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

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
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
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