Talk:Idempotence

Humorous Example

> Elevator call buttons are idempotent, though many people think they are not.

That cracked me up!

who actually believes this daesotho 20:42, 21 Oct 2004 (UTC)

What are you talking about? Dysprosia 23:32, 21 Oct 2004 (UTC)

Pronunciation?

Is there a correct pronunciation for idempotent? Is it like omnipotent (om-nip-o-tent), so id-EMP-o-tent, or is it more like the two seperate words idem + potent

I'd say EYE-dm-POT-nt, but then I speak Brit. Charles Matthews 07:57, 30 Jun 2004 (UTC)

Ditto, but then I speak Australian ;) Dysprosia 08:01, 30 Jun 2004 (UTC)

Actually the British is more like EYE-dm-PO-tnt, I guess. Charles Matthews

I'd just like to pedantically point out that it would be pronounced i-DEM-po-tent because because syllable onsets are maximized. daesotho 20:33, 21 Oct 2004 (UTC)

The opening sentence is

"In mathematics, an idempotent element (IPA [ˈaɪdɛmˌpotnt/, like eye-dem-potent) is an element which, intuitively, leaves something unchanged. "

I'd say this is quite misleading because when we're thinking of idempotents as operating on something, they don't in general leave it unchanged, they just don't change it any more when you apply them again. A better version in my opinion would be

"In mathematics an idempotent is, intuitvely, something which changes something, but when applied again to the changed version of that thing does not change it any further."

And by the way I think it's silly giving a pronounciation guide as this is just imposing a particular accent. (For what it's worth I pronounce idempotent with a short 'i', as in the word 'id'). Alex Selby

JA: I think this is an idyll question. Jon Awbrey 14:16, 14 February 2006 (UTC)[reply]

There is only one meaning given here

The article starts by claiming that there are two meanings, but clearly they are the same. The "unary operation" definition is the same because the operation involved is function composition which is a binary operation. If @ denotes function composition, then an idempotent function f is one satisfying f@f=f. This is in fact suggested in the parenthetical comment "(or for a function, composed with)" near the start, but then the article continues as if functions are special. They aren't. McKay 10:52, 29 June 2006 (UTC)[reply]

Primitive Idempotents

Primitive idempotents are important in quantum mechanics as they are the pure states in density matrix or density operator theory. The pure states can be reprsented by spinors, for example if |a> is a spinor, then |a><a| is a primitive idempotent. See the "Density Matrix Formalism" portion of Frank Porter's quantum mechanics class notes (Cal Tech): http://www.cithep.caltech.edu/~fcp/physics/quantumMechanics/

Can we add this to the discussion? What other examples of idempotents are important in physics and mathematics? If this is something that should be included, let me know or do it yourself.

CarlAB 02:57, 7 October 2006 (UTC)[reply]

Unary operation

I don't agree with this sentence: If f is a unary operation, i.e. a map f from some set X into itself... An unary operation doesn't have to be from one set to itself. Even if we are talking about idempotence. For example, if $X\subseteq Y$ , function $f:X\rightarrow Y$ might be idempotent. If I'm right, then the next few sentences would need corrections as well. -- Obradović Goran (ta^lk 20:22, 30 April 2007 (UTC)[reply]

Merge proposal (Conclusion: merge; discussion archived)

The following discussion is closed. Please do not modify it. Subsequent comments should be made in a new section.

The result was merge Idempotence (computer science) into Idempotence. Angus Lepper^{(T, C, D)} 23:27, 27 May 2007 (UTC)[reply]

I've made a proposal at Idempotence (computer science) that it be merged into this article (or simply deleted and redirected here if — as I suspect — there is no real extra content in the other article which would serve a useful purpose here). The two terms describe exactly the same concept with the exception that this article a) does not provide examples from computing and b) provides a better and more formal definition of the subject. Any comment? Angus Lepper^{(T, C, D)} 15:44, 23 May 2007 (UTC)[reply]

Support - the merge sounds like a good idea to me. --Allan McInnes (talk) 05:13, 24 May 2007 (UTC)[reply]

The following is transcluded from Wikipedia talk:WikiProject Computer science

I agree with the merge proposal. Just adding a section in Idempotence to include the (rather limited) encyclopedic content of the other article seems sufficient. Gimme danger 17:54, 23 May 2007 (UTC)

Transcluded by Angus Lepper^{(T, C, D)} 17:14, 24 May 2007 (UTC)[reply]

The discussion above is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

Typo

The first paragraph has embeded parens, and one is not closed. It is not clear how to fix this error.

Fixed, I introduced this error when merging. Angus Lepper^{(T, C, D)} 23:45, 9 June 2007 (UTC)[reply]

Physics

But the probability distributions associated with eigenstates are idempotents.

How so? The probability distribution is simply constant in time. It's not equal to its square its composition with itself. Sure, an idempotent function relates the distribution at one time to that at another, but that's because it's the identity function. This doesn't belong in an article about idempotence, any more than anything else that's constant. —Preceding unsigned comment added by 151.200.247.124 (talk) 01:36, 19 October 2007 (UTC)[reply]

Since nobody has defended or changed this section, I've removed it. 72.75.97.3 (talk) 15:13, 8 December 2007 (UTC)[reply]

Add use for web

I have heard a number of people use idempotence to describe web pages etc. A web request is idempotent if the same URL returns exactly the same page. POST actions are, by definition, not idempotent, but GET requests may be. So a Wikipedia page is idempotent only if nobody edits it. Many web pages fail to be idempotent because they include things like the current date and time. Thoughts? GhostInTheMachine (talk) 09:10, 30 April 2008 (UTC)[reply]

Theoretical computer science?

I changed the classification Category:Theoretical computer science to Category:Computer science, since the meanings in computing described here, such as relating to databases and ESP, do in my opinion not belong to the realm of theoretical computer science. However, this change was reverted. What do others think, is this TCS or just CS? --Lambiam 14:22, 7 August 2008 (UTC)[reply]

"Alternative definition", huh?

Sometimes a unary operation is called idempotent if, whenever it is applied twice to any value, it gives the original value. For example, the complex conjugation of a number is considered an idempotent operation.

I've never heard of this. Could someone please point me to some references? (Because I haven't found any.) I think this is confusing (if not bogus), and should be removed. --Matt Kovacs (talk) 17:07, 24 January 2009 (UTC)[reply]