Bounded Treewidth and Complete Monotonicity for Scott-Sokal Spanning-Tree Polynomials

Abstract

Scott and Sokal asked for a structural description of the finite graphs \(G\) for which inverse powers \(T_G^{-β}\) of the spanning-tree polynomial are completely monotone. We prove the following bounded-treewidth criterion: if \(G\) is a finite connected simple graph with \(\operatorname{tw}(G)\le k\), then \(T_G^{-β}\) is completely monotone for every \(β>(k-1)/2\). Consequently, every partial \(3\)-tree is covered throughout the first Scott--Sokal open interval \(1<β<3/2\), including finite Apollonian networks, \(K_5-e\), and the four-spoke wheel \(W_4\). The proof combines the real Riesz/Wishart integral for determinants, star--mesh elimination of simplicial vertices, and a Gaussian Laplace kernel for the degree-\(d\) star. General bounded-treewidth graphs follow by chordal completion and monotone deletion of completion edges.