Edge-reflection positivity and weighted graph homomorphisms

Edge-reflection positivity and weighted graph homomorphisms

B. Szegedy [Edge coloring models and reflection positivity, Journal of the American Mathematical Society 20 (2007) 969–988] showed that the number of homomorphisms into a weighted graph is equal to the partition function of a complex edge-coloring model. Using some results in geometric invariant theory, we characterize for which weighted graphs the edge-coloring model can be taken to be real va...

On Weighted Graph Homomorphisms

For given graphs G and H , let |Hom(G,H)| denote the set of graph homomorphisms from G to H . We show that for any finite, n-regular, bipartite graph G and any finite graph H (perhaps with loops), |Hom(G,H)| is maximum when G is a disjoint union of Kn,n’s. This generalizes a result of J. Kahn on the number of independent sets in a regular bipartite graph. We also give the asymptotics of the log...

Edge Coloring Models and Reflection Positivity

The motivation of this paper comes from statistical physics as well as from combinatorics and topology. The general setup in statistical mechanics can be outlined as follows. Let G be a graph and let C be a finite set of “states” or “colors”. We think of G as a crystal in which either the edges or the vertices are regarded as “sites” which can have states from C. In the first case we speak abou...

The complexity of signed graph and edge-coloured graph homomorphisms

The goal of this work is to study homomorphism problems (from a computational point of view) on two superclasses of graphs: 2-edge-coloured graphs and signed graphs. On the one hand, we consider the H-Colouring problem when H is a 2-edge-coloured graph, and we show that a dichotomy theorem would imply the dichotomy conjecture of Feder and Vardi. On the other hand, we prove a dichotomy theorem f...

Edge coloring models and reflection positivity Balázs Szegedy