On partition functions for 3-graphs

A cyclic graph is a graph with at each vertex a cyclic order of the edges incident with it specified. We characterize which real-valued functions on the collection of cubic cyclic graphs are partition functions of a real vertex model (P. de la Harpe, V.F.R. Jones (1993) [4]). They are characterized by ‘weak reflection positivity’, which amounts to the positive semidefiniteness of matrices based on the ‘k  -join’ of cubic cyclic graphs (for all k∈Z+k∈Z+).Basic tools are the representation theory of the symmetric group and geometric invariant theory, in particular the Hanlon–Wales theorem on the decomposition of Brauer algebras and the Procesi–Schwarz theorem on inequalities defining orbit spaces.