Recurrences for the genus polynomials of linear sequences of graphs

Abstract

Abstract Given a finite graph H, the n th member Gn of an H-linear sequence is obtained recursively by attaching a disjoint copy of H to the last copy of H in G n−1 by adding edges or identifying vertices, always in the same way. The genus polynomial Γ G (z) of a graph G is the generating function enumerating all orientable embeddings of G by genus. Over the past 30 years, most calculations of genus polynomials Γ Gn (z) for the graphs in a linear family have been obtained by partitioning the embeddings of Gn into types 1, 2, …, k with polynomials Γ G n j $\begin{array}{} \Gamma_{G_n}^j \end{array}$ (z), for j = 1, 2, …, k; from these polynomials, we form a column vector V n ( z ) = [ Γ G n 1 ( z ) , Γ G n 2 ( z ) , … , Γ G n k ( z ) ] t $\begin{array}{} V_n(z) = [\Gamma_{G_n}^1(z), \Gamma_{G_n}^2(z), \ldots, \Gamma_{G_n}^k(z)]^t \end{array}$ that satisfies a recursion Vn (z) = M(z)V n−1(z), where M(z) is a k × k matrix of polynomials in z. In this paper, the Cayley-Hamilton theorem is used to derive a k th degree linear recursion for Γ n (z), allowing us to avoid the partitioning, thereby yielding a reduction from k 2 multiplications of polynomials to k such multiplications. Moreover, that linear recursion can facilitate proofs of real-rootedness and log-concavity of the polynomials. We illustrate with examples.