The structure of a tridiagonal pair

TL;DR

This paper characterizes the structure of tridiagonal pairs of linear transformations on finite-dimensional vector spaces over algebraically closed fields, establishing their properties, symmetries, and classification data.

Contribution

It proves key structural properties of tridiagonal pairs over algebraically closed fields, including dimension, symmetry, and uniqueness results, and provides a classification framework.

Findings

Each of the extremal eigenspaces has dimension 1.

Existence of a symmetric bilinear form compatible with the pair.

Uniqueness of an anti-automorphism fixing the pair.

Abstract

Let $K$ denote a field and let $V$ denote a vector space over $K$ with finite positive dimension. We consider a pair of linear transformations $A:V \to V$ and $A^*:V \to V$ that satisfy the following conditions: (i) each of $A,A^*$ is diagonalizable; (ii) there exists an ordering $\{V_i\}_{i=0}^d$ of the eigenspaces of $A$ such that $A^* V_i \subseteq V_{i-1} + V_i + V_{i+1}$ for $0 \leq i \leq d$, where $V_{-1}=0$ and $V_{d+1}=0$; (iii) there exists an ordering $\{V^*_i\}_{i=0}^\delta$ of the eigenspaces of $A^*$ such that $A V^*_i \subseteq V^*_{i-1} + V^*_i + V^*_{i+1}$ for $0 \leq i \leq \delta$, where $V^*_{-1}=0$ and $V^*_{\delta+1}=0$; (iv)there is no subspace $W$ of $V$ such that $AW \subseteq W$, $A^* W \subseteq W$, $W \neq 0$, $W \neq V$. We call such a pair a tridiagonal pair on $V$. It is known that $d=\delta$ and for $0 \leq i \leq d$ the dimensions of $V_i, V_{d-i}, V^*_i, V^*_{d-i}$ coincide. In this paper we show that the following (i)--(iv) hold provided that $K$ is algebraically closed: (i) Each of $V_0$, $V^*_0$, $V_d$, $V^*_d$ has dimension 1. (ii) There exists a nondegenerate symmetric bilinear form $(,)$ on $V$ such that $(Au,v)=(u,Av)$ and $(A^*u,v)=(u,A^*v)$ for all $u,v \in V$. (iii) There exists a unique anti-automorphism of $End(V)$ that fixes each of $A,A^*$. (iv) The pair $A,A^*$ is determined up to isomorphism by the data $(\{\th_i\}_{i=0}^d; \{\th^*_i\}_{i=0}^d; \{\zeta_i\}_{i=0}^d)$, where $\th_i$ (resp. $\th^*_i$) is the eigenvalue of $A$ (resp. $A^*$) on $V_i$ (resp. $V^*_i$), and $\{\zeta_i\}_{i=0}^d$ is the split sequence of $A,A^*$ corresponding to $\{\th_i\}_{i=0}^d$ and $\{\th^*_i\}_{i=0}^d$.