Brown functors of directed graphs

Abstract

We prove that any digraph Brown functor -- i.e. a contravariant functor from the homotopy category of finite directed graphs to the category of abelian groups, satisfying the triviality axiom, the additivity axiom, and the Mayer-Vietoris axiom -- is representable. Furthermore, we show that the first path cohomology functor is a digraph Brown functor.

20 pages, 4 figures, simplified proofs, and additional references. arXiv admin note: significant text overlap with a part of arXiv:2003.07426