$k$-Adjoint of Hyperplane Arrangements

TL;DR

This paper introduces the $k$-adjoint of hyperplane arrangements, generalizing classical concepts, and demonstrates its equivalence to existing decompositions of the Grassmannian, with applications in combinatorial classification and invariant properties.

Contribution

It defines the $k$-adjoint of hyperplane arrangements and proves its equivalence to known Grassmannian decompositions, extending matroid and Schubert decompositions.

Findings

Unified decomposition of Grassmannian via $k$-adjoint

Classification of $k$-dimensional restrictions of arrangements

Anti-monotonicity of Whitney and independence numbers

Abstract

In this paper, we introduce the $k$-adjoint of a given hyperplane arrangement $\mathcal{A}$ associated with rank-$k$ elements in the intersection lattice $L(\mathcal{A})$, which generalizes the classical adjoint proposed by Bixby and Coullard. The $k$-adjoint of $\mathcal{A}$ induces a decomposition of the Grassmannian, which we call the $\mathcal{A}$-adjoint decomposition. Inspired by the work of Gelfand, Goresky, MacPherson, and Serganova, we generalize the matroid decomposition and refined Schubert decomposition of the Grassmannian from the perspective of $\mathcal{A}$. Furthermore, we prove that these three decompositions are exactly the same decomposition. A notable application involves providing a combinatorial classification of all the $k$-dimensional restrictions of $\mathcal{A}$. Consequently, we establish the anti-monotonicity property of some combinatorial invariants, such as Whitney numbers of the first kind and the independece numbers.