Ultra log-concavity and real-rootedness of dependence polynomials

Abstract

For some positive integer $m$, a real polynomial $P(x)=\sum _{k=0}^m{a}_k{x}^k$ with $a_k⩾0$ is called {log-concave} (resp. ultra log-concave) if $a_k^2⩾a_{k-1}{a}_{k+1}$ (resp. $a_k^2⩾\left(1+\frac{1}{k}\right)\left(1+\frac{1}{m-k}\right)\cdot$ $a_{k-1}{a}_{k+1}$) for all $1⩽k⩽m-1$. If $P(x)$ has only real roots, then it is called {real-rooted}. It is well-known that the conditions of log-concavity, ultra log-concavity and real-rootedness are ever-stronger. %A famous theorem due to Newton states that if a real polynomial with non-negative coefficients is real-rooted, then it is log-concave. For a graph $G$, a dependent set is a set of vertices which is not independent, i.e., the set of vertices whose induced subgraph contains at least one edge. The dependence polynomial of $G$ is defined as $D(G,x):=\sum _{k⩾0}{d}_k(G)x^k$, where $d_k(G)$ is the number of dependent sets of size $k$ in $G$. Horrocks proved that $D(G,x)$ is log-concave for every graph $G$ [J. Combin. Theory, Ser. B, 84 (2002) 180--185]. In the present paper, we prove that, for a graph $G$, $D(G,x)$ is ultra log-concave if $G$ is $(K_2\cup 2K_1)$-free or contains an independent set of size $|V(G)|-2$, and give the characterization of graphs whose dependence polynomials are real-rooted. Finally, we focus more attention to the problems of log-concavity about independence systems and pose several conjectures closely related the famous Mason's Conjecture.

arXiv comment16 pages, 5 figures