Orlik–Randell conjecture on cohomology of generic arrangement Milnor fibers

Let Q=H1HkQ=H_1\cdots H_k be a reduced polynomial defining a central generic arrangement in Cn\mathbb C^n, let RnR_n be the polynomial ring, let EE be the vector space defined by the displayed Jacobian expressions in the source, and let ω\omega and π\pi denote the differential form and map used in the Orlik–Randell construction. Write UrU_r for the degree-rr graded piece of UU.

Orlik–Randell conjecture. For the fiber X1=Var(Q1)X_1=\operatorname{Var}(Q-1), there is a finite-dimensional homogeneous vector space URnU\subset R_n such that

Rn=E(C[Q]U),R_n=E\oplus (\mathbb C[Q]\otimes U), UHn1(X1,C),gπ(gω)ˉU\longrightarrow H^{n-1}(X_1,\mathbb C),\qquad g\longmapsto \bar{\pi(g\omega)}

is an isomorphism, and

Ωαn1=π(Uω)dΩαn2.\Omega^{n-1}_\alpha=\pi(U\omega)\oplus d\Omega^{n-2}_\alpha.

Moreover,

νr={(r+n1n1),0rkn,displaystyle(k2n1),kn+1rk1,displaystyle(k2n1)(rk+n1n1),kr2kn2.\nu_r=\begin{cases}\displaystyle {r+n-1\choose n-1},&0\le r\le k-n,\\\\displaystyle {k-2\choose n-1},&k-n+1\le r\le k-1,\\\\displaystyle {k-2\choose n-1}-{r-k+n-1\choose n-1},&k\le r\le 2k-n-2.\end{cases}

This conjecture proposes a homogeneous description of representatives for the Milnor-fiber cohomology, analogous to the isolated-singularity case; the source gives no resolution of the full assertion.

Sources & referencesView supporting material

Primary source

Uli Walther, “Bernstein-Sato polynomial versus cohomology of the Milnor fiber for generic arrangements”, arXiv:math/0204080 (2003).

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