  We investigate group-theoretic "signatures" of odd cycles of a graph, and
their connections to topological obstructions to 3-colourability. In the case
of signatures derived from free groups, we prove that the existence of an odd
cycle with trivial signature is equivalent to having the coindex of the
hom-complex at least 2 (which implies that the chromatic number is at least 4).
In the case of signatures derived from elementary abelian 2-groups we prove
that the existence of an odd cycle with trivial signature is a sufficient
condition for having the index of the hom-complex at least 2 (which again
implies that the chromatic number is at least 4).
