A closed model structure on the category of weakly unital dg categories, II (2007.12716v1)

Abstract: In this paper, which is subsequent to our previous paper PS, we continue our study of the closed model structure on the category $\mathrm{Cat}{\mathrm{dgwu}}(\Bbbk)$ of small weakly unital dg categories (in the sense of Kontsevich-Soibelman [KS]) over a field $\Bbbk$. In [PS], we constructed a closed model structure on the category of weakly unital dg categories, imposing a technical condition on the weakly unital dg categories, saying that $\mathrm{id}_x\cdot \mathrm{id}_x=\mathrm{id}_x$ for any object $x$. Although this condition led us to a great simplification, it was redundant and had to be dropped. Here we get rid of this condition, and provide a closed model structure in full generality. The new closed model category is as well cofibrantly generated, and it is proven to be Quillen equivalent to the closed model category $\mathrm{Cat}\mathrm{dg}(\Bbbk)$ of (strictly unital) dg categories over $\Bbbk$, given by Tabuada [Tab1]. Dropping the condition $\mathrm{id}x^2=\mathrm{id}_x$ makes the construction of the closed model structure more distant from loc.cit., and requires new constructions. One of them is a pre-triangulated hull of a wu dg category, which in turn is shown to be a wu dg category as well. One example of a weakly unital dg category which naturally appears is the bar-cobar resolution of a dg category. We supply this paper with a refinement of the classical bar-cobar resolution of a unital dg category which is strictly unital (appendix B). A similar construction can be applied to constructing a cofibrant resolution in $\mathrm{Cat}\mathrm{dgwu}(\Bbbk)$.