Lorentzian polynomials and the independence sequences of graphs

We study the multivariate independence polynomials of graphs and the log-concavity of the coefficients of their univariate restrictions. Let R-W4 be the operator defined on simple and undirected graphs which replaces each edge with a caterpillar of size 4. We prove that all graphs in the image of R-W4 are what we call pre-Lorentzian, that is, their multivariate independence polynomial becomes Lorentzian after appropriate manipulations. In particular, as pre-Lorentzian graphs have log-concave (and therefore unimodal) independence sequences, our result makes progress on a conjecture of Alavi, Malde, Schwenk and Erd & odblac;s which asks if the independence sequence of trees or forests is unimodal.