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November 5

UK bird flu restrictions

With recent events in the UK and the new biosecurity restrictions, does this mean that I should stop flying my pet parrot outside (always on leash) and keep her inside for the present time? Or does this not apply to parrots? I might email the govt for clarification myself, but does anyone know? I don't even know if the goffin is susceptible to avian flu. --Iloveparrots (talk) 00:55, 5 November 2021 (UTC)

You should read the latest from GOV.UK, and decide for yourself:

• "Bird flu - Latest situation: Avian influenza prevention zone declared across Great Britain". GOV.UK. 3 November 2021. --107.15.157.44 (talk) 02:03, 5 November 2021 (UTC)

See also "Is my pet bird at risk from avian flu?".  --Lambiam 12:30, 5 November 2021 (UTC)

Mohs hardness for iodine

Is there a reported value? --- Sandbh (talk) 03:18, 5 November 2021 (UTC)

It's really hard to find, but I dug it up in this CRC Handbook. Page 713-714 contains "Physical Properties of Iodine", and has a reported Mohs hardness of 2. --Jayron32 12:22, 5 November 2021 (UTC)

November 6

Chemistry reactivity questions.

Suppose A is more reactive than B, as in, A reacts with a bunch of other substances faster than B does. Then can there be cases where B reacts with something better than A does? Note that, I imagine to make this question useful, we must make A and B in the same category of something. Like, A and B should both be metals. So if A reacts with 10 different acids better than B does, can B react with an acid that A doesn't? Or B reacts with a base that A doesn't? Can it be such that B reacts with gases better than A does?

Since this is such a specific question, I'll throw in another 1. Our article on chemiluminescence says the 1st accidental discovery was in 1877. But the source is in German. Just wondering if anyone compiles a list of discoveries of chemiluminescence in order, I'm curious to know what the 1st discovery is by theory and not by accident. I'm also wondering if anyone discoveries chemiluminescence in say, the past 20 years, by accident, or and by theory still. 67.165.185.178 (talk) 14:51, 6 November 2021 (UTC).

for your first question the answer is yes. Consider aluminium and copper. When exposed to water and air copper reacts much faster to make a green covering. But aluminium dissolves much better in acid than copper. This is because aluminium forms a protective film. Sometimes the compound produced can be unstable, so for example, lithium reacts with nitrogen, but potassium does not. Or sometimes a stable complex can be formed. For gold, it is pretty tough to react or dissolve, but with air and cyanide it can dissolve. Graeme Bartlett (talk) 21:29, 6 November 2021 (UTC)

Ducks and geese don't have crops?

See the recent edit to crop (anatomy) here. The anon rightly points out in his/her edit summary that the article states that ducks and geese don't have crops, but the article is illustrated with an image and a video showing a duck and a goose, apparently with a bulging crop. I have found some websites (e.g. this) that say that ducks and geese don't have crops too. So I have no idea. Anyone able to help? I only really know about parrots to any great extent. --Iloveparrots (talk) 20:26, 6 November 2021 (UTC)

I found lots of links saying they do, including dissected ducks. Greglocock (talk) 22:49, 6 November 2021 (UTC)

@Greglocock - I just saw you'd edited the article. I added something else myself. But... the source you added for ducks - is that not a photo of a dissected chicken? Look at the head... --Iloveparrots (talk) 22:54, 6 November 2021 (UTC)

Farm Animal Behaviour: Characteristics for Assessment of Health and Welfare (p. 234) by Ingvar Ekesbo and Stefan Gunnarsson says:

Unlike many birds, geese and ducks lack a crop.

The authors are both professors at the Swedish University of Agricultural Sciences (SLU), so probably know their onions.

Alansplodge (talk) 13:22, 7 November 2021 (UTC)

See also:-

Domestic ducks, like their wild forebear, the Mallard, do not have a crop... [1]

Ducks do not have a crop to store food... [2]

Unlike most birds, ducks and geese do not have a crop. This outpouching of the oesophagus allows some storage of food before digestion proper begins. In ducks and geese, the whole oesophagus is stretchy and expandable, .. [3]

Ducks do not have a crop to store food, unlike the chicken. Food is stored in the gullet and proventriculus in a similar way to the crop,

The last quote is from Anaesthesia for Veterinary Nurses Alansplodge (talk) 13:41, 7 November 2021 (UTC)

Here are the FAO saying that geese do have crops. Its oesophagus is relatively long, with mucous glands to lubricate the passage of food and extends into the spindle shaped crop that serves as a reservoir for food storage. DuncanHill (talk) 13:53, 7 November 2021 (UTC)

The problem is that anatomy and biology are largely 'stamp collecting' to quote Rutherford, possibly. If a crop is defined as a thing that ducks and geese don't have, then they don't have one. If a crop is defined as part of the gullet where food is stored before hitting the stomach, then they do have one. I'm not interested in arguing about stamp collecting. Greglocock (talk) 21:51, 7 November 2021 (UTC)

Naming body parts by the function they serve indeed has its value for some analyses. But for other analyses in biology it makes sense to name homologous structures consistently, so that we can recognise how they have evolved, how they develop in the embryo, and, for instance, how the nerves innervating them are wired up. Thus there is value in referring to the wing of a bird, the front leg of an elephant, and the arm of a human as the forelimb even though their functions are very different. Recognising homology is not to be dismissed as stamp collecting. Jmchutchinson (talk) 17:00, 8 November 2021 (UTC)

Perhaps the Wikipedia-like way of solving this is to record in the article that there are differing opinions? Alansplodge (talk) 00:15, 8 November 2021 (UTC)

Or differing definitions.--Shantavira|^{feed me} 09:45, 8 November 2021 (UTC)

November 7

Competition vs collaboration

From my experience it seems to me that humans tend to output the best performance in a competition, with this I mean when they have to outperform an opponent (be it a single person or a team), rather than collaboration (solving a fixed problem together before a certain deadline).

That is only my anectdotal evidence though. Are there studies (if possible, more than one) that analyse whether performance is higher in competitions rather than collaborations? --Pier4r (talk) 19:31, 7 November 2021 (UTC)

This is very hard to discuss in general, since it may depend on (1) the cultural background of the subjects; (2) the personality of the subjects; (3) the nature of the task; (4) the measure used for measuring performance; (5) the motivating factors for performing well; (6) the training, if any, the subjects have had in this kind of task. If a group of surgical professionals have to perform a number of open-heart surgeries, I feel it would not draw out their best performance if they are divided in two competing teams, where the team first to tie the knot of the final suture stitch wins the coveted flash surgery championship.  --Lambiam 09:44, 8 November 2021 (UTC)

If you'll excuse the breach of Godwin's Law, one well-known Bohemian corporal applied Social Darwinism to the governance of his country and would often allocate the same task to different departments on the basis that the best one would win. This led to duplication of effort, infighting and an inability to share information which was severely detrimental to the war effort. The outstanding example was the German nuclear weapons program, whose failure is described by a German historian thus:

Compared with the British and American war research efforts united in the Manhattan Project, to this day the prime example of 'big science', the Uranverein was only a loosely knit, decentralized network of researchers with quite different research agendas. Rather than teamwork as on the American end, on the German side we find cut-throat competition, personal rivalries, and fighting over the limited resources.

Alansplodge (talk) 11:32, 9 November 2021 (UTC)

And as requested, some research:

• Our systematic review demonstrates that there is no evidence to support the role of social enterprise as a substitute for publicly owned services. However, there is evidence to show that where social enterprise operates in a collaborative environment, enhanced outcomes can be achieved, such as connectedness, well-being and self-confidence. [4]
• Findings demonstrated a negative direct effect of competition on the range of perceived performance ratings, and a positive indirect effect of competition on team satisfaction as mediated through task conflict. [5]
• The covid-19 pandemic has once again shown the value of international cooperation and collaboration. [6]

Alansplodge (talk) 11:49, 9 November 2021 (UTC)

Not all cultures can do an Amish barnraising. But it is a proof by existence of the fact that it can be done. -- Cimon Avaro; on a pogostick. (talk) 14:11, 9 November 2021 (UTC)

The article is at barn raising Mike Turnbull (talk) 14:16, 9 November 2021 (UTC)

Somewhat related, but I feel this is true with immune systems. If 1 of Snow White's 7 dwarfs got sick, he could stay in isolation and be sick. But if the other 6 intermingle with him and they all catch that sick, then the immune system of the 7 of them combined, has a stronger advantage when the sickness is divided amongst all of them, though with replication. And the reason I use the 7 dwarfs example is because they're all unemployed people, so if 1 person is sick, it benefits for them all to be sick. 67.165.185.178 (talk) 20:17, 11 November 2021 (UTC).

Tube amps versus transistor amps - current and voltage

I remember something that I read about 20 years ago that transistor amplifiers provide a voltage whereas tube amplifiers provide a current. That doesn't make sense to me. Am I mis-remembering something, or is there some sense to it? Bubba73 ^{You talkin' to me?} 20:26, 7 November 2021 (UTC)

They both provide current and voltage, because electricity (by necessity) has both; both current and voltage are different (and related, see Ohm's law) properties of an electric circuit. --Jayron32 22:31, 7 November 2021 (UTC)

Yes, which is why I don't understand the meaning of one delivering a current and the other a voltage, unless it is a constant current or constant voltage, or something. Bubba73 ^{You talkin' to me?} 01:12, 8 November 2021 (UTC)

In general transistor amplifiers run with higher current and lower voltage than tube amplifiers. And those valve amplifiers produce a much higher voltage, but a lower current. Graeme Bartlett (talk) 09:30, 8 November 2021 (UTC)

An amplifier has an input signal and an output signal. As classified in Amplifier § Ideal, these can be defined as current signals or as voltage signals, giving four possible combinations. If the input and output signal are of the same type, this can be used to define the "gain" of the amplifier, which can be a current gain or a voltage gain. Vacuum tube amplifiers produce a voltage gain, while transistor-based amplifiers have a current signal as input, so it easier to think of them as providing a current gain. See for example Bipolar junction transistor: "A bipolar transistor allows a small current injected at one of its terminals to control a much larger current flowing between two other terminals, making the device capable of amplification or switching."  --Lambiam 09:30, 8 November 2021 (UTC)

Thanks, I must have heard something like that in lay terms. Also, IIRC, you can short a tube amps output but not leave it open, whereas you can leave open the output of a transistor amp, but not short it. I.e., a tube amp needs the resistance. Is that right? Bubba73 ^{You talkin' to me?} 04:42, 9 November 2021 (UTC)

Presumably, when you write that one cannot do certain things, you mean one shouldn't. I don't know what bad things may happen when disregarding the warning.  --Lambiam 10:38, 9 November 2021 (UTC)

Yes, I've accidentally had a tube amp on without a load for a short time without ruining it. Bubba73 ^{You talkin' to me?} 03:11, 10 November 2021 (UTC)

November 9

Scientific fraud

This is about Elisabeth Bik. How does she know there's an intention to cheat rather than a printing error? I mean, in publishing scientific papers there are many links, humans and machines. At any step of the process something could go wrong and an image be swapped for another.

E.g. in my study of sociology I have computed the same correlation as someone else did in a book I had never heard of. That was very clear to my professor. But he could not prove intention to cheat, since he provided us with a 30 years old data file. He vowed to never again provide such old files to his students.

People sometimes stumble upon the same correlation, or the same words. That's why afterwards I have always scanned myself my own papers for plagiarism. Which not only keeps the trouble away, but also provides one with new bibliographical sources. tgeorgescu (talk) 02:24, 9 November 2021 (UTC)

Speaking of "how does one know whether something was a printing error?", see this article from 2010. --184.145.50.17 (talk) 06:25, 9 November 2021 (UTC)

What makes you say that Elisabeth Bik knows there is "an intention to cheat"? When she signals issues with published research papers, she calls them "concerns". Many involve reuses of identical images alleged to have been obtained from independent experiments. While such a reuse may occasionally result from an unfortunate accidental mix-up (not a "printing error"), when this happens again and again in the publications of an author, or five times in one publication, it becomes IMO very hard to ascribe this apparent pattern to something unintentional.  --Lambiam 09:52, 9 November 2021 (UTC)

From here: "Overall, 3.8% of published papers contained problematic figures, with at least half exhibiting features suggestive of deliberate manipulation." (emphasis mine). There's the hedging of "suggestive", but clearly an indication that an attempt has been made to identify the intentional ones. Matt Deres (talk) 20:56, 9 November 2021 (UTC)

• Parts of what Bik reports on her blog can be explained by mistakes. Others, not so much. Consider this recent example of images containing repeated elements - the explanation seems to be that they took photos from elsewhere without mentioning it and photoshopped over covered parts, which is clearly a breach of ethics. Tigraan^{Click here for my talk page ("private" contact)} 11:26, 10 November 2021 (UTC)

For an explicit example: "Please note that I am not suggesting here that moving this data point from the treatment group to the placebo group was done on purpose. Mistakes can happen, in particular if there is a rush to publish, and I do not know exactly what has happened here."^[7] This is IMO not "hedging" on the part of Bik, so as to avoid being caught in commitment, but a frank appraisal of (lack of) certainty.  --Lambiam 09:44, 11 November 2021 (UTC)

Definition of Volt

Volts = energy / charge. Each galvanic reaction can add a fixed amount of electrical energy to a closed circuit. Volts equal energy per chemical reaction divided by electric charge.

$Volts={\frac {EnergyPerChemicalReaction}{ElectricCharge}}$

Within a capacitor, the relation between work and charge is:

$W={\frac {1}{2}}VQ={\frac {1}{2}}CV^{2}$

Therefore:

$Volts=2{\frac {Energy}{Charge}}={\sqrt {2{\frac {Energy}{Capacitance}}}}$

A galvanic cell can charge a capacitor until capacitor volts equal nominal volts. Therefore, energy per charge entering the capacitor can differ from energy per charge within the capacitor. Please account for the difference in energy per charge. — Preceding unsigned comment added by Vze2wgsm1 (talk • contribs) 09:00, 9 November 2021 (UTC) Vze2wgsm1 (talk) 09:05, 9 November 2021 (UTC)

Would that be a homework question? Your first challenge is that when charging a capacitor, the voltage starts off from zero and then ramps up. So that you will have to integrate the (current going in) x (voltage) over time to work out the total energy put in. Now secondly your galvanic cell puts out a "fixed" voltage. So what will happen when you attach a fixed voltage to a zero voltage? It is basically the same as shorting out your battery. So where does the energy go in a short circuit? The same effect will happen when attaching an uncharged capacity to your cell. So user:Vze2wgsm1 where will the energy go? Graeme Bartlett (talk) 11:02, 9 November 2021 (UTC)

All charges from a galvanic cell have the same energy per charge. When a 2 volt galvanic cell is charging a capacitor with zero or 1 volt, then a conflict exists between energy per volt in and energy per volt stored. Vze2wgsm1 (talk) 13:31, 9 November 2021 (UTC)

The galvanic cell provides a fixed voltage, the capacitor starts at zero voltage. But we know that the sum of all voltages over a loop in the circuit must be zero, so there must be some voltage over some other component. The wires have some finite resistance and there's the internal resistance of the galvanic cell. Sending a current through that resistance may have something to do with it. PiusImpavidus (talk) 14:24, 9 November 2021 (UTC)

Could be superconductive conductors between the galvanic cell and the capacitor. The capacitor's plates could be superconductive.Vze2wgsm1 (talk) 15:05, 9 November 2021 (UTC)

The differences are always due to energy lost via heat. The second law of thermodynamics strikes again... --Jayron32 14:25, 9 November 2021 (UTC)

No. A capacitor's heat loss is via internal resistance. $P=V^{2}/R$ Vze2wgsm1 (talk) 14:58, 9 November 2021 (UTC)

Not only energy lost to heat. The difference between the voltage across the cell and the voltage across the capacitor includes other impedances such as inductance. If you try to eliminate dissipative effects using superconductors, effects like kinetic inductance can become important. There are still dissipative effects (such as interactions with nearby dielectric materials), but these can be made very small [8]. The circuit then becomes a high-Q LC oscillator. --Amble (talk) 17:46, 9 November 2021 (UTC)

Capacitor or other discharge into a resistor can remove both the charges and the energies associated with the charges.

During oscillation within an LC circuit, all the charges and the energy associated with the charges can exhaustively convert into magnetic energy. Then polarity reverses and magnetic energy can exhaustively convert into the charges and energy associated with the charges in the capacitor. Vze2wgsm1 (talk) 19:03, 9 November 2021 (UTC)

Even if the wires and the plates of the capacitor are superconductive, you still have the internal resistance of the galvanic cell, basically the resistance of the electrolyte, which isn't superconductive. And if you get the resistance low enough, inductance will get important. Now the energy delivered by the galvanic cell not stored in the capacitor when it gets to the same voltage as the cell will be stored in the magnetic field. Then the voltage on the capacitor will overshoot the voltage of the cell, using the energy from the magnetic field. As noted above, it's an LC circuit. PiusImpavidus (talk) 09:35, 10 November 2021 (UTC)

The problem is that a 2 V galvanic cell can supply a 1 volt capacitor w/o violating the conservation of energy law.

A 2 V galvanic cell with high electrolyte resistance can deliver 1 V to an external circuit if external circuit resistance equals electrolyte resistance. Normally, electrolyte resistance is low enough to be negligible.

$NominalVolts=ExternalCircuitVolts+ElectrolyteVolts$

When galvanic cells are in series, they all add their energy to the same charge. Supplied energy per charge can be much higher than the capacitor's energy per charge.

$Vtotal=V1+V2+V3+...$

Normally, the inductance of the short wires between the cell and the capacitor is too low to hold much magnetic energy.Vze2wgsm1 (talk) 12:24, 10 November 2021 (UTC)

You first want to neglect resistance, then you want to neglect inductance, then you say it doesn't add up. Indeed, but you cannot neglect these things. Remember, things are only negligible compared to something else.

The circuit can be described as a simple loop with a voltage source, a capacitor, a resistor and an inductor. When going around this loop, the voltages (over each component) must add up to zero. This means that, if the capacitor isn't charged to the same voltage as the voltage source, there must be a non-zero voltage over the resistor and/or the inductor. Physics just tells us that in this circuit resistance and inductance cannot both be negligible. The current or its time derivative will always get large enough to get enough voltage over the resistor or the inductor to get the sum of voltages to zero, and this tells you where the energy is going. PiusImpavidus (talk) 09:47, 11 November 2021 (UTC)

Does the geoid have any concave "straight lines"? How long?

They must be so short that whether "straight line" means great circle/great ellipse/geodesic on a geoid or ellipsoid/loxodrome or intersection of one of the planes perpendicular to normal with the one of the roundoids, or coplanar with one of the centroids of one of the round surfaces or one of the points on the curving plumb bob line probably hardly affects the distance. Sagittarian Milky Way (talk) 16:18, 9 November 2021 (UTC)

Can you clarify with some context? I'm not sure what you are asking about. --OuroborosCobra (talk) 17:27, 9 November 2021 (UTC)

Is every "line segment" (a great circle is probably close enough at these tiny fractions of Earth's size) on the geoid convex? Or are there some concave ones? Probably around mountains, trenches and stuff. Extremely sightly concave.Sagittarian Milky Way (talk) 18:19, 9 November 2021 (UTC)

No, a geoid is not the surface of the earth, and so does not trace mountains and valleys and trenches. It is an idealized mathematical shape that approximates the shape the earth would take if its surface were entirely covered with water. It is related to, but slightly different than, the various Earth ellipsoids that also create mathematical approximations of the Earth's shape. What makes the geoid not an ellipsoid is that it takes into account such things as gravitational anomalies (areas of the earth where the surface gravity is higher or lower than average). However, these anomalies are NOT all that large, and their effect should not be over-estimated. As noted at Geoid, these deviations are no more than about +/- 100 meters, considering a great circle has a circumference of about 40,000,000 meters, or if you prefer a radius of 6,400,000 that means a vertical deviation of less than 100/6,400,000 or a little less than 15 parts per million. That is not enough to make even a noticeable dent in the surface. --Jayron32 19:16, 9 November 2021 (UTC)

I know all that, everyone knows valleys have concave lines. Does the geoid have them too? If the extremely smoothed out topography of the geoid can curve faster than the ~0.01 arc second/foot curvature of the Earth then you can have a concave line segment where the mean sea level bends the wrong way. Vertical deflection says plumb bobs "tilt" up to ~100 seconds of arc at asymmetric Himalayan mountains. But since big mountains are miles wide the mountain center of mass must be rather far from the point of max deflection (near the steeper slope?) so a geoid concavity may not be possible. Sagittarian Milky Way (talk) 21:04, 9 November 2021 (UTC)

I literally just explained that to you. Geoids don't follow mountains or valleys. They just don't. Even the Himalayas don't have enough of a gravitational attraction to alter the shape of the geoid great circle (or elipse, whatever) enough to alter the curvature enough for you to even notice. The most pronounced gravitational anomaly only has an effect of 15 parts per million. That will not "bend" the curve of ANY part of the geoid enough to even come close to "flat", never mind "concave". --Jayron32 12:08, 10 November 2021 (UTC)

It doesn't literally follow the land but Hawaii looks like Hawaii flattened about 300:1, not enough to cancel convexity. It is semantics if 300:1 flattening is still following. But I have a better idea: a 10 millionth scale half density Earth of common rock would theoretically have a surface gravity of 1/20 millionth g, if you find ground 24-25 inches below a flat geoid (Salton Sea?) and put one there that should be good for 648000/π/20 million or 0.0103132403 seconds of deflection. At 0.001 radii from the side surface it'll be 648000/π/(20 million*1.001²)=0.0102926447 seconds or 0.0000205956 seconds less which would take at least ~0.636 millimeters of Earth curvature to overcome and over 0.639 at the pole, which is more than 0.001 radius distance from the rock ball. If you decrease the ball size the gravity decreases proportionally but so does the distance between the side surface and x% of side surface gravity. It might still be true that any concavity is either prevented by isostasy i.e. the thicker continental crust under mountains (2.83 vs denser for the rock the continental crust floats on) or unmeasurably small but it seems any dense ball in a field would do it (theoretically). ​Sagittarian Milky Way (talk) 21:09, 10 November 2021 (UTC)

The textbook Geodesy: The Concepts (Authors P. Vanícek, E.J. Krakiwsky, 2015), pg. 91, states "One must bear in mind that, in reality, the lows on the map are not concave. As stated earlier, the geoid is a convex surface[.]" --Amble (talk) 21:21, 9 November 2021 (UTC)

So still convex. Nice to know! Sagittarian Milky Way (talk) 23:42, 9 November 2021 (UTC)

The height variation of the geoid (±100 m) is too small to overcome the positive curvature of the reference ellipsoid. Ruslik_Zero 08:54, 10 November 2021 (UTC)

This also depends on the wobbliness of the height variation. Modelling it locally to a first approximation as $-A\cos(\varepsilon d)$, in which $A\approx 100\operatorname {m}$ and $d$ denotes the horizontal distance from a given point in some fixed direction, local convexity requires the value of $AR_{\operatorname {earth} }\varepsilon ^{2}$ not to exceed $1$.  --Lambiam 10:07, 10 November 2021 (UTC)

This can be rewritten as $A(\varepsilon )>1/(R_{\operatorname {earth} }\varepsilon ^{2})$. This means that the power in perturbations with frequency $\varepsilon$ has to decrease slower than $1/\varepsilon ^{2}$ for a concavity to arise. Ruslik_Zero 14:02, 10 November 2021 (UTC)

I think we should ask a clarification: "which geoid"? There has been much talk, here, about "the" geoid, as if there is exactly one geoid that is universally agreed. I do not stand alone in my nitpickery. Our very own Wikipedia articles cite this NOAA source: "What Is The Geoid?" - which provides "just a few examples of the difficulty in defining the geoid."

Putting aside questions of "uniqueness" of "the" geoid, there is a core definitional question. "The" geoid - in fact, every geoid (and there are many to choose from, with a sampling of useful geoids explained and documented at the National Geodetic Survey website - ... every geoid is just a mathematical model that describes the shape of the Earth. It is a representation of an abstract, geometrical surface. There are many different standards; and methods; and practical uses. For today's discussion, we could probably all agree that the most useful geoid is "the" geoid represented by one of the most common gravimetric anomaly implementations. It is a shape defined and standardized using (e.g.) the World Geodetic System; it is expressible mathematically in the form of a harmonic continuation (essentially, it is a practical extension of the pure spherical harmonics representation). For this discussion, let's grant that we only care about geoids that are expressed in this format; the values of each coefficient are not of particular interest: these values differ according to each particular geoid representation and its practical purpose, (e.g. a geoid designed to represent gravitational anomaly; a geoid designed to represent gravimetric deflection; and so on). Let's ignore any of the curved surfaces that are not expressed as some form of spherical harmonic series - even if they are the same surface, represented using different formats.

And we need a second question: "what do we mean with the word convex?"

In the purview of a pure, mathematical definition - convexity and concavity have very precise mathematical meanings. Specifically, there is the definition of a convex function. Now, this is really important - when we talk about "convexity" and "concavity", we need to be really careful. For a simple shape, mathematical function convexity is intuitively identical to the visualized image of convexity. From this simplification, we get such concepts as the convex hull (which also has a precise, mathematical definition). But as we get to more sophisticated shapes, the precise mathematical meaning of the word "convex" begins to drift from our elementary intuition about "convexity."

Why does this matter? Because most of our geoid models are truncated harmonic series. If you don't immediately know why that matters, ... please accept by argument from authority that it matters a lot. A perfect representation of the "abstract" perfect geoid would require an infinite continuation of that series. Truncating that series introduces error (small but non-zero). This truncation error can change whether the surface is, or is not, convex.

Ruslik is 100% on the right track, in the effort to define the concavity or convexity, with reference to the local curvature. The catch is: upon which surface are we checking convexity? Upon the geoid or upon a truncated spherical harmonic representation of that geoid? Because that small difference matters a lot. A spherical harmonic representation is a linear combination of convex basis functions. That is not an accident; it is an on-purpose. This mathematical representation cannot be non-convex. (It is no accident that it is a convex series). This specific representation of the shape allows mathematically-inclined individuals to use standard and conventional methods of optimization to do work relating to the geoid model. The trouble falls in two categories:

1. Is this numerical representation the best representation? Truncated spherical harmonic series are not the only way to represent the ideal geoid: we can actually use different mathematical expressions to describe (and to approximate) the same shape, and depending on which expressions we choose, that shape can be represented exactly- or approximately-. We do not have to approximate this shape using a convex series of basis functions; we can write a mathematical expression of the same surface, using a different formulation, which can be locally non-convex.
2. Is this surface the best surface? Truncated spherical harmonic series are not a perfect representation of the ideal geoid - we truncated an infinite series of harmonics. We presume that the ideal shape is well-approximated using only a finite number of terms. The approximation that we use to represent the shape is always convex; is same true about the exact shape? This is independent of any concerns about how we might measure exactly; I only describe how we might define a surface: we can define a non-convex surface, and then approximate it with a truncated harmonic series, and then we can rigorously compute the approximation-error.

Now, back to why this makes any difference at all: convexity is a property of a function that can have an effect over an infinitesimal extent. And this isn't only a theoretical concern: it is a practical concern - especially if one wishes to use conventional numerical methods to work with a curve or shape! When we truncate an infinite series because the last infinite-number-of-coefficients are very small, we are pretending that we don't care about such things. That is exactly the type of fallacy that leads to profound numerical errors in practice!

Wrapping up, the take-aways are: (a) we need to be really careful about how we are describing our terms. If we use our terminology interchangeably - substituting "geoid" vs. "approximation of the geoid"; "function convexity" vs. "convex hull" vs. "shape that sort of curves inward a bit"; then we are abusing mathematical terminology in a manner that leads us to a wrong answer. And if we are ignoring "bits" that are "very small," we are committing a really serious mathematical error. Even in elementary calculus, we learn that working with infinite series leads to strange consequences: we're literally discussing the sums of infinite numbers of infinitesimally-small bits. That sum can converge to zero, or it can converge to a finite number, or it can converge to infinity. The difference is in the details.

Nimur (talk) 15:29, 10 November 2021 (UTC)

An obvious way of defining whether a surface enclosing a space is convex is to define this as the enclosed space being convex, being the 3D analogon of the definition of a closed convex curve. So then an ellipsoid is convex but a torus is not. Then a geoid represented by a truncated spherical harmonic series is not necessarily non-convex – just think of truncating immediately after the $Y_{0{,}0}$ term, in which case the geoid is a sphere.  --Lambiam 18:42, 10 November 2021 (UTC)

I was barking up the wrong tree, but a geoid represented by a truncated spherical harmonic series is also not necessarily convex.  --Lambiam 09:15, 11 November 2021 (UTC)

Regarding "convexity is a property of a function that can have an effect over an infinitesimal extent" leads one pretty quickly down the coastline paradox issue and the problem of the fractal nature of problems like this. I intentionally avoided that discussion, as it introduces a level of detail to the discussion that is unlikely to help the OP. But you've now added that detail. I doubt the OP will take any of this into consideration, or really anything you wrote, but it is the problem with trying to answer questions like this both sufficiently and accurately. --Jayron32 16:52, 10 November 2021 (UTC)

I did care if a lead BB or dense crystal at sea level could do it, though the order of magnitude of the longest such concave line on Earth would be more interesting. And now I know that for math convention convenience geoid models are defined in a way that can't represent concave local curvature (yes I know the difference between Venn diagram indentations and concave local curvature), even if the best-fit curve for mean sea level that one would choose if one were omniscient and thorough might have tiny zones of local curvature "bending the wrong way". Sagittarian Milky Way (talk) 21:45, 10 November 2021 (UTC)

Okay and now I read the rest and find out that conventional geoid models can represent concave shapes (but probably don't as the book says they're convex) Sagittarian Milky Way (talk) 21:50, 10 November 2021 (UTC)

Re. "A spherical harmonic representation is a linear combination of convex basis functions. [...] This mathematical representation cannot be non-convex.": Where are you getting this from? The individual spherical harmonics are not even positive-valued in every direction; much less does each individual harmonic represent a convex shape. Even if they did, the conclusion doesn't follow, because the coefficients can be negative. --Amble (talk) 19:06, 10 November 2021 (UTC)

I apologize for failure to cite a source; "I got it from memory," which is a pretty weak citation. I think the formal name for this result is the Funk-Hecke theorem. The proof that (certain) trigonometric polynomials are convex is Homework Problem 3.19 in the horrible CVX book.

Here is a Papers in Analysis, R.T. Seeley (1966) presenting "a concise and elementary exposition of spherical harmonics, including the Funk-Hecke theorem."

Nimur (talk) 19:21, 10 November 2021 (UTC)

Thanks. That exercise is for convex basis functions, with additional constraints on the coefficients (non-negative and ordered), none of which apply here. The spherical harmonic expansion used for the geoid can easily represent non-convex shapes. The familiar (non-convex) shapes of electron orbitals in atoms are based on a very similar expansion. --Amble (talk) 19:39, 10 November 2021 (UTC)

For those looking to build the wiki and turn Funk–Hecke theorem blue, the eponyms are Paul Funk and Erich Hecke. DMacks (talk) 04:34, 11 November 2021 (UTC)

Amble, I believe you are correct, insofar as "it is possible" to construct a geoid using non-convex basis functions. I spoke (wrote?) too broadly; my statements are not universally true. Even though I concede that point, my statements are correct in some instances: for practical purposes, some geoid models are intentionally built using only convex basis functions. If we're in disagreement about whether this specific constraint applies, all I can say is that I did open my earlier commentary with remarks to the effect - "there is not only one geoid." Among the many, many, many models, some are intentionally built to be convex; those are the ones that have been most useful to me, and in that respect, my bold statements re: "cannot be non-convex" ought not be generalized to every other model. Details, details, details... but, I concede that detail to Amble, who is correct. It is possible to use non-convex basis functions (including some spherical harmonic representations) to model a non-convex shape, and to call it a geoid, by relaxing those specific constraints.

Honestly, at this phase of my mathematical prowess, so I'm not in a position to prove- or disprove theorems - it would take a lot of time and effort, and I'm "out of currency" with respect to methods of analysis. But, as Jayron already wrote, we've probably long lost the original author of the original question in our details... but, maybe some of our other readers and contributors got something out of the endeavor! We found an actual red-link, which (to me) is a rare occurrence in this decade!

Nimur (talk) 11:27, 11 November 2021 (UTC)

November 10

2021 Madagascar food crisis

Hello all! I'll be short. Many news outlets refer to the ongoing 2021 Madagascar food crisis as famine. When should the title of the article be changed? Or conversely, it is not yet a famine? Thanks for your knowledge. CoryGlee (talk) 12:34, 10 November 2021 (UTC)

This is a discussion that should be had at Talk:2021 Madagascar food crisis. If you ask the same question there, maybe you'll get better answers from more interested people. --Jayron32 12:46, 10 November 2021 (UTC)

It should be done by adding Template:Requested move to the article's talk page; I've done this. LongHairedFop (talk) 16:53, 11 November 2021 (UTC)

Plant questions.

Q1. So 1 type of classification for plants is C4, C4, and CAM. Another classification is vascular and non-vascular, making this 3 by 2. However, there doesn't seem to be a non-vascular plant that are C4 or CAM? Are 100% of non-vascular plants are all C3?

Q2. Is there bacteria inside the vessels of plants? Say, in the xylem and phloem. Is there selectivity, for bad bacteria from good bacteria? And what do plants do to counter bacteria, or unwanted bacteria? For chemicals, humans have HOCl and H2O2, do plants have chemicals like those too? For non-chemicals, humans have macrophages, lysozymes, and white blood cells, do plants have an equivalent for that too? Thanks. 67.165.185.178 (talk) 21:48, 10 November 2021 (UTC).

Q3. And thirdly, is 2ndary lighting enough for plants? By that I mean, where the plants is not directly next to a window, so it gets reflected light only (what dowe call this kind of light). Or do indoor plants just have to be by windows? I was told regular lights at night is not enough unless the light are red or blue.

67.165.185.178 (talk) 03:37, 11 November 2021 (UTC).

These are three separate questions; I have labelled them with a number for easy reference. Courtesy links: C3 = C₃ carbon fixation; C4 = C₄ carbon fixation; CAM = crassulacean acid metabolism; vascular plant; non-vascular plant.  --Lambiam 09:04, 11 November 2021 (UTC)

For Q3, different plants have different lighting requirements. Plants from forest floors can tolerate lower light, and could grow inside a room. Check out indoor plants and Houseplant care#Light requirements. Graeme Bartlett (talk) 10:11, 11 November 2021 (UTC)

Some will also grow fine indoors but will not produce the fruit or flowers they normally would. This can have to do with the amount of light, but also with their yearly cycle, they may not detect when spring or autumn happens, for instance. When massively produced, with that knowledge (and that of their nutritional and pollination requirements) it's also possible to simulate faster seasons artificially. —PaleoNeonate – 23:53, 11 November 2021 (UTC)

November 11

Atmospheric escape from moons; what effects of their host planet's gravity?

So there is a lot of scientific literature on atmospheric escape (the process by which atmospheres of planets slowly evaporate into space) and the role radiation, gravity etc. play but I can't find anything specific about moons. The thing I wonder about is whether being a moon rather than a planet would change the escape process; after all the escaping atmosphere would have to overcome not just the gravity of the moon but also that of the planet. Jo-Jo Eumerus (talk) 16:44, 11 November 2021 (UTC)

There's definitely literature on atmospheric escape from moons such as Titan: you can see some discussion in atmosphere of Titan and the references there. The presence of the planet can be important, for example in the Galilean moons [9], [10], where the Jovian magnetosphere contributes to sputtering. From the abstracts they describe some portion leaving the Jovian system and another portion contributing to the Jovian magnetosphere. --Amble (talk) 17:27, 11 November 2021 (UTC)

• ec The article on Io (moon) says that Jupiter's "magnetosphere of Jupiter sweeps up gases and dust from Io's thin atmosphere at a rate of 1 tonne per second" and "end up in various neutral (non-ionized) clouds and radiation belts in Jupiter's magnetosphere and, in some cases, are eventually ejected from the Jovian system". Perhaps some gets drawn down into Jupiter too. Abductive (reasoning) 17:33, 11 November 2021 (UTC)

That's for the magnetosphere; does Jupiter's gravity also play a role? This is an old paper speculating about the effect of Saturn's gravity on Titam.JoJo Eumerus mobile (main talk) 17:40, 11 November 2021 (UTC)

That's interesting. I hadn't heard about the idea of recapture by Titan. --Amble (talk) 17:47, 11 November 2021 (UTC)

To answer this question you need to compare the vertical extent of the atmosphere to Hill radius of the satellite in the gravity field of the planet. Ruslik_Zero 20:39, 11 November 2021 (UTC)

Gunk buildup on computer mouse

Why mouse is particularly prone to gunk buildup (on the top and sides) compared to some other frequently used items, such as pens, cup handles, kitchen utensils, etc? Does it have something to do with material the mouse is made from? 212.180.235.46 (talk) 17:57, 11 November 2021 (UTC)

I'd guess it has to do with differences in use. You keep your hands in near constant contact with a mouse for hours on end, day after day, and don't necessarily wash your hands before each use. You definitely don't clean your mouse between every use, and may go days or weeks or longer without cleaning it. You keep your hands on the mouse even as your palms get sweaty. Now, compare that to the other items you listed. You only use a pen in brief spurts while writing. You only hold the cup handle for brief moments while taking a sip, and you probably wash your cup between every beverage. You only hold kitchen utensils for the brief moments of usage, i.e. knife and fork while cutting steak, but that doesn't last for hours on end. You then wash them between meals. Many/most (hopefully) also wash their hands before starting a meal. --OuroborosCobra (talk) 19:25, 11 November 2021 (UTC)

Storage and dust might also be factors, as the other items listed are not stored in ways that so enable the accumulation of dust (which could mix with sweat to form the 'gunk'). PaleCloudedWhite (talk) 20:52, 11 November 2021 (UTC)

I suppose the main ingredient is accumulated sebum mixed with dead skin cells and fine dust. The surface of a mouse should not be slippery and is often somewhat matt (non-glossy), which may facilitate the adhesion of sebum – in addition to the factors already mentioned.  --Lambiam 21:24, 11 November 2021 (UTC)

If it's salty there's dried sweat too. I don't feel like tasting. Sagittarian Milky Way (talk) 23:09, 11 November 2021 (UTC)

Sets of chemical elements

This may seem a funny question at first glance, but is it, in fact, rather a coincidence that the number of categories in the most common classification of element sets in the periodic system is exactly ten, or were these sets deliberately designed in a manner to amount to the round figure of ten? (Cf. Names for sets of chemical elements, however, listing twelve IUPAC-recommended classes with a pertinent citation, which lacks a corresponding reference entry, though.)--Hildeoc (talk) 22:22, 11 November 2021 (UTC)

Some of the 12 overlap. Sagittarian Milky Way (talk) 22:51, 11 November 2021 (UTC)

While it's true that 10 is a nice round number in the decimal system, a coincidence requires two (or more) events or occurrences. I don't think that the number of categories in the periodic table can be classed as a remarkable concurrence in and of itself. nagualdesign 23:09, 11 November 2021 (UTC)

November 12

Mechanical hierarchy - fixing appliances.

Q1. When it comes to HVAC, PTAC (fixing air conditioners, central air), is there a hierarchy for most-roundedness? I personally think fixing microwaves is the most well-rounded, especially if you also know how to fix washing machines. So if you know how to fix microwaves, it should be easy to learn how to fix something else?

Q2. I'd also like to know what's 2 different fields with low-overlap? For example, someone who is an electrician, does not know how to fix ovens. And someone who fixes ovens, does not necessarily know how to be an electrician? So if you train to fix ovens and an electrician, not much overlap?

So, to summarize, Q1 = someone who can fix A, can fix A and B, but someone who can only fix B, does not know how to fix A. Would microwaves be a good candidate? And Q2 = A and B do not overlap. Thanks. 67.165.185.178 (talk) 03:59, 12 November 2021 (UTC).

The term "fix" has too broad a meaning to answer this. Let's stay with the microwave. Fix it by replacing the broken turntable belt. Sure. Easy. Fix it by repairing a blown circuit board with your soldering iron - not so much. By "fix" do you mean replace a faulty component with a part you found on ebay or do you mean taking your multi-meter and oscilloscope any diving into the guts and diagnosing the the faulty component on the circuit board, remove a chip or blown capacitor and solder a new one in? 41.165.67.114 (talk) 06:56, 12 November 2021 (UTC)