Signed a-polynomials of graphs and Poincar'e polynomials of real toric manifolds

摘要

Choi and Park introduced an invariant of a finite simple graph, called emph{signed a-number}, arising from computing certain topological invariants of some specific kinds of real toric manifolds. They also found the signed a-numbers of path graphs, cycle graphs, complete graphs, and star graphs. We introduce a emph{signed a-polynomial} which is a generalization of the signed a-number and gives emph{a-, b-, and c-numbers}. The signed a-poly-nomial of a graph $G$ is related to the Poincar'e polynomial $P_{M(G)}(z)$, which is the generating function for the Betti numbers of the real toric manifold $M(G)$. We give the generating functions for the signed a-polynomials of not only path graphs, cycle graphs, complete graphs, and star graphs, but also complete bipartite graphs and complete multipartite graphs. As a consequence, we find the Euler characteristic number and the Betti numbers of the real toric manifold $M(G)$ for complete multipartite graphs $G$.