On the twisted tensor product of small dg categories

Abstract: Given two small dg categories $C,D$, defined over a field, we introduce their (non-symmetric) twisted tensor product $C\overset{\sim}{\otimes} D$. We show that $-\overset{\sim}{\otimes} D$ is left adjoint to the functor $Coh(D,-)$, where $Coh(D,E)$ is the dg category of dg functors $D\to E$ and their coherent natural transformations. This adjunction holds in the category of small dg categories (not in the homotopy category of dg categories $\mathrm{Hot}$). We show that for $C,D$ cofibrant, the adjunction descends to the corresponding adjunction in the homotopy category. Then comparison with a result of To\"{e}n shows that, for $C,D$ cofibtant, $C\overset{\sim}{\otimes} D$ is isomorphic to $C\otimes D$, as an object of the homotopy category $\mathrm{Hot}$.