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Long ago I took an oral exam Algebra and my professor asked me the following: "Let $G$ be an abelian group of order 17020. What is its commutator subgroup $G’$?" At first I focused on factoring the number, but in a few seconds I realized with a smile that he said abelian and of course gave the right answer. Afterwards, I found the question very funny.
On another occasion I sat down with another professor over lunch and we discussed group theory and suddenly he quick-wittedly remarked: can you classify all groups $G$ with only a single non-normal subgroup $H$? Of course, such an $H$ must be normal by definition and without saying anything we could not resist to roar with laughter …
Have you also come across some of these sorts of math jokes?

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    This is related to [this](http://mathoverflow.net/questions/46837/advanced-math-jokes-closed) and [this](http://mathoverflow.net/questions/1083/do-good-math-jokes-exist-closed) MO thread which is pretty much an exhaustive list.2012-06-25
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    Also see [this AMS notice](http://www.ams.org/notices/200501/fea-dundes.pdf).2012-06-25
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    Not a math joke, but if you google, you'll find a lot of otherwise smart people speaking very seriously about the difficulty of factoring primes. In fact, we just had that on a separate post [here](http://math.stackexchange.com/questions/163041/formula-for-n-xy-where-n-is-given-and-x-y-are-both-unknown-prime-numb#comment375617_163041). But they weren't funny *on purpose*, so I guess it doesn't count.2012-06-26
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    @Arturo Wow, that MSE comment 3 hours ago is already number 3 in Google matches for "factoring large primes". It is surprising how often that phrase appears, even in refereed publications, e.g. in CACM '89 "The recent reports of factoring large primes with a network of microcomputers suggest that this is a problem well-adapted to such a scheme".2012-06-26
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    @BillDubuque: If I understand the way that Google displays results, the fact that you and I visit MSE often may affect how high it shows up in the search on our computers (assuming you keep some cookies associated to MSE). That is, my understanding is that the exact same search may result in different rankings depending on local information on the computer doing the search. I could be wrong, though....2012-06-26
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    @Arturo Yes, but it still ranks number 3 even if not signed in to Google, so I think it has to do with the general high pagerank Google assigns MSE.2012-06-26

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