Waldhausen category

In mathematics a Waldhausen category (after Friedhelm Waldhausen) is a category C with a zero object equipped with cofibrations co(C) and weak equivalences we(C), both containing all isomorphisms, both compatible with pushout, and co(C) containing the unique morphisms

from the zero-object to any object A.

To be more precise about the pushouts, we require when

is a cofibration and

is any map, that we have a push-out

where the map

is a cofibration:

A category C is equipped with bifibrations if it has cofibrations and its opposite category C^OP has so also. In that case, we denote the fibrations of C^OP by quot(C). In that case, C is a biWaldhausen category if C has bifibrations and weak equivalences such that both (C, co(C), we) and (C^OP, quot(C), we^OP) are Waldhausen categories.

As examples one may think of exact categories, where the cofibrations are the admissible monomorphisms. Another example is the full subcategory of cofibrant objects in a pointed model categories, that is, the full subcategory consisting of those objects ${\displaystyle X}$ for which ${\displaystyle 0\to X}$ is a cofibration. (The bifibrant objects do not in general form a Waldhausen category, as a pushout of fibrant objects need not be fibrant. For more information on this second example see the paper by Sagave in the references)

...
Wikipedia