## Advanced Studies in Pure Mathematics

- Adv. Stud. Pure Math.
- Arrangements of Hyperplanes — Sapporo 2009, H. Terao and S. Yuzvinsky, eds. (Tokyo: Mathematical Society of Japan, 2012), 75 - 107

### Vanishing products of one-forms and critical points of master functions

Daniel C. Cohen, Graham Denham, Michael Falk, and Alexander Varchenko

#### Abstract

Let $\mathcal{A}$ be an affine hyperplane arrangement in $\mathbb{C}^{\ell}$ with complement $U$. Let $f_1, \dots, f_n$ be linear polynomials defining the hyperplanes of $\mathcal{A}$, and $A^{\cdot}$ the algebra of differential forms generated by the one-forms $\mathrm{d} \log f_1 , \dots, \mathrm{d} \log f_n$. To each $\lambda \in \mathbb{C}^n$ we associate the master function $\Phi = \prod^{n}_{i=1} f^{\lambda_i}_i$ on $U$ and the closed logarithmic one-form $\omega = \mathrm{d} \log \Phi$. We assume $\omega$ is a general element of a rational linear subspace $D$ of $A^1$ of dimension $q \gt 1$ such that the map $\bigwedge^k(D) \rightarrow A^k$ given by multiplication in $A^{\cdot}$ is zero for all $p \lt k \leq q$, and is nonzero for $k = p$. With this assumption, we prove the critical locus $\mathrm{crit}(\Phi)$ of $\Phi$ has components of codimension at most $p$, and these are intersections of level sets of $p$ rational master functions. We give conditions that guarantee $\mathrm{crit}(\Phi)$ is nonempty and every component has codimension equal to $p$, in terms of syzygies among polynomial master functions. If $\mathcal{A}$ is $p$-generic, then $D$ is contained in the degree $p$ resonance variety $\mathcal{R}^p(\mathcal{A})$—in this sense the present work complements previous work on resonance and critical loci of master functions. Any arrangement is 1-generic; we give a precise description of $\mathrm{crit}(\Phi_{\lambda})$ in case $\lambda$ lies in an isotropic subspace $D$ of $A^1$, using the multinet structure on $\mathcal{A}$ corresponding to $D \subseteq \mathcal{R}^1 (\mathcal{A})$. This is carried out in detail for the Hessian arrangement. Finally, for arbitrary $p$ and $\mathcal{A}$, we establish necessary and sufficient conditions for a set of integral one-forms to span such a subspace, in terms of nested sets of $\mathcal{A}$, using tropical implicitization.

#### Article information

**Source***Arrangements of Hyperplanes — Sapporo 2009*, H. Terao and S. Yuzvinsky, eds. (Tokyo: Mathematical Society of Japan, 2012), 75-107

**Dates**

Received: 17 October 2010

Revised: 27 March 2011

First available in Project Euclid:
24 November 2018

**Permanent link to this document**

https://projecteuclid.org/
euclid.aspm/1543085005

**Digital Object Identifier**

doi:10.2969/aspm/06210075

**Mathematical Reviews number (MathSciNet)**

MR2933793

**Zentralblatt MATH identifier**

1260.32007

**Subjects**

Primary: 32S22: Relations with arrangements of hyperplanes [See also 52C35]

Secondary: 55N25: Homology with local coefficients, equivariant cohomology 52C35: Arrangements of points, flats, hyperplanes [See also 32S22] 14T04

**Keywords**

Arrangement of hyperplanes master function critical locus resonance variety Orlik–Solomon algebra tropicalization

#### Citation

Cohen, Daniel C.; Denham, Graham; Falk, Michael; Varchenko, Alexander. Vanishing products of one-forms and critical points of master functions. Arrangements of Hyperplanes — Sapporo 2009, 75--107, Mathematical Society of Japan, Tokyo, Japan, 2012. doi:10.2969/aspm/06210075. https://projecteuclid.org/euclid.aspm/1543085005