Additivity and Fiber Sequences for Combinatorial K-Theory

The (A)CGW categories of Campbell and Zakharevich show how finite sets and varieties behave like the objects of an exact category for the purpose of algebraic $K$-theory. These structures admit a well-behaved Q-construction akin to Quillen’s, and satisfy analogues of the D'evissage and Localization theorems. In this work, we modify Campbell and Zakharevich’s axioms to obtain a framework called ECGW categories that allows for an $S_{\bullet }$-construction akin to Waldhausen’s, and show how it produces a K-theory spectrum which satisfies an analogue of the Additivity Theorem. We also define a notion of ``relative ECGW categories’’ which have weak equivalences determined by a subcategory of acyclic objects satisfying minimal conditions; these satisfy analogues of the Fibration and Localization Theorems that generalize previous versions in the literature. We illustrate these results with examples including exact categories, extensive categories, algebraic varieties, and polytopes up to scissors congruence.