English

Leonard pairs having specified end-entries

Rings and Algebras 2014-09-16 v1

Abstract

Fix an algebraically closed field F\mathbb{F} and an integer d3d \geq 3. Let VV be a vector space over F\mathbb{F} with dimension d+1d+1. A Leonard pair on VV is an ordered pair of diagonalizable linear transformations A:VVA: V \to V and A:VVA^* : V \to V, each acting in an irreducible tridiagonal fashion on an eigenbasis for the other one. Let {vi}i=0d\{v_i\}_{i=0}^d (resp.\ {vi}i=0d\{v^*_i\}_{i=0}^d) be such an eigenbasis for AA (resp.\ AA^*). For 0id0 \leq i \leq d define a linear transformation Ei:VVE_i : V \to V such that Eivi=viE_i v_i=v_i and Eivj=0E_i v_j =0 if jij \neq i (0jd)(0 \leq j \leq d). Define Ei:VVE^*_i : V \to V in a similar way. The sequence Φ=(A,{Ei}i=0d,A,{Ei}i=0d)\Phi =(A, \{E_i\}_{i=0}^d, A^*, \{E^*_i\}_{i=0}^d) is called a Leonard system on VV with diameter dd. With respect to the basis {vi}i=0d\{v_i\}_{i=0}^d, let {thi}i=0d\{\th_i\}_{i=0}^d (resp.\ {ai}i=0d\{a^*_i\}_{i=0}^d) be the diagonal entries of the matrix representing AA (resp.\ AA^*). With respect to the basis {vi}i=0d\{v^*_i\}_{i=0}^d, let {θi}i=0d\{\theta^*_i\}_{i=0}^d (resp.\ {ai}i=0d\{a_i\}_{i=0}^d) be the diagonal entries of the matrix representing AA^* (resp.\ AA). It is known that {θi}i=0d\{\theta_i\}_{i=0}^d (resp. {thi}i=0d\{\th^*_i\}_{i=0}^d) are mutually distinct, and the expressions (θi1θi+2)/(θiθi+1)(\theta_{i-1}-\theta_{i+2})/(\theta_i-\theta_{i+1}), (θi1θi+2)/(θiθi+1)(\theta^*_{i-1}-\theta^*_{i+2})/(\theta^*_i - \theta^*_{i+1}) are equal and independent of ii for 1id21 \leq i \leq d-2. Write this common value as β+1\beta + 1. In the present paper we consider the "end-entries" θ0\theta_0, θd\theta_d, θ0\theta^*_0, θd\theta^*_d, a0a_0, ada_d, a0a^*_0, ada^*_d. We prove that a Leonard system with diameter dd is determined up to isomorphism by its end-entries and β\beta if and only if either (i) β±2\beta \neq \pm 2 and qd11q^{d-1} \neq -1, where β=q+q1\beta=q+q^{-1}, or (ii) β=±2\beta = \pm 2 and Char(F)2\text{Char}(\mathbb{F}) \neq 2.

Keywords

Cite

@article{arxiv.1409.4333,
  title  = {Leonard pairs having specified end-entries},
  author = {Kazumasa Nomura},
  journal= {arXiv preprint arXiv:1409.4333},
  year   = {2014}
}

Comments

arXiv admin note: substantial text overlap with arXiv:1408.2180

R2 v1 2026-06-22T05:57:03.384Z