In the literature, I've mostly seen two quasicategories coming from $\text{Ch}_R$:

  1. By considering $\text{Ch}_R$ with weak equivalences $\mathcal W = \text{quasi-isomorphisms}$, we can consider its Dwyer-Kan localization $L^H(\text{Ch}_R)$, a simplicial category. Then, from the Quillen equivalence $$|-|:\text{sSet}_{\text{Joyal}}\leftrightarrows\text{sSet-Cat}_{\text{Bergner}}:N_\Delta,$$ we can define, taking a fibrant replacement if necessary, $$ N_\Delta(L^H(\text{Ch}_R))\in\text{quasicategories} $$

  2. In the Dold-Kan equivalence $N:\text{sAb}\leftrightarrows\text{Ch}_R:\Gamma.$, $\Gamma$ is a monoidal functor (between cartesian categories), thus it induces a functor $$ \Gamma_*:\text{Ch}_R\text{-Cat}\to \text{sAb-Cat}. $$ The underlying set of a simplicial abelian group is a Kan complex thus $$ N_\Delta(\Gamma_*(\text{Ch}_R))\in\text{quasicategories} $$

Are the quasicategories $N_\Delta(L^H(\text{Ch}_R)$ and $N_\Delta(\Gamma_*(\text{Ch}_R)$ equivalent? Is this trivial?

Related: how to make the category of chain complexes into an $\infty$-category.

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    $\begingroup$ Are you restricting to complexes of projectives? If not, then the latter doesn't turn quasi-isomorphisms into equivalences - its homotopy category is the "chain homotopy category" $K(R)$ instead of the derived category $D(R)$. $\endgroup$ Jan 25, 2021 at 23:04
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    $\begingroup$ @TylerLawson: This is an answer, In my opinion $\endgroup$
    – Leo Alonso
    Jan 26, 2021 at 0:01
  • $\begingroup$ @TylerLawson Thank you for the answer, the question was wrong and I updated Example 2. What about now $\endgroup$ Jan 26, 2021 at 14:46
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    $\begingroup$ @DanielPlácido Your edit does not address Tyler Lawson's point, which is still correct: in your example 2 you're only inverting homotopy equivalences of complexes, not quasi-isomorphisms. $\endgroup$ Jan 26, 2021 at 15:34

1 Answer 1


The two categories you describe are not equivalent in the fashion that you hope.

No matter what kind of simplicial category $C$ is, the quasicategory $N_\Delta(C)$ has an explicit description of its homotopy category: namely, it has the same objects as $C$, and $$ Hom_{hN_\Delta C}(X,Y) = \pi_0 Hom_C(X,Y). $$ (This is true even if we need to take a fibrant replacement of $C$ first.)

Consider first case (1). Whenever $C$ is a category with a subcategory $W$ of weak equivalences, the Dwyer-Kan localization $L^H C$ has morphism spaces that are explicitly described: the 0-simplices are composites of morphisms in $C$ with formal inverses of morphisms in $C$, and the 1-simplices are natural transformations ("hammocks") built out of commutative diagrams involving these morphisms in $C$ or $W$. Taking $\pi_0$, we find that the homotopy category is formed by taking the morphisms in $C$ and formally inverting the morphisms in $W$: it is the localization. $$ hN_\Delta L^H C \cong C[W^{-1}] $$

Now consider case 2. If $C$ is a dg category, then we can also be explicit about the simplices in $Hom_{\Gamma_* C}(X,Y)$. Namely, a 0-simplex is a 0-cycle $f$ in the Hom-complex $\underline{Hom}_C(X,Y)$, and an edge from $f$ to $g$ is a 1-chain $h$ in $\underline{Hom}_C(X,Y)$ with $\partial h = g-f$. Taking $\pi_0$, we find that the homotopy category is formed by taking the morphisms in $C$ and modding out by the relation of chain homotopy equivalence $$ Hom_{hN_\Delta \Gamma_* C}(X,Y) \cong H_0 \underline{Hom}_C(X,Y). $$ This makes it the "chain homotopy category".

Applied to $Ch(R)$, the first construction is the derived category $D(R)$, and the second construction is the chain homotopy category $K(R)$. There is a natural functor $K(R) \to D(R)$, but it is not an equivalence: for example, there is a map of complexes from $(\dots \to 0 \to \Bbb Z \to \Bbb Z \to 0)$ to $(\dots \to 0 \to 0 \to \Bbb Z/2 \to 0)$ that is a quasi-isomorphism; there are no nonzero maps of chain complexes in the opposite direction in $Ch(\Bbb Z)$ and hence no nonzero maps in $K(\Bbb Z)$ either.

I believe that if you restrict to the subcategory of "cofibrant" complexes (e.g. bounded-below complexes of projectives) then you get an equivalence. Most of the versions of this result that I know, however, are for simplicial model categories (which $Ch_R$ is not), and so I don't have a reference handy.

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    $\begingroup$ One can use a trick to answer your last question in the affirmative: an exact functor between stable ∞-category is an equivalence iff it induces an equivalence on the homotopy categories, and it's quite easy to see that both the dg-nerve of cofibrant complexes and the DK localization are stable, and that the functor preserves cofibers and direct sums (the latter because it is the DK-localization of a stable model category). $\endgroup$ Jan 29, 2021 at 18:42
  • $\begingroup$ @DenisNardin Good point! $\endgroup$ Jan 30, 2021 at 1:26

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