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Anyone have any to contribute? https://plus.google.com/+NilesJohnson/posts/P47niHmk9eU
This one is not directly about category theory, but rather sheaf theory. (I don’t have a Google+ account, feel free to relay if deemed sufficiently relevant and funny.)
A sheaf and a presheaf are walking down the street. Suddently the presheaf spots the sheafification functor in the distance. “Quick! Hide! The sheafification functor is coming!”, warns the presheaf. The sheaf however stays calm and proudly explains, “I’m already a sheaf. The sheafification functor can’t harm me.” Eventually the sheafification functor arrives and reduces the sheaf to the terminal one. What happened? It was the sheafification functor with respect to the trivial topology where any family is regarded as a covering family.
Edit: Not a joke, but arguably still funny (in allusion to the well-known phrase “right is where your thumb is on the left”): Right is where the tensor product is exact.
I find it funny that the annual Category Theory conference makes available only the abstracts of the contributed talks.
I've heard the sheafification joke about $\mathrm{e}^x$ and a constant function meeting a differential operator. (Punchline: it's $\partial/\partial{y}$.) Both jokes, I suppose, are about the importance of not making assumptions about the context.
I wonder if I could use that joke to explain to my calculus students why they shouldn’t write $y'$.
Once I was thinking what the category of my ex-girlfriends should be. Then I realized I was treating women as objects.
(And then I realized that this would pass as a punch line.)
That’s a really good one, Nikolaj!
I learned the following play of words from Yuri Sulyma (at the previous week’s Topos à l’IHES):
Let $\mathcal{C}$ be a category. Let $X,Y \in \mathcal{C}$ be objects. Assume that the induced representable presheaves $Hom(\cdot,X)$ and $Hom(\cdot,Y)$ are naturally isomorphic. Then how do you prove that $X$ and $Y$ are themselves isomorphic? Yo ned a lemma for that.
I’ve learned this one from Ingo Blechschmidt while on a walk through a forest at night in october of 2019, back when I first got into category theory. When I laughed, he called it the best reaction he got yet.
What does a category theorist say during meditation? Hooooommmm
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