Bernstein-Sato ideals for free hyperplane arrangements

Abstract: Let $f=(f_1,\dots,f_r)$ be a complete factorization of a central hyperplane arrangement $D$ in $X=\mathbb{C}<sup>n$. For a monoid ideal $K\subseteq \mathbb{N}<sup>r$ we study the Bernstein-Sato ideal $B<sup>K_f$ of $f$ along $K$, that is, the $\mathbb{C}[s]$-annihilator of $$\mathcal{D}<em>X[s]f^s/\sum</em>{m\in K}\mathcal{D}<em>X[s]f<sup>{s+m}$$. When $D$ is free we compute two families of these ideals with the help of AI. For the unit shift $K=\langle e_i\rangle$ we prove that $B<sup>{-e_i}_f$ is generated by an explicit product of linear forms indexed by the dense edges of $D$ contained in $D_i$. This determines all the Bernstein-Sato ideals $$B^{{a,b}_f=\operatorname{Ann}}</em>{\mathbb{C}[s]}\mathcal{D}_X[s]f<sup>{s-a}/\mathcal{D}_X[s]f<sup>{s-b}$$, $a\geq b$, of a free arrangement, generalizing formulas of Maisonobe (2016) and Bath (2020). The main new ingredient identifies the multiplicities of the relative characteristic cycle of $$\mathcal{D}_X[s]f<sup>s/\mathcal{D}_X[s]f<sup>{s+e_i}$$ along the conormal bundle of the origin with the coefficients of the Hilbert series of an Artinian complete intersection attached to a generic Ziegler restriction of $D$; the total multiplicity computed in Wu (2022) then forces all the resulting coefficientwise upper bounds to be equalities. For the coordinate monoid ideal $K=\langle e_1,\dots,e_r\rangle$ we show that $B<sup>K_f$ is generated by one Euler relation for each irreducible factor of the essential quotient of $D$. Finally, we show that the zero locus of a Bernstein-Sato ideal along a monoid ideal need not be a finite union of translated linear subvarieties, even for a reduced free arrangement in $\mathbb{C}<sup>2$: for $f=(x,y,x+y,x+2y)$ and $K=\langle 3e_1,3e_2\rangle$ we compute $B<sup>K_f$ exactly and find an irreducible quadric component. This disproves a conjecture due to Budur.