  To define a minimal mathematical framework for isolating some of the
characteristic properties of quantum entanglement, we introduce a
generalization of the tensor product of graphs. Inspired by the notion of a
density matrix, the generalization is a simple one: every graph can be obtained
by addition modulo two, possibly with many summands, of tensor products of
adjacency matrices. In this picture, we are still able to prove a combinatorial
analogue of the Peres-Horodecki criterion for testing separability.
