Modular Equalities for Complex Reflection Arrangements

Abstract

We compute the combinatorial Aomoto-Betti numbers ${\beta }_p(\mathcal{A})$ of a complex reflection arrangement. When $\mathcal{A}$ has rank at least 3, we find that ${\beta }_p(\mathcal{A})\le 2$, for all primes $p$. Moreover, ${\beta }_p(\mathcal{A})=0$ if $p>3$, and ${\beta }_2(\mathcal{A})\ne 0$ if and only if $\mathcal{A}$ is the Hesse arrangement. We deduce that the multiplicity $e_d(\mathcal{A})$ of an order $d$ eigenvalue of the monodromy action on the first rational homology of the Milnor fiber is equal to the corresponding Aomoto-Betti number, when $d$ is prime. We give a uniform combinatorial characterization of the property $e_d(\mathcal{A})\ne 0$, for $2\le d\le 4$. We completely describe the monodromy action for full monomial arrangements of rank 3 and 4. We relate $e_d(\mathcal{A})$ and ${\beta }_p(\mathcal{A})$ to multinets, on an arbitrary arrangement.