The characteristic ideal of a finite, connected, regular graph

Abstract

Let $\Phi(x,y)\in\mathbb{C}[x,y]$ be a symmetric polynomial of partial degree $d$. The graph $G(\Phi)$ is defined by taking $\mathbb{C}$ as set of vertices and the points of $\mathbb{V}(\Phi(x,y))$ as edges. We study the following problem: given a finite, connected, $d$-regular graph $H$, find the polynomials $\Phi(x,y)$ such that $G(\Phi)$ has some connected component isomorphic to $H$ and, in this case, if $G(\Phi)$ has (almost) all components isomorphic to $H$. The problem is solved by associating to $H$ a characteristic ideal which offers a new perspective to the conjecture formulated in a previous paper, and allows to reduce its scope. In the second part, we determine the characteristic ideal for cycles of lengths $\le 5$ and for complete graphs of order $\le 6$. This results provide new evidence for the conjecture.