On complex roots of the independence polynomial
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It is known from the work of Shearer (1985) (and also Scott and Sokal (2005)) that the independence polynomial Z_G(λ) of a graph G of maximum degree at most d+1 does not vanish provided that |λ|≤d^d/(d+1)^d+1. Significant extensions of this result have recently been given in the case λ≥ 0 by Peters and Regts (2019) and Bencs and Csikvári (arxiv:1807.08963). In this paper, our motivation is to further extend these results and find zero free regions when λ≤ 0. We begin by giving new geometric criteria for establishing zero-free regions as well as for carrying out semi-rigorous numerical explorations. We then provide two examples of the (rigorous) use of these criteria, by establishing two new zero-free regions in the left-half plane. We also improve upon the results of Bencs and Péter Csikvári (arxiv:1807.08963) for the right half-plane using our framework. By a direct application of the interpolation method of Barvinok, combined with extensions due to Patel and Regts, these results also imply deterministic polynomial time approximation algorithms for the independence polynomial of bounded degree graphs in the new zero-free regions.
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