Circular bidiagonal pairs

TL;DR

This paper classifies pairs of linear maps called circular bidiagonal pairs, which have matrices with specific bidiagonal and diagonal forms, revealing two infinite families of solutions.

Contribution

It provides a classification of circular bidiagonal pairs up to affine equivalence, identifying two infinite families of solutions.

Findings

Two infinite families of circular bidiagonal pairs identified

Complete classification up to affine equivalence

Explicit descriptions of the solution families

Abstract

A square matrix is said to be circular bidiagonal whenever (i) each nonzero entry is on the diagonal, or the subdiagonal, or in the top-right corner; (ii) each subdiagonal entry is nonzero, and the entry in the top-right corner is nonzero. Let $\mathbb F$ denote a field, and let $V$ denote a nonzero finite-dimensional vector space over $\mathbb F$. We consider an ordered pair of $\mathbb F$-linear maps $A: V \to V$ and $A^*: V \to V$ that satisfy the following two conditions: (i) there exists a basis for $V$ with respect to which the matrix representing $A$ is circular bidiagonal and the matrix representing $A^*$ is diagonal; (ii) there exists a basis for $V$ with respect to which the matrix representing $A^*$ is circular bidiagonal and the matrix representing $A$ is diagonal. We call such a pair a circular bidiagonal pair on $V$. We classify the circular bidiagonal pairs up to affine equivalence. There are two infinite families of solutions, which we describe in detail.