Duality for Groups (1950)

This is an early paper by Mac Lane that lead to interesting results (such as abelian categories) and paved the way for the notion of categorical duality in early category theory.

Interestingly, he did not cover the case of (not necessarily abelian) groups and attempting to phrase the category of groups in self-dual terms necessarily would lead to additional structure on the category.

Nevertheless, the abelian case turned out to lead to the whole field of abelian categories as studied by many great mathematicians (such as Grothendieck). This paper is one of the earliest full exhibitions of how to approach duality in the category theoretic sense.