$A_{\infty}$-structures in monoidal DG categories and strong homotopy unitality

Abstract

We define $A_{\infty}$-structures -- algebras, coalgebras, modules, and comodules -- in an arbitrary monoidal DG category or bicategory by rewriting their definitions in terms of unbounded twisted complexes. We develop new notions of strong homotopy unitality and bimodule homotopy unitality to work in this level of generality. For a strong homotopy unital $A_{\infty}$-algebra we construct Free-Forgetful homotopy adjunction, its Kleisli category, and its derived category of modules. Analogous constructions for $A_{\infty}$-coalgebras require bicomodule homotopy counitality. We define homotopy adjunction for $A_{\infty}$-algebra and $A_{\infty}$-coalgebra and show such pair to be derived module-comodule equivalent. As an application, we obtain the notions of an $A_{\infty}$-monad and of an enhanced exact monad. We also show that for any adjoint triple $(L,F,R)$ of functors between enhanced triangulated categories the adjunction monad $RF$ and the adjunction comonad $LF$ are derived module-comodule equivalent.