Computations associated with the resonance arrangement

Abstract: The resonance arrangement $\mathcal{A}<em>n$ is the arrangement of hyperplanes in $\mathbb{R}<sup>n$ given by all hyperplanes of the form $\sum</em>{i \in I} x_i = 0$, where $I$ is a nonempty subset of ${1,\dots,n}$. We consider the characteristic polynomial $\chi(\mathcal{A}_n; t)$ of the resonance arrangement, whose value $R_n$ at $-1$ is of particular interest, and corresponds to counts of generalized retarded functions in quantum field theory, among other things. No formula is known for either the characteristic polynomial or $R_n$, though $R_n$ has been computed up to $n=8$. By exploiting symmetry and using computational methods, we compute the characteristic polynomial of $\mathcal{A}_9$, and thus obtain $R_9$. The coefficients of the characteristic polynomial are also equal to the so-called Betti numbers of the complexified hyperplane arrangement; that is, the coefficient of $t<sup>{n-i}$ is denoted by the Betti number $b_i(\mathcal{A}_n)$. Explicit formulas are known for the Betti numbers up to $b_3(\mathcal{A}_n)$. Using computational methods, we also obtain an explicit formula for $b_4(\mathcal{A}_n)$, which gives the $t<sup>{n-4}$ coefficient of the characteristic polynomial.