Modified Orlik's conjecture for characteristic-polynomial divisibility

Let A{\mathcal{A}} be an \ell-arrangement and let HAH\in{\mathcal{A}}. Write A=A{H}{\mathcal{A}}'={\mathcal{A}}\setminus\{H\}, and let π(A;t)\pi({\mathcal{A}};t) denote the Poincaré polynomial. Assume that A{\mathcal{A}} is free and that

π(AH;t) divides π(A;t).\pi({\mathcal{A}}^H;t)\text{ divides }\pi({\mathcal{A}};t).

Equivalently, assume that π(A;t)\pi({\mathcal{A}}';t) divides π(A;t)\pi({\mathcal{A}};t). Modified Orlik's conjecture. There exists an arrangement A{\mathcal{A}} and HAH\in{\mathcal{A}} such that, for the triple (A,A,AH)({\mathcal{A}},{\mathcal{A}}',{\mathcal{A}}^H), (1) π(AH;t)=i=11(1+dit)\pi({\mathcal{A}}^H;t)=\prod_{i=1}^{\ell-1}(1+d_it) with d1,,d1Zd_1,\ldots,d_{\ell-1}\in\mathbb{Z}; (2) π(AH;t)\pi({\mathcal{A}}^H;t) divides both π(A;t)\pi({\mathcal{A}};t) and π(A;t)\pi({\mathcal{A}}';t); and (3) neither A{\mathcal{A}} nor A{\mathcal{A}}' is free, or A{\mathcal{A}} is free and A{\mathcal{A}}' is not free.

The source introduces this as a conjectural counterexample to the stronger modified Orlik problem. The stated belief is that the problem is too strong and that such an arrangement exists; no resolution is supplied in the provided text.

Sources & referencesView supporting material

Primary source

Takuro Abe, “Restrictions of free arrangements and the division theorem”, arXiv:1603.03863 (2016).

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