p)</math> if and only if <math>p</math> is [[irregular prime|Euler irregular prime]]
::* <math>\varphi(n)=1</math> for <math>n=1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 24, 25, 27, 30, 33, 35, 42, 45</math> (is there any other such <math>n</math>)?
::* <math>\varphi(n)</math> is prime for <math>n=23, 26, 28, 32, 36, 37, 38, 39, 40, 43, 46, 49, 51, 53, 54, 63, 64, 66, 69, 75, 81, 96, 105, 118, 128, ...</math> (are there infinitely many such <math>n</math>?)
::Is there an algorithm to calculate <math>\gamma(n)</math> quickly? [[Special:Contributions/61.229.100.34|61.229.100.34]] ([[User talk:61.229.100.34|talk]]) 21:14, 8 December 2024 (UTC)


== Can we say anything special about every pair of functions f,g, satisfying f(g(x))=f(x) for every x? ==
Does a solution for <math>\mathbf A^3=\mathbf I</math> exist, where <math>\mathbf I\ne\mathbf A\in\mathbb R^{3\times3}</math>? I have been trying to find a solution for hours. Thanks, [[Special:Contributions/85.250.176.130|85.250.176.130]] ([[User talk:85.250.176.130|talk]]) 08:18, 24 August 2011 (UTC)
:<math>\mathbf A=\begin{pmatrix}
-\frac 1 2 & -\frac{\sqrt 3}2 & 0 \\
\frac{\sqrt 3}2 & -\frac 1 2 & 0 \\
0 & 0 & 1 \end{pmatrix}</math>
[[User:Bo Jacoby|Bo Jacoby]] ([[User talk:Bo Jacoby|talk]]) 08:36, 24 August 2011 (UTC).
::This is a perfect example of where it's easier to think of [[Function composition|composition]] of [[Linear map|linear transformations]], rather than [[Matrix multiplication|multiplication of matrices]]. What linear transformation composed with itself 3 times will return the identity matrix? Well, clearly, if you rotate around an arbitrary axis with angle 120°, then you get such a function, and then you only need to confirm that the function is, in fact, a linear transformation. --[[User:COVIZAPIBETEFOKY|COVIZAPIBETEFOKY]] ([[User talk:COVIZAPIBETEFOKY|talk]]) 13:09, 24 August 2011 (UTC)
A simpler solution is
:<math>\mathbf A=\begin{pmatrix}0&1&0\\0&0&1\\1&0&0\end{pmatrix}</math>
[[User:Bo Jacoby|Bo Jacoby]] ([[User talk:Bo Jacoby|talk]]) 16:46, 24 August 2011 (UTC).


Especially, is there an accepted term for such a pair?
:... which is a rotation about the line ''x'' = ''y'' = ''z''. [[User:Gandalf61|Gandalf61]] ([[User talk:Gandalf61|talk]]) 17:04, 24 August 2011 (UTC)


Here are three simple examples, for two functions f,g, satisfying the above, and defined for every natural number:
== Problem from Spivak's ''Calculus'' ==


Example #1:
Let ''f'' be two-times differentiable with ''f''(0) = 0, ''f''(1) = 1, and ''f''{{'}}(0) = ''f''{{'}}(1) = 0. Prove that |''f''{{'}}{{'}}(''a'')| ≥ 4 for some ''a'' in (0,1). Spivak hints that we should try to prove that either ''f''{{'}}{{'}}(''a'') ≥ 4 for some ''a'' in (0,1/2), or ''f''{{'}}{{'}}(''a'') ≤ &minus;4 for some ''a'' in (1/2, 1), but I'm not sure how to do that. I managed to prove the claim for the case when ''f'' has a maximum point over (0,1) in (0, 1/2), but I'm not convinced this is the approach he's suggesting. Thanks for the help. —<strong>[[User:Anonymous Dissident|<span style="font-family:Script MT Bold;color:DarkRed">Anonymous Dissident</span>]]</strong>[[User_talk:Anonymous Dissident|<sup><span style="font-family:Verdana;color:Gray">Talk</span></sup>]] 21:44, 24 August 2011 (UTC)
:f is constant.
:Try a proof by contradiction. Suppose <math>f'' < 4</math> on the interval <math>(0, 1/2)</math>. Using <math>f'(0) = 0</math> and <math>f(0) = 0</math>, find a bound for <math>f(1/2)</math>. Repeat using <math>f'' > -4</math> on <math>(1/2, 1)</math>, <math>f'(1) = 0</math>, <math>f(1) = 1</math> to get a different bound. Conclude a contradiction.--[[User:Antendren|Antendren]] ([[User talk:Antendren|talk]]) 22:04, 24 August 2011 (UTC)
::Success! Cheers. Maybe I should stop considering contradiction as a last resort... —<strong>[[User:Anonymous Dissident|<span style="font-family:Script MT Bold;color:DarkRed">Anonymous Dissident</span>]]</strong>[[User_talk:Anonymous Dissident|<sup><span style="font-family:Verdana;color:Gray">Talk</span></sup>]] 08:11, 25 August 2011 (UTC)
:::Actually no. I thought I had it but now things went wrong. I began with the false premises. Then I used the mean value theorem on the interval (0, ''x'') to show that ''f''{{'}}(''x'') < 4''x'' for all ''x'' in (0, 1/2], and then the MVT again to show ''f''(''x'') < 4''x''<sup>2</sup>, whence ''f''(1/2) < 1. Similar methods on [1/2, 1) lead merely to ''f''(1/2) > 0, which isn't helpful. What have I done wrong? —<strong>[[User:Anonymous Dissident|<span style="font-family:Script MT Bold;color:DarkRed">Anonymous Dissident</span>]]</strong>[[User_talk:Anonymous Dissident|<sup><span style="font-family:Verdana;color:Gray">Talk</span></sup>]] 12:00, 25 August 2011 (UTC)
::::The bound <math>f(x)<4x^2</math> is too weak. Hint: <math>(2x^2)'=4x</math>. -- [[User:Meni Rosenfeld|Meni Rosenfeld]] ([[User Talk:Meni Rosenfeld|talk]]) 13:38, 25 August 2011 (UTC)
:Let x=f(t) be the position of a car standing still on the position x=0, then speeding up at t=0 at maximum acceleration x ' '=4 until t=1/2, when it brakes at maximum deceleration x ' '=&minus;4, until t=1. Then x=1 and x '=0. This is the only motion with |x ' '|&le;4 that satifies the boundary conditions. But f(t) is not two times differentiable at the time t=1/2 when speeding up is suddenly changed to braking. A two times differentiable motion satisfies the sharp inequality f ' '(a)>4 for some a. [[User:Bo Jacoby|Bo Jacoby]] ([[User talk:Bo Jacoby|talk]]) 13:52, 25 August 2011 (UTC).
::Given that the problem as stated is even easier, calling only for proof that |''f''{{'}}{{'}}(''a'')| ≥ 4 somewhere (not strictly greater), how far can the differentiability condition be relaxed? Would it be sufficient that ''f'' be a differentiable function, twice differentiable almost everywhere, or would ''f'' need to be twice differentiable at all by finitely many locations? -- [[Special:Contributions/110.49.248.74|110.49.248.74]] ([[User talk:110.49.248.74|talk]]) 00:11, 26 August 2011 (UTC)
:::yes. If f'' is unbounded, we're done. Otherwise f'' is in L^1, so the FTC holds. [[User:Sławomir Biały|<span style="text-shadow:grey 0.3em 0.3em 0.1em; class=texhtml">Sławomir Biały</span>]] ([[User talk:Sławomir Biały|talk]]) 00:30, 26 August 2011 (UTC)


Example #2:
= August 25 =
:f(x)=g(x), and is the smallest even number, not greater than x.


Example #3:
== Logarithmic Spiral ==
:f(x)=1 if x is even, otherwise f(x)=2.
:g(x)=x+2.
[[Special:Contributions/2A06:C701:746D:AE00:ACFC:490:74C3:660|2A06:C701:746D:AE00:ACFC:490:74C3:660]] ([[User talk:2A06:C701:746D:AE00:ACFC:490:74C3:660|talk]]) 09:31, 8 December 2024 (UTC)


: One way to consider such a pair is dynamically. If you consider the dynamical system <math>x\mapsto g(x)</math>, then the condition can be stated as "<math>f</math> is constant on <math>g</math>-orbits". More precisely, let <math>D</math> be the domain of <math>f,g</math>, which is also the codomain of <math>g</math>. Define an equivalence relation on <math>D</math> by <math>x\sim y</math> if <math>g^a(x)=g^b(y)</math> for some positive integers <math>a,b</math>. Then <math>f</math> is simply a function on the set of equivalence classes <math>D/\sim</math> (=space of orbits). In ergodic theory, such a function <math>f</math> is thought of as an "observable" or "function of state", being the mathematical analog of a thermodynamic observable such as temperature. [[User:Tito Omburo|Tito Omburo]] ([[User talk:Tito Omburo|talk]]) 11:52, 8 December 2024 (UTC)
What's the reason why the logarithmic spiral is so common in nature? The article mentions self-similarity, but could someone be more specific? [[Special:Contributions/74.15.137.168|74.15.137.168]] ([[User talk:74.15.137.168|talk]]) 02:49, 25 August 2011 (UTC)
::After you've mentioned temprature, could you explain what are f,g, as far as temprature is concerned? Additionally, could you give another useful example from physics for such a pair of functions? [[Special:Contributions/2A06:C701:746D:AE00:ACFC:490:74C3:660|2A06:C701:746D:AE00:ACFC:490:74C3:660]] ([[User talk:2A06:C701:746D:AE00:ACFC:490:74C3:660|talk]]) 19:49, 8 December 2024 (UTC)
:Only God knows the true reason why, but man can guess. Symmetries may reuse code in the DNA. These symmetries are often broken in later stages of development or evolution. There is a high level of symmetry between arm and leg on a human, and even higher symmetry between right arm and left arm. There is also a kind of symmetry between a small boy and a big boy. The organs grow. The change of teeth is an exception. The logarithmic spiral has a continuous symmetry group, and so it may grow using a short piece of DNA code. [[User:Bo Jacoby|Bo Jacoby]] ([[User talk:Bo Jacoby|talk]]) 08:00, 25 August 2011 (UTC).
:::This equation is just the definition of function ''g''. For instance if function ''f'' has the inverse function ''f''<sup>−1</sup> then we have ''g(x)=x''. [[User:Ruslik0|Ruslik]]_[[User Talk:Ruslik0|<span style="color:red">Zero</span>]] 20:23, 8 December 2024 (UTC)
::I think the fundamental underlying reason is that [[exponential growth]] and [[exponential decay]] are common in nature, and when something changes exponentially while circling around a fixed center, you naturally get a logarithmic spiral. [[User:Looie496|Looie496]] ([[User talk:Looie496|talk]]) 16:03, 25 August 2011 (UTC)
::: If f is the temperature, and g is the evolution of an ensemble of particles in thermal equilibrium (taken at a single time, say one second later), then because temperature is a function of state, one has <math>f(x)=f(g(x))</math> for all ensembles x. Another example from physics is when <math>g</math> is a Hamiltonian evolution. Then the functions <math>f</math> with this property (subject to smoothness) are those that (Poisson) commute with the Hamiltonian, i.e. "constants of the motion". [[User:Tito Omburo|Tito Omburo]] ([[User talk:Tito Omburo|talk]]) 20:33, 8 December 2024 (UTC)
:::Oh okay, so because <math>\frac{dr}{d \theta} = \frac{dr}{dt} \frac{dt}{d \theta} = \frac{v}{\omega} \propto r</math>, logarithmic spirals are common? [[Special:Contributions/74.15.137.168|74.15.137.168]] ([[User talk:74.15.137.168|talk]]) 17:18, 25 August 2011 (UTC)
::::Thx. [[Special:Contributions/2A06:C701:746D:AE00:ACFC:490:74C3:660|2A06:C701:746D:AE00:ACFC:490:74C3:660]] ([[User talk:2A06:C701:746D:AE00:ACFC:490:74C3:660|talk]]) 10:43, 9 December 2024 (UTC)
::::That's a reasonable way of looking at it, but it also ignores all the processes (developmental, evolutionary, geo-chemical, etc) that go into forming such spirals in nature (which others allude to above). You may be interested in reading up on [[phyllotaxis]] and links therein (this is the start of the whole 'pineapple bracts follow the fibonacci sequence' business). You might also get better answers on the science desk, this isn't really a math question :) [[User:SemanticMantis|SemanticMantis]] ([[User talk:SemanticMantis|talk]]) 20:29, 25 August 2011 (UTC)
:Let <math>f</math> be a function from <math>X</math> to <math>Y</math> and <math>g</math> a function from <math>X</math> to <math>X.</math> Using the notation for [[function composition]], the property under discussion can concisely be expressed as <math>f\circ g=f.</math> An equivalent but verbose way of saying the same is that the [[preimage]] of any set <math>B \subseteq Y</math> under <math>f</math> is [[Closure (mathematics)|closed]] under the application of <math>g.</math> &nbsp;--[[User talk:Lambiam#top|Lambiam]] 08:54, 9 December 2024 (UTC)
::Thx. [[Special:Contributions/2A06:C701:746D:AE00:ACFC:490:74C3:660|2A06:C701:746D:AE00:ACFC:490:74C3:660]] ([[User talk:2A06:C701:746D:AE00:ACFC:490:74C3:660|talk]]) 10:43, 9 December 2024 (UTC)


== IEEE Xplore paper claim to acheive exponentiation inversion suitable for pairing in polynomial time. Is it untrustworthy ? ==
== Proof too easy? ==


I just found https://ieeexplore.ieee.org/abstract/document/6530387. Given the multiplicative group factorization in the underlying finite field of a target bn curve, they claim to acheive exponentiation inversion suitable for pairing inversion in seconds on a 32 bits cpu.
Good morning. I was to show rigourously that f(x)=''x''<sup>3</sup>-4''x''+6 is not one to one (injective, I think it's also called?). This is my proof (contradiction), but something doesn't sit right with me about it; it seems too easy. Have a look:<br />
Suppose that f(m)=f(n) if and only if m=n<br />
<math>m^3-4m+6=n^3-4n+6</math> (m=n)<br />
<math>(m^3-n^3)-(4m-4n)=0</math> (6s cancel, bring both to one side)<br />
<math>(m-n)(m^2+mn+n^2)-(m-n)4=0</math> (factoring)<br />
<math>(m-n)(m^2+mn+n^2-4)=0</math> (grouping)<br />
<math>m^2+mn+n^2-4=0</math> (The expression is zero when either the first or the second factor is zero; I'm not interested in the first because it is the same as the hypothesis)<br />
By the quadratic formula:<br />
<math>(*)\qquad m=\frac{-n\pm\sqrt{n^2-4(n^2-4)}}{2}</math><br />
<math>n=\frac{-n\pm\sqrt{n^2-4(n^2-4)}}{2}</math> (m=n)<br />
<math>\rightarrow n=\frac{2\sqrt{3}}{3}</math>
However, putting n back into <math>(*)</math>, I get <math>m=\frac{2\sqrt{3}}{3}, \frac{-4\sqrt{3}}{3}</math>, contradiction
Therefore, f(m)=f(n) for at least one pair (m,n): m≠n [[Special:Contributions/203.117.33.23|203.117.33.23]] ([[User talk:203.117.33.23|talk]]) 23:24, 25 August 2011 (UTC)
Is this proof legit? Thanks so much, and sorry for wasting so much of your time. PS: What happened to all the other pairs, because in the graph there is obviously more than one. THanks again.
:Didn't look too closely at your proof above, but you're probably meant to just notice that ''f(2) = f(0)''. [[User:Staecker|Staecker]] ([[User talk:Staecker|talk]]) 00:00, 26 August 2011 (UTC)
:OK now I looked at your proof. It's definitely suspicious. Your assumption, in order to derive a contradiction, is: "''f(n)=f(m)'' iff ''n=m''". By your argument (which is fine) this is equivalent to "<math>(m-n)(m^2+mn+n^2-4)=0</math> iff ''m=n''". Your use of the quadratic formula is fine, but why then substitute ''n'' for ''m''? You're not assuming that ''m=n'', you're assuming the more complicated "iff" statement. [[User:Staecker|Staecker]] ([[User talk:Staecker|talk]]) 00:06, 26 August 2011 (UTC)


On 1 side, the paper is supposed to be peer reviewed by the iee Xplore journal and they give examples on 100 bits. On the other side, in addition to the claim, their algorithm 2 and 3 are very implicit, and as an untrained student, I fail to understand how to implement them, though I fail to understand things like performing a Weil descent.
:The first derivative has two distinct real roots, so there are portions with positive and negative slope, which can't be true for a one to one function. I don't know what your standard of rigour has to be, but something could be worked up round this fact.←[[Special:Contributions/86.155.185.195|86.155.185.195]] ([[User talk:86.155.185.195|talk]]) 12:03, 26 August 2011 (UTC)


Is the paper untrustworthy, or would it be possible to get code that can be run ? [[Special:Contributions/2A01:E0A:401:A7C0:152B:F56C:F8A8:D203|2A01:E0A:401:A7C0:152B:F56C:F8A8:D203]] ([[User talk:2A01:E0A:401:A7C0:152B:F56C:F8A8:D203|talk]]) 18:53, 8 December 2024 (UTC)
= August 26 =


About the paper, I agree to share the paper privately [[Special:Contributions/2A01:E0A:401:A7C0:152B:F56C:F8A8:D203|2A01:E0A:401:A7C0:152B:F56C:F8A8:D203]] ([[User talk:2A01:E0A:401:A7C0:152B:F56C:F8A8:D203|talk]]) 18:54, 8 December 2024 (UTC)
== Limit to infinity ==


= December 9 =
If <math>\lim_{x \rightarrow \infty}f(x)</math> exists, is it always true that <math>\lim_{x \rightarrow \infty} f(x) = \lim_{x \rightarrow \infty} f(x+1)</math>? Intuitively I would think so, but I'm not sure how I would prove it.[[User:Widener|Widener]] ([[User talk:Widener|talk]]) 06:43, 26 August 2011 (UTC)
:Yes. This is easy to show using the definition of limit. -- [[User:Meni Rosenfeld|Meni Rosenfeld]] ([[User Talk:Meni Rosenfeld|talk]]) 07:16, 26 August 2011 (UTC)
::Same ''N'' ? [[User:Widener|Widener]] ([[User talk:Widener|talk]]) 08:12, 26 August 2011 (UTC)
:::That depends on what exactly you mean by ''N'' (that is, which limit it applies to). You can certainly use ''N<sub>2</sub>'' = ''N<sub>1</sub>''-1, but ''x''>''N'' ⇒ ''x''>''N''-1. -- [[Special:Contributions/110.49.248.138|110.49.248.138]] ([[User talk:110.49.248.138|talk]]) 20:02, 26 August 2011 (UTC)


== If the [[Mersenne number]] 2^p-1 is prime, then must it be the smallest [[Mersenne prime]] == 1 mod p? ==
== FLAW in Cauchy's proof for the inequality of arithmetic and geometric means?! ==


If the [[Mersenne number]] 2^p-1 is prime, then must it be the smallest [[Mersenne prime]] == 1 mod p? (i.e. there is no prime q < p such that 2^q-1 is also a [[Mersenne prime]] == 1 mod p) If p is prime (no matter 2^p-1 is prime or not), 2^p-1 is always == 1 mod p. However, there are primes p such that there is a prime q < p such that 2^q-1 is also a [[Mersenne prime]] == 1 mod p:
The article [[inequality of arithmetic and geometric means]] gives the following proof by Cauchy, for the subcase when ''n'' = 2.<br />
<math>
\begin{align}
x_1 & \ne x_2 \\[3pt]
x_1 - x_2 & \ne 0 \\[3pt]
\left( x_1 - x_2 \right) ^2 & > 0 \\[3pt]
x_1^2 - 2 x_1 x_2 + x_2^2 & > 0 \\[3pt]
x_1^2 + 2 x_1 x_2 + x_2^2 & > 4 x_1 x_2 \\[3pt]
\left( x_1 + x_2 \right) ^2& > 4 x_1 x_2 \\[3pt]
\Bigl( \frac{x_1 + x_2}{2} \Bigr)^2 & > x_1 x_2 \\[3pt]
\frac{x_1 + x_2}{2} & > \sqrt{x_1 x_2}
\end{align}
</math> <br />
How is the last step in this proof valid? [[User:Widener|Widener]] ([[User talk:Widener|talk]]) 08:18, 26 August 2011 (UTC)
:By assumption, <math>x_1</math> and <math>x_2</math> are non-negative, so <math>x_1x_2</math> and <math>\frac{x_1+x_2}{2}</math> are non-negative. Square-root is an increasing function on the non-negative reals, so it preserves order.--[[Special:Contributions/121.74.96.240|121.74.96.240]] ([[User talk:121.74.96.240|talk]]) 08:26, 26 August 2011 (UTC)
::Okay, so generally, what is the proof that <math>x^2 > y \Rightarrow x > \sqrt{y}</math> for <math>x,y \ge 0</math> ? [[User:Widener|Widener]] ([[User talk:Widener|talk]]) 08:38, 26 August 2011 (UTC)
:::Instead of showing that, I'll show <math> x^2 > y^2 \Rightarrow x > y</math>. Replace <math>y</math> with <math>\sqrt{y}</math> for what you want.
:::Now, suppose not. Then <math>x \le y</math>. So <math>x^2 \le xy \le y^2</math>. Contradiction.--[[User:Antendren|Antendren]] ([[User talk:Antendren|talk]]) 08:55, 26 August 2011 (UTC)


* 2^19-1 is [[Mersenne prime]] == 1 mod 73 (p=73, q=19)
== Cumulative position probabilities as a result of tossing [[dice]] ==
* 2^31-1 is [[Mersenne prime]] == 1 mod 151 (p=151, q=31)
* 2^61-1 is [[Mersenne prime]] == 1 mod 151 (p=151, q=61)
* 2^17-1 is [[Mersenne prime]] == 1 mod 257 (p=257, q=17)
* 2^31-1 is [[Mersenne prime]] == 1 mod 331 (p=331, q=31)
* 2^61-1 is [[Mersenne prime]] == 1 mod 331 (p=331, q=61)
* 2^127-1 is [[Mersenne prime]] == 1 mod 337 (p=337, q=127)
* 2^89-1 is [[Mersenne prime]] == 1 mod 353 (p=353, q=89)
* 2^89-1 is [[Mersenne prime]] == 1 mod 397 (p=397, q=89)


but for these primes p, 2^p-1 is not prime, and my question is: Is there a prime p such that 2^p-1 is a prime and there is a prime q < p such that 2^q-1 is also a [[Mersenne prime]] == 1 mod p?
Let's say you roll a die or dice and move a token forward by that number of positions. What's the probability of hitting each position, if multiple throws are allowed ? Let me give you an example with one die:


* If 2^11-1 is prime, then this is true, since 2^11-1 is == 1 mod 31 and 2^31-1 is prime, but 2^11-1 is not prime
Chance of reaching position 1 = 1/6 (the chance of rolling a 1)
* If 2^23-1 or 2^67-1 is prime, then this is true, since 2^23-1 and 2^67-1 are == 1 mod 89 and 2^89-1 is prime, but 2^23-1 and 2^67-1 are not primes
* If 2^29-1 or 2^43-1 or 2^71-1 or 2^113-1 is prime, then this is true, since 2^29-1 and 2^43-1 and 2^71-1 and 2^113-1 are == 1 mod 127 and 2^127-1 is prime, but 2^29-1 and 2^43-1 and 2^71-1 and 2^113-1 are not primes
* If 2^191-1 or 2^571-1 or 2^761-1 or 2^1901-1 is prime, then this is true, since 2^191-1 and 2^571-1 and 2^761-1 and 2^1901-1 are == 1 mod 2281 and 2^2281-1 is prime, but 2^191-1 and 2^571-1 and 2^761-1 and 2^1901-1 are not primes
* If 2^1609-1 is prime, then this is true, since 2^1609-1 is == 1 mod 3217 and 2^3217-1 is prime, but 2^1609-1 is not prime


Another question: For any prime p, is there always a [[Mersenne prime]] == 1 mod p? [[Special:Contributions/220.132.216.52|220.132.216.52]] ([[User talk:220.132.216.52|talk]]) 19:03, 9 December 2024 (UTC)
Chance of reaching position 2 = 1/6 (the chance of rolling a 2) + 1/36 (the chance of rolling a 1, twice in a row)


: Neither question is easy. For the first, relations <math>2^{q-1}\equiv 1\pmod p</math> would imply that the integer 2 is not a primitive root mod p, and that its order divides <math>q-1</math> for the prime q. This is a sufficiently infrequent occurrence that it seems ''likely'' that all Mersenne numbers could be ruled out statistically, but not enough is known about their distribution. For the second, it is not even known if there are infinitely many Mersenne primes. [[User:Tito Omburo|Tito Omburo]] ([[User talk:Tito Omburo|talk]]) 19:23, 9 December 2024 (UTC)
So, the chances of hitting each spot should go up with increasing distance from the starting spot, approaching a limit. Note that the chances, when added up, will be more than 100%, since the token will land on multiple spots. So, do we have a chart of these probabilities, for one die and for when rolling a pair of dice ? Also, what is the theoretical limit probability for the cases of one die or two dice ? [[User:StuRat|StuRat]] ([[User talk:StuRat|talk]]) 18:26, 26 August 2011 (UTC)
::I found that: 2^9689-1 is the smallest Mersenne prime == 1 mod 29, 2^44497-1 is the smallest Mersenne prime == 1 mod 37, 2^756839-1 is the smallest Mersenne prime == 1 mod 47, 2^57885161-1 is the smallest Mersenne prime == 1 mod 59, 2^4423-1 is the smallest Mersenne prime == 1 mod 67, 2^9941-1 is the smallest Mersenne prime == 1 mod 71, 2^3217-1 is the smallest Mersenne prime == 1 mod 97, 2^21701-1 is the smallest Mersenne prime == 1 mod 101, and none of the 52 known Mersenne primes are == 1 mod these primes p < 1024: 79, 83, 103, 173, 193, 197, 199, 227, 239, 277, 307, 313, 317, 349, 359, 367, 373, 383, 389, 409, 419, 431, 443, 461, 463, 467, 479, 487, 503, 509, 523, 547, 563, 587, 599, 613, 647, 653, 659, 661, 677, 709, 727, 733, 739, 743, 751, 757, 769, 773, 797, 809, 821, 823, 827, 829, 839, 853, 857, 859, 863, 887, 907, 911, 919, 929, 937, 941, 947, 971, 977, 983, 991, 1013, 1019, 1021 [[Special:Contributions/220.132.216.52|220.132.216.52]] ([[User talk:220.132.216.52|talk]]) 20:51, 9 December 2024 (UTC)
::Also,
::* 2^19937-1 is [[Mersenne prime]] == 1 mod 2^16+1
::* 2^521-1 is [[Mersenne prime]] == 1 mod 2^13-1
::* 2^3021377-1 is [[Mersenne prime]] == 1 mod 2^17-1
::* 2^2281-1 is [[Mersenne prime]] == 1 mod 2^19-1
::* 2^21701-1 is [[Mersenne prime]] == 1 mod 2^31-1
::* 2^19937-1 is [[Mersenne prime]] == 1 mod 2^89-1
::* 2^86243-1 is [[Mersenne prime]] == 1 mod 2^107-1
::but none of these primes p has 2^p-1 is known to be prime, the status of 2^(2^89-1)-1 and 2^(2^107-1)-1 are still unknown (see [[double Mersenne number]]), but if at least one of them is prime, then will disprove this conjecture (none of the 52 known Mersenne primes are == 1 mod 2^61-1 or 2^127-1), I think that this conjecture may be as hard as the [[New Mersenne conjecture]]. [[Special:Contributions/220.132.216.52|220.132.216.52]] ([[User talk:220.132.216.52|talk]]) 20:55, 9 December 2024 (UTC)
::Also, for the primes p < 10000, there is a prime q < p such that 2^q-1 is also a [[Mersenne prime]] == 1 mod p only for p = 73, 151, 257, 331, 337, 353, 397, 683, 1321, 1613, 2113, 2731, 4289, 4561, 5113, 5419, 6361, 8191, 9649 (this sequence is not in [[OEIS]]), however, none of these primes p have 2^p-1 prime. [[Special:Contributions/220.132.216.52|220.132.216.52]] ([[User talk:220.132.216.52|talk]]) 02:23, 10 December 2024 (UTC)


= December 10 =
:With one standard six-sided die, your token advances an average of 3.5 positions per roll, so the limit of its probability of landing on any one position far enough along is 2/7. -- [[Special:Contributions/110.49.248.138|110.49.248.138]] ([[User talk:110.49.248.138|talk]]) 20:20, 26 August 2011 (UTC)


== More on the above conjecture ==
:Also note (again for one die), that the probability of the token landing on any one position is 1/6 times the sum of the probabilities of landing on the previous six positions (which, as you pointed out, will sum to greater than unity). This yields an easy to calculate algorithm. For my HP-48G, I wrote the program << 6 DUPN + + + + + 6 / >>, seeded the stack with 0 0 0 0 0 1, and ran the program repeatedly, yielding:
1: 0.166667
2: 0.194444
3: 0.226852
4: 0.264660
5: 0.308771 HI
6: 0.360232
7: 0.253604 LO
8: 0.268094
9: 0.280369
10: 0.289288
11: 0.293393 HI
12: 0.290830
...
38: 0.28571632
39: 0.28571480
40: 0.28571365
...
:This agrees with your probabilities for positions 1 and 2, and approaches the limit of 2/7 ≈ 0.28571429. Note that 6 is the most likely position to land on. For two dice, you will have to weight your average appropriately, but it is easy to see that the limit is 1/7. -- [[Special:Contributions/110.49.227.131|110.49.227.131]] ([[User talk:110.49.227.131|talk]]) 20:59, 26 August 2011 (UTC)


Above I posed:
::Interesting. I was wondering if it would behave like this, first rising above the limit, then dropping below, and continuing to bounce back and forth at ever reducing magnitudes as it approaches the limit. If you'd be willing to do the same for 2 dice, I'd like to see those results, too. [[User:StuRat|StuRat]] ([[User talk:StuRat|talk]]) 21:30, 26 August 2011 (UTC)
:{{serif|'''Conjecture'''. ''Every prime number can be written in one of the three forms <math>a^2+b^2,</math> <math>2a^2+b^2</math> and <math>|2a^2-b^2|.</math>''}}
If true, it implies no natural prime is a prime in the ring <math>\mathbb Z[e^{\pi i/4}]</math>.


The absolute-value bars are not necessary. A number that can be written in the form <math>-(2a^2-b^2)</math> is also expressible in the form <math>+(2a^2-b^2).</math>
:::Sure. For two dice the odds of throwing a 1, 2, ..., 12 are, in thirty-sixths, 0, 1, 2, 3, 4, 5, 6, 5, 4, 3, 2, 1. So my quick and dirty [[HP-48G]] program becomes << 12 DUPN 0 * SWAP 1 * + SWAP 2 * + SWAP 3 * + SWAP 4 * + SWAP 5 * + SWAP 6 * + SWAP 5 * + SWAP 4 * + SWAP 3 * + SWAP 2 * + SWAP 1 * + 36 / >> (I suppose that I really should be using a loop here), and the stack is initialized to 0 0 0 0 0 0 0 0 0 0 0 1.
1: 0
2: 0.027778
3: 0.055556
4: 0.084105
5: 0.114198
6: 0.146626
7: 0.182227 HI
8: 0.166346
9: 0.155526
10: 0.147784
11: 0.141275
12: 0.134199
13: 0.124704 LO
14: 0.138567
15: 0.145771
16: 0.148355 HI
17: 0.147890
18: 0.145672
19: 0.142886
20: 0.140764
21: 0.140745 LO
22: 0.141558
...
48: 0.142850
49: 0.142855
50: 0.142859
...
:::peaking on Lucky Number 7 and with the limit of 1/7 = 0.142857... . I am curious about the period of the oscillations about the limit -- whether and how it changes as the values approach the limit. My sense is that the period remains fairly steady at between 8 and 9. I'm sure that this type of hunting behavior is well studied in numerical analysis. -- [[Special:Contributions/110.49.235.126|110.49.235.126]] ([[User talk:110.49.235.126|talk]]) <span style="font-size: smaller;" class="autosigned">—Preceding [[Wikipedia:Signatures|undated]] comment added 02:35, 27 August 2011 (UTC).</span><!--Template:Undated--> <!--Autosigned by SineBot-->


It turns out (experimentally; no proof) that a number that can be written in two of these forms can also be written in the third form. The conjecture is not strongly related to the concept of primality, as can be seen in this reformulation:
::::Thanks. I hope you don't mind, but I went ahead and labelled the HI and LO points above. The distances between them seem rather chaotic. I wonder if they smooth out more later on. [[User:StuRat|StuRat]] ([[User talk:StuRat|talk]]) 05:50, 27 August 2011 (UTC)


:{{serif|'''Conjecture'''. ''A natural number that cannot be written in any one of the three forms <math>a^2+b^2,</math> <math>2a^2+b^2</math> and <math>2a^2-b^2</math> is composite.}}
:::::(Edit Conflict) I don't mind at all. I've added a couple of extra points to the lists so that they end one point after an extremum, moving both of your "questioned" labels, but they are still too short to gather more than an impression on initial behavior, and they are probably approaching the point of being too long to post on this ref desk. My statement about the 8-9 period was based on casual observation of positions out in the range of 40 - 60, but I really need to run this on a computer to get better analysis -- and to avoid the hassle of copying figures over from a calculator. I'll do a bit more work on this tonight, and contact you via your [[User talk:StuRat#Cumulative position probabilities as a result of tossing dice|talk page]]. -- [[Special:Contributions/110.49.225.10|110.49.225.10]] ([[User talk:110.49.225.10|talk]]) 07:12, 27 August 2011 (UTC)
The first few numbers that cannot be written in any one of these three forms are
:<math>15,</math> <math>21,</math> <math>30,</math> <math>35,</math> <math>39,</math> <math>42,</math> <math>55,</math> <math>60,</math> <math>69,</math> <math>70,</math> <math>77,</math> <math>78,</math> <math>84,</math> <math>87,</math> <math>91,</math> <math>93,</math> <math>95.</math>
They are indeed all composite, but why this should be so is a mystery to me. What do <math>2310=2\times 3\times 5\times 7\times 11,</math> <math>5893=71\times 83</math> and <math>7429=17\times 19\times 23,</math> which appear later in the list, have in common? I see no pattern.


It seems furthermore that the [[primorial]]s, starting with <math>5\#=30,</math> make the list. (Checked up to <math>37\!\#=7420738134810.</math>) &nbsp;--[[User talk:Lambiam#top|Lambiam]] 19:23, 10 December 2024 (UTC)
:::::[[Generating function]]s can often be used to investigate such questions. In the first case, if the denominator is taken to be 6<sup>n</sup>, then a generating function for the numerator is given by 1/(1-x-6x<sup>2</sup>-6<sup>2</sup>x<sup>3</sup>-6<sup>3</sup>x<sup>4</sup>-6<sup>4</sup>x<sup>5</sup>-6<sup>5</sup>x<sup>6</sup>. The denominator this function has a factor of 1-6x. The limit of probability can be found using [[partial fractions]]: the generating function can be written (2/7)/(1-6x)+(another term) and as long as the roots (real and complex) of the denominator of the other term have absolute value greater than 1/6, the 1-6x term dominates. I don't see an obvious way of proving this, though it's apparently the case given the table above. Note that the anon's proof above that the limit is 2/7 has a subtle flaw in that it assumes the limit exists in the first place. The analysis of two die case can be done similarly but is more complex.--[[User:RDBury|RDBury]] ([[User talk:RDBury|talk]]) 07:01, 27 August 2011 (UTC)
::::::Wouldn't another approach be to consider the numbers in batches of 6 (or 12 for the two dice case), and construct a matrix A that calculates the probabilities of the next 6 given the probabilities of the current 6 (this matrix would be fairly complex, since you'd have to consider up to 6 tosses of the die)? Then hope it's diagonalizable, and if so, compute the limit.--[[User:Antendren|Antendren]] ([[User talk:Antendren|talk]]) 08:32, 27 August 2011 (UTC)
:::::::That's a great idea! We can even simplify the matrix by instead calculating the 6 position probabilities counting back from ''the very next position'', given the 6 counting back from the current position. Thus, your A is my M<sup>6</sup>. For the one die case:
[ 1/6 1/6 1/6 1/6 1/6 1/6 ] [ 1 ]
[ 1 0 0 0 0 0 ] [ 0 ]
[ 0 1 0 0 0 0 ] [ 0 ]
M = [ 0 0 1 0 0 0 ], S = [ 0 ].
[ 0 0 0 1 0 0 ] [ 0 ]
[ 0 0 0 0 1 0 ] [ 0 ]
:::::::And for the two dice case:
[ 0 1/36 2/36 3/36 4/36 5/36 6/36 5/36 4/36 3/36 2/36 1/36 ] [ 1 ]
[ 1 0 0 0 0 0 0 0 0 0 0 0 ] [ 0 ]
[ 0 1 0 0 0 0 0 0 0 0 0 0 ] [ 0 ]
[ 0 0 1 0 0 0 0 0 0 0 0 0 ] [ 0 ]
[ 0 0 0 1 0 0 0 0 0 0 0 0 ] [ 0 ]
[ 0 0 0 0 1 0 0 0 0 0 0 0 ] [ 0 ]
M = [ 0 0 0 0 0 1 0 0 0 0 0 0 ], S = [ 0 ].
[ 0 0 0 0 0 0 1 0 0 0 0 0 ] [ 0 ]
[ 0 0 0 0 0 0 0 1 0 0 0 0 ] [ 0 ]
[ 0 0 0 0 0 0 0 0 1 0 0 0 ] [ 0 ]
[ 0 0 0 0 0 0 0 0 0 1 0 0 ] [ 0 ]
[ 0 0 0 0 0 0 0 0 0 0 1 0 ] [ 0 ]
:::::::Then the probability of landing on position ''n'' is the top value of M<sup>n</sup> S, which, given S, is the top left value of M<sup>n</sup>. Note that the column vectors:
[ 2/7; 2/7; 2/7; 2/7; 2/7; 2/7]
:::::::and
[ 1/7; 1/7; 1/7; 1/7; 1/7; 1/7; 1/7; 1/7; 1/7; 1/7; 1/7; 1/7]
:::::::are fixed points under their respective M -- that is, they are eigenvectors with eigenvalue 1. Does that help? (That would seem to be a necessary condition for them to be limits, but not necessarily sufficient.) -- [[Special:Contributions/110.49.242.138|110.49.242.138]] 1([[User talk:110.49.242.138|talk]]) 12:17, 27 August 2011 (UTC)


:Quick note, for those like me who are curious how numbers of the form <math>-(2a^{2}-b^{2})</math> can be written into a form of <math>2a^{2}-b^{2}</math>, note that <math>2a^{2} - b^{2} = (2a + b)^{2} - 2(a + b)^{2}</math>, and so <math>2a^{2} - b^{2} = -p \Rightarrow p = 2(a + b)^{2} - (2a + b)^{2}</math>. [[User:GalacticShoe|GalacticShoe]] ([[User talk:GalacticShoe|talk]]) 02:20, 11 December 2024 (UTC)
::::::::The eigenvectors above are overspecific, given that any scalar multiple of an eigenvector is also an eigenvector. So what I should have pointed out is that the "one-vector" (sometimes called the "summing vector")
:A prime is expressible as the sum of two squares if and only if it is congruent to <math>1 \!\!\!\pmod 4</math>, as per [[Fermat's theorem on sums of two squares]]. A prime is expressible of the form <math>2a^{2} + b^{2}</math> if and only if it is congruent to <math>1, 3 \!\!\pmod 8</math>, as per [[OEIS:A002479]]. And a prime is expressible of the form <math>2a^{2} - b^{2}</math> if and only if it is congruent to <math>1, 7 \!\!\pmod 8</math>, as per [[OEIS:A035251]]. Between these congruences, all primes are covered. [[User:GalacticShoe|GalacticShoe]] ([[User talk:GalacticShoe|talk]]) 05:59, 11 December 2024 (UTC)
[1; 1; ...1]
::More generally, a number is ''not'' expressible as:
::::::::is an eigenvector with eigenvalue 1 for any such matrix with a finitely supported probability distribution, given in block form:
::# <math>a^{2} + b^{2}</math> if it has a prime factor congruent to <math>3 \!\!\!\pmod 4</math> that is raised to an odd power (equivalently, <math>3, 7 \!\!\pmod 8</math>.)
::# <math>2a^{2} + b^{2}</math> if it has a prime factor congruent to <math>5, 7 \!\!\pmod 8</math> that is raised to an odd power
::# <math>2a^{2} - b^{2}</math> if it has a prime factor congruent to <math>3, 5 \!\!\pmod 8</math> that is raised to an odd power
::It is easy to see why expressibility as any two of these forms leads to the third form holding, and also we can see why it's difficult to see a pattern in numbers that are expressible in none of these forms, in particular we get somewhat-convoluted requirements on exponents of primes in the factorization satisfying congruences modulo 8. [[User:GalacticShoe|GalacticShoe]] ([[User talk:GalacticShoe|talk]]) 06:17, 11 December 2024 (UTC)
:::Thanks. Is any of this covered in some Wikipedia article? &nbsp;--[[User talk:Lambiam#top|Lambiam]] 10:06, 11 December 2024 (UTC)
Assume p is 3 mod 4. Suppose that (2|p)=1. Then <math>x^4+1\equiv (x^2+\lambda x +1)(x^2- \lambda x + 1)\pmod p</math> where <math>\lambda^2-2\equiv 0</math>. Because the cyclotomic ideal <math>(p, \zeta^2+ \lambda\zeta+1)</math> has norm <math>p^2</math> and is stable under the Galois action <math>\zeta\mapsto 1/\zeta</math> it is generated by a single element <math>a\zeta^2+b\zeta+a</math>, of norm <math>(2a^2-b^2)^2</math>.


If (2|p)=-1, then the relevant ideal is stable under <math>\zeta\mapsto -1/\zeta</math> and so is generated by <math>a\zeta^2+b\zeta-a</math>, of norm <math>(2a^2+ b^2)^2</math>. [[User:Tito Omburo|Tito Omburo]] ([[User talk:Tito Omburo|talk]]) 14:43, 11 December 2024 (UTC)
p_1 ... p_n-1 | p_n
--------------+----
|
I | 0
|


= December 11 =
::::::::where sum(p_i) = 1. So the point should have been that fixed points exist, not that the specific limit alone is a fixed point. -- [[Special:Contributions/110.49.241.90|110.49.241.90]] ([[User talk:110.49.241.90|talk]]) 23:44, 28 August 2011 (UTC)
::::::That the roots of the denominator of the second term have absolute value at least 1/6 can be proved with [[winding number]]s. If z traces the circle |z|=1/6, given by z=1/6(cos t + i sin t), then the curve traced by the denominator is given by w=1 + 5/6 cos t + 4/6 cos 2t + 3/6 cos 3t + 2/6 cos 4t + 1/6 cos 5t + i(5/6 sin t + 4/6 sin 2t + 3/6 sin 3t + 2/6 sin 4t + 1/6 sin 5t). I claim that the real part of w is always at least 1/2, in fact 1/2 + 5/6 cos t + 4/6 cos 2t + 3/6 cos 3t + 2/6 cos 4t + 1/6 cos 5t = 1/6(cos 5/2t + cos 3/2t + cos 1/2)<sup>2</sup> as can be shown using various trigonometric identities. Then the curve traced by w stays to the right of the imaginary axis and therefore its winding number around 0 is 0, implying that there are no roots of the polynomial inside the original circle.--[[User:RDBury|RDBury]] ([[User talk:RDBury|talk]]) 08:25, 27 August 2011 (UTC)


== Unique normal ultrafilter ==
OK, thanks for the feedback, everyone. 110.49.225.10 placed 100 lines of output for the 1 die case (and 150 lines of output for the 2 dice case) on my talk page. [[User:StuRat|StuRat]] ([[User talk:StuRat|talk]]) 07:21, 28 August 2011 (UTC)


So I'm supposed to know the answer to this, I suppose, but I don't seem to :-)
== I have a question about fractals. ==


"Everyone knows" that, in <math>L[U]</math>, [[Gödel's constructible universe]] relative to an [[ultrafilter]] <math>U</math> on some [[measurable cardinal]] <math>\kappa</math>, there is only a single [[normal ultrafilter]], namely <math>U</math> itself. See for example [[John R. Steel]]'s monograph [https://math.berkeley.edu/~steel/papers/comparisonlemma.03.07.22.pdf here], at Theorem 1.7.
My question is does anyone know if fractals has found any applications in psychology? I was asking this because I took a random system of thought (Theology, actually); and it seems to be based on a fractal. Please let me know. Thanks! [[User:Lighthead|<b><font color="#CCCC00">'''Lighthead'''</font></b>]] [[User talk:Lighthead|<sup>þ</sup>]] 22:49, 26 August 2011 (UTC)


So I guess that must mean that the [[product measure]] <math>U\times U</math>, meaning you fix some identification between <math>\kappa\times\kappa</math> and <math>\kappa</math> and then say a set has measure 1 if measure 1 many of its vertical sections have measure 1, must ''not'' be normal. (Unless it's somehow just equal to <math>U</math> but I don't think it is.)
By the way I realize that Theology is not exactly quantifiable. Roughly speaking. [[User:Lighthead|<b><font color="#CCCC00">'''Lighthead'''</font></b>]] [[User talk:Lighthead|<sup>þ</sup>]] 22:51, 26 August 2011 (UTC)


But is there some direct way to see that? Say, a <s>continuous</s> function <math>f:\kappa\to\kappa</math> with <math>\forall\alpha f(\alpha)<\alpha</math> such that <s>the set of fixed points of <math>f</math> is not in the ultrafilter</s> no singleton has a preimage under <math>f</math> that's in the ultrafilter? I haven't been able to come up with it. --[[User:Trovatore|Trovatore]] ([[User talk:Trovatore|talk]]) 06:01, 11 December 2024 (UTC)
Let me explain so that I don't look like a ''complete'' idiot. Let's take Judeo-Christian Philosophy. God (1). Angels (.5) Humans (.25) Animals, perhaps (.125) And so on. [[User:Lighthead|<b><font color="#CCCC00">'''Lighthead'''</font></b>]] [[User talk:Lighthead|<sup>þ</sup>]] 23:00, 26 August 2011 (UTC)
:Keep explaining- I still don't see anything like fractals in what you're describing. (And I don't know if fractals have applications in psychology- I doubt it.) [[User:Staecker|Staecker]] ([[User talk:Staecker|talk]]) 23:21, 26 August 2011 (UTC)


::Well maybe I don't understand fractals then. I'm not mathematical. And thanks for doubting, I'm sure that if people would've doubted in the first place the idea of fractals would've gotten off the ground. Mandelbrot was very popular. Especially in his views. [[User:Lighthead|<b><font color="#CCCC00">'''Lighthead'''</font></b>]] [[User talk:Lighthead|<sup>þ</sup>]] 23:52, 26 August 2011 (UTC)


:::The concept of a fractal is that a part of the system has the same structure as the whole system, but at a smaller scale. For your example to represent a fractal, humans would have to be parts of God and also have the same structure as God -- in Christian theology humans may be considered to have the same form of God, but they are not considered parts of God. So that isn't really a fractal in any sense that I understand. I'm not aware of any applications of fractals to psychology. (There are claims to have found fractal structure in brain electrical activity, if that can be counted as psychology.) [[User:Looie496|Looie496]] ([[User talk:Looie496|talk]]) 00:16, 27 August 2011 (UTC)


:Well, as for religions, perhaps the "repeating the same pattern at different scales" part may apply to some time scales in the Hindu and native Central, and South American religions, in particular. I believe they have the "cycles within cycles" concept as central parts of their religions. [[User:StuRat|StuRat]] ([[User talk:StuRat|talk]]) 00:39, 27 August 2011 (UTC)


= December 15 =
::As to the people that thought that I was saying nonsense; I meant Theology from the socio-psychological standpoint. That's actually why I gave the disclaimer that I realize that ''God'' is not quantifiable. I don't believe that God is a construct, but I was attacking it from the standpoint that those from the scientific standpoint at least objectively see it as a construct. I think that StuRat came close to understanding what I'm talking about. And by the way it's not too far fetched to see psychological processes as mathematical. How do you explain game theory, and how they've found the actions of terrorist cells and dictatorial governments to follow mathematical principles? People like Staecker should avoid the knee-jerk reaction of treating everything that people say as nonsense. I might not be very mathematical, but I do have a capacity to think theoretically. [[User:Lighthead|<b><font color="#CCCC00">'''Lighthead'''</font></b>]] [[User talk:Lighthead|<sup>þ</sup>]] 03:00, 27 August 2011 (UTC)


== What is the cause of this paradox? ==
:::I actually don't want to get too far into Theology because that was just an example; but even under the Judeo-Christian perspective, God is said to be a part of everything and therefore a part of us (from what Looie496 was saying). And also to what Looie496 was saying, yeah, electrical brain activity would count from what I'm saying. Electrical brain activity has been researched as perhaps an indicator of what people are thinking. That seems to answer most if not all of what I'm saying. [[User:Lighthead|<b><font color="#CCCC00">'''Lighthead'''</font></b>]] [[User talk:Lighthead|<sup>þ</sup>]] 04:12, 27 August 2011 (UTC)


I recently completed a calculus term, in which one of the last units involving how much one aspect of an object was changing in relation to time at a certain point, given the rate of change of another aspect. Many specific questions could be analyzed as a right triangle with one leg (the x) remaining constant and the other leg (the y) growing at a specified rate. When it came time to solve for the value of the dz/dt (the rate of the hypotenuse’s growth with respect to time) at a certain point, it ended up as less than the provided dy/dt. Here’s an illustration:
= August 27 =


The x is the distance from me to a tower. This remains constant.
== Show that the set of bounded functions is infinite dimensional ==


The y is the distance from the tower to a flying bird.
Define the set of bounded functions as the set of functions for which there exists <math>c \in \mathbb{R}</math> such that <math>|f(x)|<c</math> for all real x. Anyway I guess the approach is to show that, given any arbitrary finite linearly independent subset of this set of functions, show that their exists another bounded function which is not in the span of them. How do you do that, or is there a better approach? [[User:Widener|Widener]] ([[User talk:Widener|talk]]) 03:38, 27 August 2011 (UTC)
:Hint: Think about the characteristic functions of singletons &mdash; functions that are zero everywhere except at one point, where they take the value 1. Given a finite set of such functions, can you see a way to find a bounded function not in their span? --[[User:Trovatore|Trovatore]] ([[User talk:Trovatore|talk]]) 03:46, 27 August 2011 (UTC)
:: (or more to the point, a function that ''is'' in the span of all characteristic functions of singletons, but ''not'' in the span of the given subset) --[[User:Trovatore|Trovatore]] ([[User talk:Trovatore|talk]]) 03:49, 27 August 2011 (UTC)
::: Obviously any other characteristic function of singletons will do. But that only shows that the set of characteristic functions of singletons is infinite dimensional, it does not show that all bounded functions are infinite dimensional. [[User:Widener|Widener]] ([[User talk:Widener|talk]]) 03:55, 27 August 2011 (UTC)
:::: The span of the set of characteristic functions of singletons is a subspace of the space of all bounded functions. --[[User:Trovatore|Trovatore]] ([[User talk:Trovatore|talk]]) 04:02, 27 August 2011 (UTC)
::::: I'm guessing that if A is a subspace of B and A is infinite dimensional, that implies B is infinite dimensional. Can you prove that? [[User:Widener|Widener]] ([[User talk:Widener|talk]]) 04:08, 27 August 2011 (UTC)
:::::: Try it the other direction. Take a subspace of a finite-dimensional space. Can you show it's finite-dimensional? --[[User:Trovatore|Trovatore]] ([[User talk:Trovatore|talk]]) 04:10, 27 August 2011 (UTC)
::::::: Okay thanks. [[User:Widener|Widener]] ([[User talk:Widener|talk]]) 04:23, 27 August 2011 (UTC)


The dy/dt is the speed at which the bird is flying from the tower.
== Previous problem with generalized dice ==


The z is my distance from the bird.
Suppose in the problem above, instead of a fair die you use a weighted die, so the probability of getting i is p<sub>i</sub> (i>0). We don't need to assume the number of sides is 6 or even finite. The argument given above shows that if limit of the probability (call this q<sub>n</sub>) that position n will be reached exists, then that limit is 1/E(i), provided of course that E(i) exists. This won't always be the case though, for example if p<sub>2</sub>=1 and all other p<sub>i</sub> are 0, then the probability of reaching n is 0 if n is odd and 1 if n is even, so there is no limit. There are also distributions with no expected value. But if you add conditions to allow for these possibilities then is it true that the limit always exists? Specifically, prove or disprove: If gcf{i: p<sub>i</sub>>0}=1 then lim(n→∞) q<sub>n</sub> = 1/E(i) if E(i) exists and lim(n→∞) q<sub>n</sub> = 0 if E(i) does not exist.--[[User:RDBury|RDBury]] ([[User talk:RDBury|talk]]) 09:07, 27 August 2011 (UTC)


In this illustration, the distance between me and the bird is increasing at a slower rate than the speed at which the bird itself is flying. What is the cause of this paradox? [[User:Primal Groudon|Primal Groudon]] ([[User talk:Primal Groudon|talk]]) 19:43, 15 December 2024 (UTC)
== Can you solve this? ==
:I do not see any paradox here. [[User:Ruslik0|Ruslik]]_[[User Talk:Ruslik0|<span style="color:red">Zero</span>]] 20:30, 15 December 2024 (UTC)

What is 12/20 - 15/20 <small><span class="autosigned">— Preceding [[Wikipedia:Signatures|unsigned]] comment added by [[User:Lightylight|Lightylight]] ([[User talk:Lightylight|talk]] • [[Special:Contributions/Lightylight|contribs]]) 20:28, 27 August 2011 (UTC)</span></small><!-- Template:Unsigned --> <!--Autosigned by SineBot-->

:Yes I can solve it :) Did you try it out, what is 12-15 dollars for instance? How about if the dollars are replaced by nickels (5 cents or a twentieth of a dollar) [[User:Dmcq|Dmcq]] ([[User talk:Dmcq|talk]]) 20:34, 27 August 2011 (UTC)

:Holy shit, I am blown away!
:You have presented ''the'' problem whose solution will solve... '''''ALL OF MATHEMATICS'''''. --[[User:COVIZAPIBETEFOKY|COVIZAPIBETEFOKY]] ([[User talk:COVIZAPIBETEFOKY|talk]]) 21:06, 27 August 2011 (UTC)

::Is the sarcasm really necessary? It's a perfectly valid question, although it does seem a little strange age-group wise. <span style="color:#3A3A3A">'''Grandiose''' </span><span style="color:gray">([[User:Grandiose|me]], [[User_talk:Grandiose|talk]], [[Special:Contributions/Grandiose|contribs]]) </span> 21:08, 27 August 2011 (UTC)
:::Yes. --[[User:COVIZAPIBETEFOKY|COVIZAPIBETEFOKY]] ([[User talk:COVIZAPIBETEFOKY|talk]]) 00:51, 28 August 2011 (UTC)
:The only possible source of confusion on this problem is the order of operations. The standard order of operations specifies that the division be done before the subtraction:

(12/20) - (15/20) = (12-15)/20

:You could get different answers, though, if you used a different order of operations:

<u>12/(20-15)</u>
20

:But, assuming you meant to use the standard order of operations, just complete the first one, noting that you will get a negative fraction. [[User:StuRat|StuRat]] ([[User talk:StuRat|talk]]) 21:58, 27 August 2011 (UTC)
::Its a bit deeper than that. Problems like this are one of the biggest stumbling blocks in mathematical development, I guess about half the population, many adults included, would not be able to do it. Conceptually it represents the first generalisation of number after the integers, it involves a whole new techniques for manipulating numbers which follow counter intuitive rules. Why do you only subtract the tops not the bottoms as well? Why are the add/subtract rules so different to multiply rules? How come two different fractions are equal to each other. And to make matters worse the sums going to have a negative result as well, what the heck does minus half an apple mean?
::So to try and give some meaning, take a sheet of paper, fold it in four one way, fold it in five the other way. Cut along the fold so you get 20 bits. All the bits together make one whole, each bit is 1/20th. Try the easier sum 15/20 - 12/20. Count out 15 bits take away 12, how many is left, whats that as a fraction?. Play with 1/2, 1/4, 1/5 all of which can be made from the bits, add and subtract them and see what happens. --[[User:Salix alba|Salix]] ([[User talk:Salix alba|talk]]): 23:22, 27 August 2011 (UTC)

== Commas in non-decimal numbers ==

Is it okay to use commas to separate digits in numbers that are not in base ten? For example, instead of writing 10001000100011100011001011011010000100111011101100110000000011100010110010010<sub>2</sub>, could one write 10,001,000,100,011,100,011,001,011,011,010,000,100,111,011,101,100,110,000,000,011,100,010,110,010,010<sub>2</sub>? -[[User:Metroman|Metroman]] ([[User talk:Metroman|talk]]) 23:54, 27 August 2011 (UTC)
:While the comma is a commonly used [[digit group separator]], there are many options, and your choice should be based on context. The comma is certainly a permissible choice for your own use, but a space or an underscore ("_") is more commonly used with binary numbers. The space or [[thin space]] is more likely to be seen in general texts, and the underscore in computer science. The underscore is a permissible digit group separator symbol for constants (of any base, including decimal) in many computer languages because the comma is reserved to separate values.
:Also note that you need not group only by threes as you are used to in decimal (although several cultures use different groupings of two or four in decimal numbers, and may even vary the group size within a single number as with Indians writing 7,00,00,00,000 where you may be used to 7,000,000,000 -- see [[lakh]]). Grouping a binary number in three suggests that while you are writing it in binary, you are representing it in support of [[octal]]. These days it is more common to group in fours, suggesting support of [[hexadecimal]], or eights, marking [[byte]] (or [[octet (computing)|octet]]) boundaries. -- [[Special:Contributions/110.49.242.14|110.49.242.14]] ([[User talk:110.49.242.14|talk]]) 01:16, 28 August 2011 (UTC)

:It's problematic. See [[Digit group separator]]. Groups of 3 hints at decimal numbers. I wouldn't use binary groups of 3 except in special circumstances where [[octal]] numbers are very common. [[User:PrimeHunter|PrimeHunter]] ([[User talk:PrimeHunter|talk]]) 01:18, 28 August 2011 (UTC)

= August 28 =

== Kernel of a transformation on the set of polynomials ==

Given integers <math>n,m \ge 1</math> Let <math>x_1,...,x_n</math> be distinct points in <math>\mathbb{R}</math> and <math>\mathbb{P}_m</math> denote the set of polynomials of degree at most <math>m</math>. Define a transformation <math>T:\mathbb{P}_m \rightarrow \mathbb{R}^n</math> by <math>T(f) = \begin{bmatrix} f(x_1) \\ \vdots \\ f(x_n) \end{bmatrix}</math> where <math> f \in \mathbb{P}_m</math>. What is the dimension of the kernel of T?<br /><br />
The case m<n is trivial. Otherwise any such polynomial in the kernel of T can be written as <math>a \left(\prod_{i=1}^n (x-x_i)\right) \left(\prod_{i=n+1}^m (x-c_i)\right)</math> where <math>c_i \in \mathbb{C}, a \in \mathbb{R}</math> are arbitrary (free?) variables. Is it sufficient to claim that the answer is <math>m-n+1</math> because that is the number of free variables, or do I need to show something more (like find a basis for the kernel) ? [[User:Widener|Widener]] ([[User talk:Widener|talk]]) 11:57, 28 August 2011 (UTC)
:You know the dimension of the image from the question on Aug. 21. Use dim(ker)+dim(im)=dim(domain).--[[User:RDBury|RDBury]] ([[User talk:RDBury|talk]]) 15:27, 28 August 2011 (UTC)
::Yes, I know that, but I would like to know if it can be done without knowing dim(im) beforehand. [[User:Widener|Widener]] ([[User talk:Widener|talk]]) 17:38, 28 August 2011 (UTC)
:::You've more or less already answered your own question: any polynomial in the kernel can be written (uniquely) as <math>\left(\prod_{i=1}^n (x-x_i)\right)p(x)</math> for some <math>p\in P_{m-n}</math>. [[User:Sławomir Biały|<span style="text-shadow:grey 0.3em 0.3em 0.1em; class=texhtml">Sławomir Biały</span>]] ([[User talk:Sławomir Biały|talk]]) 17:49, 28 August 2011 (UTC)

== Partial Derivative of Volume of Cone ==

Hello. Why is the partial derivative of the volume of a cone not its surface area unlike the ball (solid sphere)? Thanks in advance. --[[User:Mayfare|Mayfare]] ([[User talk:Mayfare|talk]]) 15:24, 28 August 2011 (UTC)
:The reason it works for a ball is that a small change in radius radius results in an shell with uniform thickness; it has approximately the same volume as area times the thickness. That reasoning fails for a cone. It also depends on the how you measure the size of the sphere. For example the formulas for area and volume in terms of diameter are V=1/6 π d<sup>3</sup> and A = π d<sup>2</sup>; the derivative is off by a factor of 2 in that case. There are reasons that the derivative of volume formula is the area formula for a sphere, but it's kind of a one-off.--[[User:RDBury|RDBury]] ([[User talk:RDBury|talk]]) 15:39, 28 August 2011 (UTC)

== Cone terminology ==

There are four different things I think of as a "cone":

1) What most people call a "cone", that is, the portion trimmed between a plane and a vertex point.

2) A "semi-infinite cone", trimmed only to the vertex point.

3) An "infinite cone", with two lobes, unbounded at both ends.

4) A "conic [[frustrum]]", with a single lobe trimmed at both ends by planes.

So, am I using the proper terminology to describe these ? Is there a way to specify the first one that makes it clear I'm not talking about the others ? [[User:StuRat|StuRat]] ([[User talk:StuRat|talk]]) 17:49, 28 August 2011 (UTC)

:See the disambiguation page [[cone]]. Those aren't too different and are only a small subset of its meanings. [[User:Dmcq|Dmcq]] ([[User talk:Dmcq|talk]]) 18:13, 28 August 2011 (UTC)

== Romanian math ==

I noticed that in several international math journals, a disproportionally large number of problems were submitted by Romanians. Why is this? Are Romanians naturally better at math or something? --[[Special:Contributions/76.211.88.37|76.211.88.37]] ([[User talk:76.211.88.37|talk]]) 19:41, 28 August 2011 (UTC)
:No they're terrible at maths and can't solve any problems so they're trying to get other people to help them ;-) They've done a lot of fine maths over the years. Also mathematics has a good public reputation there, the original maths olympiad was held by Romania for instance. [[User:Dmcq|Dmcq]] ([[User talk:Dmcq|talk]]) 21:20, 28 August 2011 (UTC)
::You find that a lot of the old [[Eastern Block]] countries are the same. They weren't touched by commercialism like the West was. Things like a good education still come before being popular and famous. In the West, being clever isn't cool. <span style="white-space:nowrap;">— [[User:Fly by Night|<span style="font-family:Segoe print">'''''Fly by Night'''''</span>]] <font color="#000000">([[User talk:Fly by Night|<span style="font-family:Segoe print">talk</span>]])</font></span> 22:55, 28 August 2011 (UTC)

== A question about <math> \zeta (0) </math> ==

I know that when evaluated this produces <math> - \frac{1}{2} </math> </br> </br>
I'm not quite sure why this is the answer, but I was wondering if there was any significance of this being -.5 and whether or not there are any other values z such that <math>\zeta(z) = -\frac{1}{2} </math> And if there were, what the significance of THAT would be. As you can probably tell, I'm new to this function :-) [[User:Aacehm|Aacehm]] ([[User talk:Aacehm|talk]]) 21:14, 28 August 2011 (UTC)

Latest revision as of 20:30, 15 December 2024

Welcome to the mathematics section
of the Wikipedia reference desk.
Select a section:
Want a faster answer?

Main page: Help searching Wikipedia

   

How can I get my question answered?

  • Select the section of the desk that best fits the general topic of your question (see the navigation column to the right).
  • Post your question to only one section, providing a short header that gives the topic of your question.
  • Type '~~~~' (that is, four tilde characters) at the end – this signs and dates your contribution so we know who wrote what and when.
  • Don't post personal contact information – it will be removed. Any answers will be provided here.
  • Please be as specific as possible, and include all relevant context – the usefulness of answers may depend on the context.
  • Note:
    • We don't answer (and may remove) questions that require medical diagnosis or legal advice.
    • We don't answer requests for opinions, predictions or debate.
    • We don't do your homework for you, though we'll help you past the stuck point.
    • We don't conduct original research or provide a free source of ideas, but we'll help you find information you need.



How do I answer a question?

Main page: Wikipedia:Reference desk/Guidelines

  • The best answers address the question directly, and back up facts with wikilinks and links to sources. Do not edit others' comments and do not give any medical or legal advice.
See also:


December 4

[edit]

How much is this

[edit]

37.162.46.235 (talk) 11:00, 4 December 2024 (UTC)[reply]

1 for n even, 0 for n odd. For see Grandi's series. --Wrongfilter (talk) 11:18, 4 December 2024 (UTC)[reply]
I hope this is not homework. Note that (for finite ) this is a finite geometric series.  --Lambiam 21:51, 4 December 2024 (UTC)[reply]


December 6

[edit]

Is there anything that would prevent peforming Weil Descent on binary curves of large characteristics ?

[edit]

The ghs attack involve creating an hyperlliptic curve cover for a given binary curve. The reason the attack fails most of the time is the resulting genus grows exponentially relative to the curve’s degree.

We don’t hear about the attack on finite fields of large characteristics since such curves are already secure by being prime. However, I notice a few protocol relies on the discrete logarithm security on curves with 400/500 bits modulus resulting from extension fields of characteristics that are 200/245bits long.

Since the degree is most of the time equal to 3 or 2, is there anything that would prevent creating suitable hyperelliptic cover for such curves in practice ? 2A01:E0A:401:A7C0:28FE:E0C4:2F97:8E08 (talk) 12:09, 6 December 2024 (UTC)[reply]

December 7

[edit]

Mathematical operation navigation templates

[edit]
RDBury is right, this discussion belongs at Wikipedia talk:WikiProject Mathematics
The following discussion has been closed. Please do not modify it.

If anyone with some mathematical expertise is interested, I'd appreciate some additional input at Talk:Exponentiation#funny table at end. The question is whether our articles on various mathematical operations could use a navigational template (aka "{{Navbox}}"). Our Exponentiation article tried to use {{Mathematical expressions}} for this purpose, but it doesn't really work. I've created {{Mathematical operations}} as a potential alternative, but the categorization and presentation I've created is probably naïve. (The whole effort may or not be worth it at all.) —scs (talk) 00:36, 7 December 2024 (UTC)[reply]

Wikipedia talk:WikiProject Mathematics is a better forum for this kind of thing, since it's focused on Wikipedia's mathematical articles. --RDBury (talk) 04:07, 7 December 2024 (UTC)[reply]
@RDBury: Excellent point. Thanks. —scs (talk) 13:49, 7 December 2024 (UTC)[reply]

December 8

[edit]

For each positive integer , which primes are still primes in the ring ?

[edit]

For each positive integer , which primes are still primes in the ring ? When , is the original integer ring, when , is the ring of Gaussian integers, when , is the ring of Eisenstein integers, and the primes in the Gaussian integers are the primes , and the primes in the Eisenstein integers are the primes , but how about larger ? 218.187.66.163 (talk) 04:50, 8 December 2024 (UTC)[reply]

A minuscule contribution: for the natural Gaussian primes and are composite:
So is the least remaining candidate.  --Lambiam 09:00, 8 December 2024 (UTC)[reply]
It is actually easy to see that is composite, since is a perfect square:
Hence, writing by abuse of notation for we have:
More in general, any natural number that can be written in the form is not prime in This also rules out the Gaussian primes and  --Lambiam 11:50, 8 December 2024 (UTC)[reply]
So which primes are still primes in the ring ? How about and ? 220.132.216.52 (talk) 06:32, 9 December 2024 (UTC)[reply]
As I wrote, this is only a minuscule contribution. We do not do research on command; in fact, we are actually not supposed to do any original research here.  --Lambiam 09:23, 9 December 2024 (UTC)[reply]
Moreover, is also a perfect square. (As in the Gaussian integers, the additive inverse of a square is again a square.) So natural numbers of the form are also composite. This further rules out and A direct proof that, e.g., is composite: There are no remaining candidates below and I can in fact not find any larger ones either. This raises the conjecture:
Every prime number can be written in one of the three forms and
Is this a known theorem? If true, no number in is a natural prime. (Note that countless composite numbers cannot be written in any of these forms; to mention just a few: )  --Lambiam 11:46, 9 December 2024 (UTC)[reply]
I'll state things a little more generally, in the cyclotomic field . (Your n is twice mine.) A prime q factors as , where each is a prime ideal of the same degree , which is the least positive integer such that . (We have assumed that q does not divide n, because if it did, then it would ramify and not be prime. Also note that we have to use ideals, because the cyclotomic ring is not a UFD.) In particular, stays prime if and only if generates the group of units modulo . When n is a power of two times an odd composite, the group of units is not cyclic, and so the answer is never. When n is a prime or twice a prime, the answer is when q is a primitive root mod n. If n is 4 times a power of two times a prime, the answer is never. Tito Omburo (talk) 11:08, 8 December 2024 (UTC)[reply]
For your , and are the same, as well as and , this is why I use instead of . 61.229.100.34 (talk) 20:58, 8 December 2024 (UTC)[reply]
Also, what is the class number of the cyclotomic field ? Let be the class number of the cyclotomic field , I only know that:
  • for (is there any other such )?
  • If divides , then also divides , thus we can let
  • For prime , divides if and only if is Bernoulli irregular prime
  • For prime , divides if and only if is Euler irregular prime
  • for (is there any other such )?
  • is prime for (are there infinitely many such ?)
Is there an algorithm to calculate quickly? 61.229.100.34 (talk) 21:14, 8 December 2024 (UTC)[reply]

Can we say anything special about every pair of functions f,g, satisfying f(g(x))=f(x) for every x?

[edit]

Especially, is there an accepted term for such a pair?

Here are three simple examples, for two functions f,g, satisfying the above, and defined for every natural number:

Example #1:

f is constant.

Example #2:

f(x)=g(x), and is the smallest even number, not greater than x.

Example #3:

f(x)=1 if x is even, otherwise f(x)=2.
g(x)=x+2.

2A06:C701:746D:AE00:ACFC:490:74C3:660 (talk) 09:31, 8 December 2024 (UTC)[reply]

One way to consider such a pair is dynamically. If you consider the dynamical system , then the condition can be stated as " is constant on -orbits". More precisely, let be the domain of , which is also the codomain of . Define an equivalence relation on by if for some positive integers . Then is simply a function on the set of equivalence classes (=space of orbits). In ergodic theory, such a function is thought of as an "observable" or "function of state", being the mathematical analog of a thermodynamic observable such as temperature. Tito Omburo (talk) 11:52, 8 December 2024 (UTC)[reply]
After you've mentioned temprature, could you explain what are f,g, as far as temprature is concerned? Additionally, could you give another useful example from physics for such a pair of functions? 2A06:C701:746D:AE00:ACFC:490:74C3:660 (talk) 19:49, 8 December 2024 (UTC)[reply]
This equation is just the definition of function g. For instance if function f has the inverse function f−1 then we have g(x)=x. Ruslik_Zero 20:23, 8 December 2024 (UTC)[reply]
If f is the temperature, and g is the evolution of an ensemble of particles in thermal equilibrium (taken at a single time, say one second later), then because temperature is a function of state, one has for all ensembles x. Another example from physics is when is a Hamiltonian evolution. Then the functions with this property (subject to smoothness) are those that (Poisson) commute with the Hamiltonian, i.e. "constants of the motion". Tito Omburo (talk) 20:33, 8 December 2024 (UTC)[reply]
Thx. 2A06:C701:746D:AE00:ACFC:490:74C3:660 (talk) 10:43, 9 December 2024 (UTC)[reply]
Let be a function from to and a function from to Using the notation for function composition, the property under discussion can concisely be expressed as An equivalent but verbose way of saying the same is that the preimage of any set under is closed under the application of  --Lambiam 08:54, 9 December 2024 (UTC)[reply]
Thx. 2A06:C701:746D:AE00:ACFC:490:74C3:660 (talk) 10:43, 9 December 2024 (UTC)[reply]

IEEE Xplore paper claim to acheive exponentiation inversion suitable for pairing in polynomial time. Is it untrustworthy ?

[edit]

I just found https://ieeexplore.ieee.org/abstract/document/6530387. Given the multiplicative group factorization in the underlying finite field of a target bn curve, they claim to acheive exponentiation inversion suitable for pairing inversion in seconds on a 32 bits cpu.

On 1 side, the paper is supposed to be peer reviewed by the iee Xplore journal and they give examples on 100 bits. On the other side, in addition to the claim, their algorithm 2 and 3 are very implicit, and as an untrained student, I fail to understand how to implement them, though I fail to understand things like performing a Weil descent.

Is the paper untrustworthy, or would it be possible to get code that can be run ? 2A01:E0A:401:A7C0:152B:F56C:F8A8:D203 (talk) 18:53, 8 December 2024 (UTC)[reply]

About the paper, I agree to share the paper privately 2A01:E0A:401:A7C0:152B:F56C:F8A8:D203 (talk) 18:54, 8 December 2024 (UTC)[reply]

December 9

[edit]

If the Mersenne number 2^p-1 is prime, then must it be the smallest Mersenne prime == 1 mod p?

[edit]

If the Mersenne number 2^p-1 is prime, then must it be the smallest Mersenne prime == 1 mod p? (i.e. there is no prime q < p such that 2^q-1 is also a Mersenne prime == 1 mod p) If p is prime (no matter 2^p-1 is prime or not), 2^p-1 is always == 1 mod p. However, there are primes p such that there is a prime q < p such that 2^q-1 is also a Mersenne prime == 1 mod p:

but for these primes p, 2^p-1 is not prime, and my question is: Is there a prime p such that 2^p-1 is a prime and there is a prime q < p such that 2^q-1 is also a Mersenne prime == 1 mod p?

  • If 2^11-1 is prime, then this is true, since 2^11-1 is == 1 mod 31 and 2^31-1 is prime, but 2^11-1 is not prime
  • If 2^23-1 or 2^67-1 is prime, then this is true, since 2^23-1 and 2^67-1 are == 1 mod 89 and 2^89-1 is prime, but 2^23-1 and 2^67-1 are not primes
  • If 2^29-1 or 2^43-1 or 2^71-1 or 2^113-1 is prime, then this is true, since 2^29-1 and 2^43-1 and 2^71-1 and 2^113-1 are == 1 mod 127 and 2^127-1 is prime, but 2^29-1 and 2^43-1 and 2^71-1 and 2^113-1 are not primes
  • If 2^191-1 or 2^571-1 or 2^761-1 or 2^1901-1 is prime, then this is true, since 2^191-1 and 2^571-1 and 2^761-1 and 2^1901-1 are == 1 mod 2281 and 2^2281-1 is prime, but 2^191-1 and 2^571-1 and 2^761-1 and 2^1901-1 are not primes
  • If 2^1609-1 is prime, then this is true, since 2^1609-1 is == 1 mod 3217 and 2^3217-1 is prime, but 2^1609-1 is not prime

Another question: For any prime p, is there always a Mersenne prime == 1 mod p? 220.132.216.52 (talk) 19:03, 9 December 2024 (UTC)[reply]

Neither question is easy. For the first, relations would imply that the integer 2 is not a primitive root mod p, and that its order divides for the prime q. This is a sufficiently infrequent occurrence that it seems likely that all Mersenne numbers could be ruled out statistically, but not enough is known about their distribution. For the second, it is not even known if there are infinitely many Mersenne primes. Tito Omburo (talk) 19:23, 9 December 2024 (UTC)[reply]
I found that: 2^9689-1 is the smallest Mersenne prime == 1 mod 29, 2^44497-1 is the smallest Mersenne prime == 1 mod 37, 2^756839-1 is the smallest Mersenne prime == 1 mod 47, 2^57885161-1 is the smallest Mersenne prime == 1 mod 59, 2^4423-1 is the smallest Mersenne prime == 1 mod 67, 2^9941-1 is the smallest Mersenne prime == 1 mod 71, 2^3217-1 is the smallest Mersenne prime == 1 mod 97, 2^21701-1 is the smallest Mersenne prime == 1 mod 101, and none of the 52 known Mersenne primes are == 1 mod these primes p < 1024: 79, 83, 103, 173, 193, 197, 199, 227, 239, 277, 307, 313, 317, 349, 359, 367, 373, 383, 389, 409, 419, 431, 443, 461, 463, 467, 479, 487, 503, 509, 523, 547, 563, 587, 599, 613, 647, 653, 659, 661, 677, 709, 727, 733, 739, 743, 751, 757, 769, 773, 797, 809, 821, 823, 827, 829, 839, 853, 857, 859, 863, 887, 907, 911, 919, 929, 937, 941, 947, 971, 977, 983, 991, 1013, 1019, 1021 220.132.216.52 (talk) 20:51, 9 December 2024 (UTC)[reply]
Also,
but none of these primes p has 2^p-1 is known to be prime, the status of 2^(2^89-1)-1 and 2^(2^107-1)-1 are still unknown (see double Mersenne number), but if at least one of them is prime, then will disprove this conjecture (none of the 52 known Mersenne primes are == 1 mod 2^61-1 or 2^127-1), I think that this conjecture may be as hard as the New Mersenne conjecture. 220.132.216.52 (talk) 20:55, 9 December 2024 (UTC)[reply]
Also, for the primes p < 10000, there is a prime q < p such that 2^q-1 is also a Mersenne prime == 1 mod p only for p = 73, 151, 257, 331, 337, 353, 397, 683, 1321, 1613, 2113, 2731, 4289, 4561, 5113, 5419, 6361, 8191, 9649 (this sequence is not in OEIS), however, none of these primes p have 2^p-1 prime. 220.132.216.52 (talk) 02:23, 10 December 2024 (UTC)[reply]

December 10

[edit]

More on the above conjecture

[edit]

Above I posed:

Conjecture. Every prime number can be written in one of the three forms and

If true, it implies no natural prime is a prime in the ring .

The absolute-value bars are not necessary. A number that can be written in the form is also expressible in the form

It turns out (experimentally; no proof) that a number that can be written in two of these forms can also be written in the third form. The conjecture is not strongly related to the concept of primality, as can be seen in this reformulation:

Conjecture. A natural number that cannot be written in any one of the three forms and is composite.

The first few numbers that cannot be written in any one of these three forms are

They are indeed all composite, but why this should be so is a mystery to me. What do and which appear later in the list, have in common? I see no pattern.

It seems furthermore that the primorials, starting with make the list. (Checked up to )  --Lambiam 19:23, 10 December 2024 (UTC)[reply]

Quick note, for those like me who are curious how numbers of the form can be written into a form of , note that , and so . GalacticShoe (talk) 02:20, 11 December 2024 (UTC)[reply]
A prime is expressible as the sum of two squares if and only if it is congruent to , as per Fermat's theorem on sums of two squares. A prime is expressible of the form if and only if it is congruent to , as per OEIS:A002479. And a prime is expressible of the form if and only if it is congruent to , as per OEIS:A035251. Between these congruences, all primes are covered. GalacticShoe (talk) 05:59, 11 December 2024 (UTC)[reply]
More generally, a number is not expressible as:
  1. if it has a prime factor congruent to that is raised to an odd power (equivalently, .)
  2. if it has a prime factor congruent to that is raised to an odd power
  3. if it has a prime factor congruent to that is raised to an odd power
It is easy to see why expressibility as any two of these forms leads to the third form holding, and also we can see why it's difficult to see a pattern in numbers that are expressible in none of these forms, in particular we get somewhat-convoluted requirements on exponents of primes in the factorization satisfying congruences modulo 8. GalacticShoe (talk) 06:17, 11 December 2024 (UTC)[reply]
Thanks. Is any of this covered in some Wikipedia article?  --Lambiam 10:06, 11 December 2024 (UTC)[reply]

Assume p is 3 mod 4. Suppose that (2|p)=1. Then where . Because the cyclotomic ideal has norm and is stable under the Galois action it is generated by a single element , of norm .

If (2|p)=-1, then the relevant ideal is stable under and so is generated by , of norm . Tito Omburo (talk) 14:43, 11 December 2024 (UTC)[reply]

December 11

[edit]

Unique normal ultrafilter

[edit]

So I'm supposed to know the answer to this, I suppose, but I don't seem to :-)

"Everyone knows" that, in , Gödel's constructible universe relative to an ultrafilter on some measurable cardinal , there is only a single normal ultrafilter, namely itself. See for example John R. Steel's monograph here, at Theorem 1.7.

So I guess that must mean that the product measure , meaning you fix some identification between and and then say a set has measure 1 if measure 1 many of its vertical sections have measure 1, must not be normal. (Unless it's somehow just equal to but I don't think it is.)

But is there some direct way to see that? Say, a continuous function with such that the set of fixed points of is not in the ultrafilter no singleton has a preimage under that's in the ultrafilter? I haven't been able to come up with it. --Trovatore (talk) 06:01, 11 December 2024 (UTC)[reply]



December 15

[edit]

What is the cause of this paradox?

[edit]

I recently completed a calculus term, in which one of the last units involving how much one aspect of an object was changing in relation to time at a certain point, given the rate of change of another aspect. Many specific questions could be analyzed as a right triangle with one leg (the x) remaining constant and the other leg (the y) growing at a specified rate. When it came time to solve for the value of the dz/dt (the rate of the hypotenuse’s growth with respect to time) at a certain point, it ended up as less than the provided dy/dt. Here’s an illustration:

The x is the distance from me to a tower. This remains constant.

The y is the distance from the tower to a flying bird.

The dy/dt is the speed at which the bird is flying from the tower.

The z is my distance from the bird.

In this illustration, the distance between me and the bird is increasing at a slower rate than the speed at which the bird itself is flying. What is the cause of this paradox? Primal Groudon (talk) 19:43, 15 December 2024 (UTC)[reply]

I do not see any paradox here. Ruslik_Zero 20:30, 15 December 2024 (UTC)[reply]