TITLE: You must be joking ... math and fun
QUESTION [6 upvotes]: 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?

REPLY [19 votes]: I’m fond of the first exercise in the second edition of Edward Scheinerman’s Mathematics: A Discrete Introduction:

Simplify the following algebraic expression: $$(x-a)(x-b)(x-c)\dots(x-z)$$