Ext in pre-Abelian categories
Fred Richman, Elbert A. Walker · Pacific Journal of Mathematics · 1977
A natural definition for Έxt is given in an arbitrary pre-abelian category.Ext is an additive bifunctor from the category to abelian groups, the five lemma and the 3x3 lemma hold, and Ext w may be defined in the usual projectiveless way to yield the standard exact sequence of Ext's.Examples are given of what Ext is in various specific preabelian categories.