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Algebraic varieties, stacks, sheaves, schemes, moduli spaces, complex geometry, quantum cohomology.

13 votes

Why does Hom need an identity in the definition of the category?

As Fernando points out, you can't talk about isomorphisms in a semicategory, which means that they won't be as much use as categories in describing universes of mathematical objects. But the category …
Finn Lawler's user avatar
  • 3,622
4 votes
Accepted

When is a colimit of a subcollection the same as the overall colimit?

Dylan's comment is right. More generally, if $D \colon J \to C$ is a diagram and $L \colon J' \to J$ a functor, then the colimit of D is isomorphic to the colimit of DL if and only if L is (co)final. …
Finn Lawler's user avatar
  • 3,622
1 vote

Double Direct Systems and Switching of Direct Limits

Your indices are hard to read, and I think you might have mixed some of them up. If you mean what I think you mean, then Mac Lane's result is exactly what you need. The two colimits are canonically …
Finn Lawler's user avatar
  • 3,622