On the interlace polynomials of forests

The interlace polynomials were introduced by Arratia, Bollobás and Sorkin (2004) [3], [4] and [5]. These invariants generalize to arbitrary graphs some special properties of the Euler circuits of 2-in, 2-out digraphs. Among many other results, Arratia, Bollobás and Sorkin (2004) [3], [4] and [5] give explicit formulas for the interlace polynomials of certain types of graphs, including paths; it is natural to wonder whether or not it is possible to extend these formulas to larger classes of graphs. We give a combinatorial description of the interlace polynomials of trees and forests.