# Conceptual and practical reasons and consequences of inverting weak equivalences

Although dealing with this in one or other form for many years, to my shame this question only struck me now.

One of the most radical differences between categories of "algebraic" and "topological" kind is absence/presence of nontrivial bimorphisms, i. e. non-invertible morphisms which are both mono and epi. Such guys exist in e. g. distributive lattices or rings, posets or topological spaces; they are absent in e. g. groups or compact Hausdorff spaces. Important case is that of quasitoposes: presence of nontrivial bimorphism is what distinguishes them from toposes.

Now, in homotopical algebra one inverts weak equivalences, and "morally" it very much looks like inverting bimorphisms, although there is no rigorous reason at all that weak equivalences be related to bimorphisms at all, and this is the first thing I don't quite understand:

Is there any connection between inverting weak equivalences and inverting bimorphisms?

Does one obtain any recognizable category by inverting bimorphisms of, say, posets or commutative rings?

Independently whether weak equivalences are "like bimorphisms" or not - ok but what are they anyway? That is, in what way do we improve a, say, model category when we invert weak equivalences? Another example where my ignorance is especially apparent is the case of homotopy categories of complexes versus derived categories. In the former we identify homotopic maps, in the latter - additionally invert quasiisomorphisms. In both cases we obtain triangulated categories. I have no idea how the latter (i. e. the derived category) is a "better" category than the former (i. e. the homotopy category). They both are triangulated, which basically means that all "nice" category-theoretic properties related to limits, etc. are destroyed anyway. So what I don't understand here is this -

Are there some abstract category-theoretic properties which are improved by passing from a homotopy category of chain complexes to the derived category?

• Quick and maybe wrong: you usually invert classes which are 2-out-of-3, and bimorphisms -to my knowledge- not always are. So there's no hope to invert only bimorphisms in general, and... how do you control the size and shape of the smallest 2-outof-3 class containing bimorphisms in $\cal C$? – Fosco Loregian Sep 16 at 9:16
• @FoscoLoregian Yes I agree. I just seek the "bimorphism-like" reason to invert weak equivalences. Non-invertible bimorphisms are "bad" things and you might want to invert them. I don't know whether it is a good idea to do this. And I don't know whether non-invertible weak equivalences are "bad" in the "same spirit" as bimorphisms. – მამუკა ჯიბლაძე Sep 16 at 9:36
• I guess a naive explanation of why you want to invert weak equivalences is that many functors are homotopical with respect to natural choices of weak equivalences, or vice-versa, many weak equivalence classes arise as classes of maps in the domain of functors (homotopy with or without coefficients, cohomology...) which are quite natural to define in algebraic topology. Also every reflective localization is, in the end, a "homotopy category". – Fosco Loregian Sep 16 at 10:34
• @FoscoLoregian I guess I would rather exclude "local" aspect from the picture: say, when you have a map of sheaves on a space and you call it weak equivalence because it induces an isomorphism between stalks at certain points of interest, this kind of thing, this I would like to exclude. There are analogous situations in homotopy theory too but I am rather interested in the cases when something has to be inverted since it so to say induces an isomorphism "at all possible points", like e. g. quasiisomorphisms do. – მამუკა ჯიბლაძე Sep 16 at 11:10

As for the difference between $D(R)$ and $K(R)$, this is detailed very nicely in Weibel's book "An introduction to homological algebra," as well as many other places. Both categories are useful in homological algebra and representation theory, but $D(R)$ is more useful because it captures the "true" homological algebra rather than a naive one (because, in homological algebra, it's more common to work up to quasi-isomorphism). It's used to compute derived functors a la Grothendieck, and for zillions of other purposes. Just googling "why is the derived category better?" or "what is the derived category good for" you find tons of books, articles, lecture notes, blog posts, mathoverflow answers, etc.