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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.

15 votes
Accepted

Constructing a "geometric" model structure on Cat by localizing the "categorical" model stru...

No, there are far too many "canonical cofibrations" for that. For example, let $A$ be a category with two objects and two parallel arrows between them. Then there is a unique functor $A \to [1]$ that …
Karol Szumiło's user avatar
12 votes
Accepted

Is the localisation of a product of categories the product of the localisation?

It is true for arbitrary products of model categories (or just cofibration categories) as proven in Theorem 7.1.1 of http://arxiv.org/abs/math/0610009v4. It is also true for finite products of arbitr …
Karol Szumiło's user avatar
12 votes
Accepted

When do colimits agree with homotopy colimits?

I don't think we can expect to have one general answer to this question, only a collection of unrelated specialized results. Here are two more: In the category of simplicial sets all filtered colimi …
Karol Szumiło's user avatar
10 votes
Accepted

Relative version of Quillen's theorem A

The condition that appears in the assumption of what you call "Relative Theorem A" was introduced by Grothendieck in Pursuing Stacks. It is a part of the definition of a basic localizer, i.e. a class …
Karol Szumiło's user avatar
9 votes
Accepted

Is there a model structure on (strict?) Monoidal Categories?

I'm not sure about the case of general monoidal categories. (Although I seem to recall a remark that there is no such structure since the category of monoidal categories is not cocomplete and a suitab …
Karol Szumiło's user avatar
8 votes
Accepted

"Joyal type" model structure for (n,1)-categories?

I don't know if this will work with the definition of $(n, 1)$-categories exactly as stated by Lurie, but it will work with a reasonable modification. Definition 2.3.4.1 from HTT basically says that …
Karol Szumiło's user avatar
7 votes
Accepted

Is the category of $G$-spaces a model category?

Yes, assuming that by "expected $G$-homotopy extension property" you mean the Serre $G$-cofibrations, not the Hurewicz $G$-cofibrations. A very explicit reference is Proposition A.1.18 in Schwede's Gl …
Karol Szumiło's user avatar
7 votes

Can one make the category of pairs of topological spaces a model category?

As Tyler Lawson points out you can use the category of all diagrams on $[1]$. Then the projective and injective model structures are both instances of Reedy model structures. This is discussed in Sect …
Karol Szumiło's user avatar
6 votes
Accepted

recognising weak equivalences of simplicial sets

$\newcommand{\Ex}{\mathrm{Ex}}\newcommand{\Sd}{\mathrm{Sd}}$Consider the square $$\begin{CD} X @>{\sim}>> \Ex^\infty X \\ @V{f}VV @VV{\Ex^\infty f}V \\ Y @>>{\sim}> \Ex^\infty Y \textrm{.} \\ \end{CD …
Karol Szumiło's user avatar
6 votes

Alternative characterization of homotopy equivalence

EDIT: This is an old question, but I have stumbled upon it by accident and realized that my answer is wrong. It turns out that genuine homotopy equivalences cannot be characterized in terms of their h …
Karol Szumiło's user avatar
6 votes
Accepted

Homotopy pushout independent of factorization and symmetric in cofibration category

I don't remember how Baues does this exactly, but all facts of this sort follow from the Gluing Lemma (see Lemma 1.4.1 in this paper) and "Brown type factorization". By this I mean the following const …
Karol Szumiło's user avatar
5 votes
Accepted

How to simplify the proof of right-properness?

To complete the argument you need to apply K. Brown's Lemma. Call your model category $\mathcal{M}$, then the map $Z \to Y$ induces a pullback functor $\mathcal{M} \downarrow Y \to \mathcal{M} \downar …
Karol Szumiło's user avatar
5 votes

Reedy model structures on oplax limits

You may be thinking of Johnson, Mark W. On modified Reedy and modified projective model structures. Theory Appl. Categ. 24 (2010), No. 8, 179–208. but his constructions (Definitions 3.3 and 5.2) hav …
Karol Szumiło's user avatar
4 votes
Accepted

Saturated classes and cofibrantly generated model structures

It doesn't make a difference as long as we restrict attention to compactly generated saturated classes, i.e. cofibrantly generated ones where generators can be chosen to have $\aleph_0$-small domains. …
Karol Szumiło's user avatar
3 votes

Morphisms every pushout of which is a weak equivalence

Here's a suggestion. Hopkins uses the name flat map for a morphism such that every pushout along it is a homotopy pushout. Rezk uses the name sharp map for the dual notion. (Roughly speaking, I think …
Karol Szumiło's user avatar

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