Reflection positivity, rank connectivity, and homomorphism of graphs
Freedman, M. · Lovasz, L. · Schrijver, A.
Original · EN
It is shown that a graph parameter can be realized as the number of homomorphisms into a fixed (weighted) graph if and only if it satisfies two linear algebraic conditions: reflection positivity and exponential rank-connectivity. In terms of statistical physics, this can be viewed as a characterization of partition functions of vertex models.
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