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Earlier someone was asking about the category "Ens" described in Categories for the Working Mathematician. My question is more basic: What does Ens stand for? Most of the categories have names that are self explanatory, and maybe this one does too. But for the life of me I can't think of it, and a Google search was no help.

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Ecole normale supérieur. – Yuval Filmus Jun 18 '11 at 20:43
@yuval I actually considered this as searching for Ens mathematics returned only that result. But do you have a reference that this was Mac Lane's intent. Because Ensemble makes better sense. Or are you just kidding? – Joe Jun 18 '11 at 20:50
Mac Lane wrote Ens, not ENS. The latter has a different connotation in math. – Yuval Filmus Jun 18 '11 at 20:55
The school Yuval refers to is so superior that it even gets a final "e": École Normale Supérieure. @Matt: And I really, really hope he is kidding! – Georges Elencwajg Jun 19 '11 at 0:20
up vote 9 down vote accepted

It stands for "ensembles". Here is the other question, btw.

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Thank you. I still don't think this is obvious, and Mac Lane could have put a sentence in there explaining. Just wondering, how do you find out what it meant? – Joe Jun 18 '11 at 19:35
@Matt: I completely agree, this isn't obvious, and I was surprised to see that Mac Lane hadn't explained it anywhere. I honestly am not sure where I learned this, I think my professor mentioned it to me in office hours. I am also surprised at the lack of references Google turns up about this. – Zev Chonoles Jun 18 '11 at 19:37
@Matt: I think it is obvious, but only because I happened to know that the French for set (in the mathematical sense!) is ensemble. Perhaps Mac Lane neglected to explain it because the working mathematician of the time was expected to know French? – Zhen Lin Jun 18 '11 at 23:01

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