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A model category is a category equipped with notions of weak equivalences, fibrations and cofibrations allowing to run arguments similar to those of classical homotopy theory.

33 votes
Accepted

Why is Voevodsky's motivic homotopy theory 'the right' approach?

(Don't be afraid about the word "$\infty$-category" here: they're just a convenient framework to do homotopy theory in). I'm going to try with a very naive answer, although I'm not sure I understan …
Denis Nardin's user avatar
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3 votes
Accepted

Kan complexes and semigroups

(2) is true (and so (1) is false). To see it, note that every horn $\Lambda^n_i\to S$ to a constant simplicial set must be constant, and so it can be filled by the constant horn $\Delta^n\to S$. Equi …
Denis Nardin's user avatar
  • 16.5k
4 votes
Accepted

Does the existence of a derived functor imply existence of model structure?

This is not quite the answer to your question as you pose it. I hope it will be useful anyway. By and large I am just expanding user337830 comments. Everything will use homological grading (what can I …
Denis Nardin's user avatar
  • 16.5k
6 votes

Geometric Meaning of Different K-theories

Fibrant replacement is essentially sheavification with respect to the corresponding topology. So étale K-theory is nothing more than the part of K-theory that satisfies étale descent. Concretely (and …
Denis Nardin's user avatar
  • 16.5k
8 votes
Accepted

Homotopy function complex for quasi-categories

Yes, you can compute the mapping spaces in ∞-categories by taking the biggest Kan subcomplex of the internal hom. The trick is not to use the Joyal model structure, but instead the model structure on …
Denis Nardin's user avatar
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3 votes
Accepted

A distinguished triangle of mapping spectra arising from recollement

I'm going to do a proof assuming we are in a stable $\infty$-category (I'm pretty sure this is almost equivalent to your "sufficiently rich" situation anyway). In your case $F=j_!j^!$ and $G=i_*i^*$. …
Denis Nardin's user avatar
  • 16.5k