Approximation in K-theory for Waldhausen Quasicategories

Abstract

We prove a series of Approximation Theorems in the setting of Waldhausen quasicategories. These theorems, inspired by Waldhausen's 1985 Approximation Theorem, give sufficient conditions for an exact functor of Waldhausen quasicategories to induce a level-wise weak homotopy equivalence of K-theory spectra. The Pre-Approximation Theorem, which holds in the general setting of quasicategories without Waldhausen structures, provides sufficient conditions for a functor F:A->B to restrict to an equivalence of the maximal infinity-groupoids in A and B. Our Approximation Theorems follow from the Pre-Approximation Theorem. The Approximation Theorem in the quasicategorical setting most analogous to Waldhausen's is: if an exact functor F:A -> B satisfies Waldhausen's App 1 and App 2, and the domain A admits colimits of the aforementioned type and F preserves them, then K(F) is a level-wise equivalence. As a corollary, if F is an exact functor with ho(F) an equivalence of ordinary categories, and every morphism in the domain A is a cofibration, then K(F) is a level-wise equivalence. We then introduce a version of App 2 called Cofibration App 2 that only requires factorization of cofibrations Fa >-> b as (equiv) o F(cofibration) and prove an analogous Cofibration Approximation Theorem, and a corollary for certain functors that induce an equivalence of cofibration homotopy categories. We also prove that S_n^infinity is Waldhausen equivalent to \overline{\mathcal{F}_{n-1}^infinity} using the mid anodyne maps known as spine inclusions, and clarify how hypotheses and notions in Waldhausen structures are related in new ways in the context of quasicategories.