## 4 comments on “What’s woolly and equivalent to the Axiom of Choice?*”

1. Among all the infinite number of pairs of shoes in Hilbert’s famous Grand Hotel, there’s one clumpy rubber-soled black pair worthy of special mention. These shoes are Russell’s Pair of Docs.

2. Mark Wildon told me this one years ago, I don’t know who invented it:

A lecturer is proving a theorem about p-groups. At the end of the proof, a student says:

“I just have one question: I can see how this uses the general properties of groups, but where does the p-ness enter?”

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A genuine example from Littlewood’s Miscellany: at the end of a paper, he wrote “thus sigma should be made as small as possible”. In the final printed paper, this last sentence was missing; but at the bottom of the page was a tiny little dot, which (after using a magnifying glass) turned out to be the smallest printed sigma he had ever seen.

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Some more: not jokes, but genuine examples which are hilarious if you have schoolboy humour, i.e. accidental dodgy innuendo spoken by lecturers:

A female undergraduate asks a lecturer (with very bad handwriting) what “**** 2.1” says, where **** are the indecipherable symbols. The lecturer replies:

“Prop. 2.1! This is a proposition! You haven’t had any of those yet, have you?”

A fluid dynamics lecturer is talking about divergence and its geometrical meaning.

“In an incompressible fluid, the divergence is zero at each point where there is no fluid entering or leaving.

“But the divergence is positive at this point; this means there’s a fluid source. So, imagine a little man sitting at this point, releasing lots of fluid…”

{…and he helpfully drew a little stick man at the point…}

“…but at this point, the divergence is negative; so this is a fluid sink. Imagine a little woman sitting at this point, sucking up the fluid…”

{…by which point, over half the audience were in fits, and he looked rather puzzled as he continued to draw a little, unfortunately non-obscene, stick woman}.

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Not a joke, but a quotation, by someone famous but I can’t remember who:

“A mathematician is like a blind man in a very dark room looking for a black cat which isn’t there.”