A characterization of Leonard pairs using the parameters [formula omitted

Two linear transformations each tridiagonal with respect to an eigenbasis of the other; the TD–D canonical form and the LB–UB canonical form

In this paper, we introduce the concepts of compatibility and companion for Leonard pairs. These concepts are roughly described as follows. Let $\mathbb{F}$ denote a field, and let $V$ denote a vector space over $\mathbb{F}$ with finite positive dimension.A Leonard pair on $V$ is an ordered pair of diagonalizable $\mathbb{F}$-linear maps $A : V \to V$ and $A^* : V \to V$ that each act in an irreducible tridiagonal fashion on an eigenbasis for the other one. Leonard pairs $A,A^*$ and $B,B^*$ on $V$ are said to be compatible whenever $A^* = B^*$ and $[A,A^*] = [B,B^*]$, where $[r,s] = r s - s r$. For a Leonard pair $A,A^*$ on $V$, by a companion of $A,A^*$ we mean an $\mathbb{F}$-linear map $K: V \to V$ such that $K$ is a polynomial in $A^*$ and $A-K, A^*$ is a Leonard pair on $V$. The concepts of compatibility and companion are related as follows. For compatible Leonard pairs $A,A^*$ and $B,B^*$ on $V$, define $K = A-B$. Then $K$ is a companion of $A,A^*$. For a Leonard pair $A,A^*$ on $V$ and a companion $K$ of $A,A^*$,define $B = A-K$ and $B^* = A^*$. Then $B,B^*$ is a Leonard pair on $V$ that is compatible with $A,A^*$. Let $A,A^*$ denote a Leonard pair on $V$. We find all the Leonard pairs $B, B^*$ on $V$ that are compatible with $A,A^*$.For each solution $B, B^*$, we describe the corresponding companion $K = A-B$.

We first summarize the basic structure of the outer distribution module of a completely regular code. Then, employing a simple lemma concerning eigenvectors in association schemes, we propose to study the tightest case, where the indices of the eigenspace that appear in the outer distribution module are equally spaced. In addition to the arithmetic codes of the companion paper, this highly structured class includes other beautiful examples and we propose the classification of $Q$-polynomial completely regular codes in the Hamming graphs. A key result is Theorem 3.10 which finds that the $Q$-polynomial condition is equivalent to the presence of a certain Leonard pair. This connection has impact in two directions. First, the Leonard pairs are classified and we gain quite a bit of information about the algebraic structure of any code in our class. But also this gives a new setting for the study of Leonard pairs, one closely related to the classical one where a Leonard pair arises from each thin/dual-thin irreducible module of a Terwilliger algebra of some $P$- and $Q$-polynomial association scheme, yet not previously studied. It is particularly interesting that the Leonard pair associated to some code $C$ may belong to one family in the Askey scheme while the distance-regular graph in which the code is found may belong to another.

Let V denote a vector space with finite positive dimension. We consider an ordered pair of linear transformations and that satisfy (i) and (ii) below. There exists a basis for V with respect to which the matrix representing A is irreducible tridiagonal and the matrix representing is diagonal. There exists a basis for V with respect to which the matrix representing is irreducible tridiagonal and the matrix representing A is diagonal. We call such a pair a Leonard pair on V. In the literature, there are some parameters that are used to describe Leonard pairs called the intersection numbers , , , and the dual eigenvalues . In this paper, we provide two characterizations of Leonard pairs. For the first characterization, the focus is on the and . For the second characterization, the focus is on the , , and .

In Linear algebra, the concept of Leonard pair (LP) was motivated by the theory of Q-polynomial distance-regular graphs. In this dissertation, we will first give a brief introduction to LPs and to two closely-related classes of objects: (i) bipartite Leonard pairs (BLPs) and (ii) almost bipartite Leonard pairs (ABLPs). Taking these as departure points, we will introduce a new class of object - doubly almost bipartite Leonard pairs (DABLPs). The primary aim of our work is to fully classify (up to isomorphism) this new family. In addition, since there is known to be a natural correspondence between Leonard pairs and families of orthogonal polynomials, we reveal which families of orthogonal polynomials correspond to the DABLPs. Several related objects, such as Leonard triples, modular Leonard triples, spin Leonard pairs, and near-bipartite Leonard pairs have corresponding notions for the doubly almost bipartite case. These analogous objects are also defined and briefly explored.

The universal DAHA of type [formula omitted] and Leonard pairs of q-Racah type

The picard group of a reduced G-Algebra

We will describe the space of bilinear functions on a pair of finite-dimensional vector spaces as the tensor product space of the duals of those two vector spaces. We then consider tensor product spaces of more than two vector spaces. Finally we define tensor product spaces in the general case so that they reduce to spaces of multilinear functions in finite dimensions.

The non-zero component graph of a vector space with finite dimension over a finite field F is the graph \(G~ = ~\left( {V,E} \right)\), where vertices of G are the non-zero vectors in V, two of which are adjacent if they have at least one basis vector with non-zero coefficient common in their basic representation. In this paper, we discuss certain properties of the non-zero component graphs of vector spaces with finite dimension over finite fields and their graph invariants.

In this chapter we discuss in detail the relations between the parameters of Q-polynomial distance-regular graphs and their Q-sequences. We start with a reformulation of the Q-polynomial property defined in §4.1E, which is the source of most of our results. This leads to a three-term recurrence relation for Q-sequences (Theorem 8.1.2) discovered by Leonard [485], and a representation of the intersection array of Q-polynomial graphs in terms of 5 parameters only (Proposition 8.1.5). We mention Leonard’s explicit parameter formulae and results of Bannai & Ito [33] on the integrality of eigenvalues for large diameters based on these formulae. We also determine which of the known distance-regular graphs are Q-polynomial.

Gabor analysis is playing an important role in time-frequency analysis for the last 60 years. The fundamental concept of Gabor frames has become a focus also in the finite dimensional setting. Here, we study which sets of discrete time frequency shifts admit a proper choice of window vector, so that they generate an orthonormal Gabor basis in the underlying finite dimensional vector space. We fully characterize such time-frequency shift sets in the case that the vector space dimension is a prime number.

Two commuting operators associated with a tridiagonal pair

The classification of Leonard triples of Racah type

Nonsymmetric Askey–Wilson polynomials and Q-polynomial distance-regular graphs

Let 𝔽 denote a field and let V denote a vector space over 𝔽 with finite positive dimension. We consider a pair of linear transformations A : V → V and A* : V → V that satisfy the following conditions: (i) each of A, A* is diagonalizable; (ii) there exists an ordering [Formula: see text] of the eigenspaces of A such that A* Vi ⊆ Vi-1 + Vi + Vi+1 for 0 ≤ i ≤ d, where V-1 = 0 and Vd+1 = 0; (iii) there exists an ordering [Formula: see text] of the eigenspaces of A* such that [Formula: see text] for 0 ≤ i ≤ δ, where [Formula: see text] and [Formula: see text]; (iv) there is no subspace W of V such that AW ⊆ W, A* W ⊆ W, W ≠ 0, W ≠ V. We call such a pair a tridiagonal pair on V. It is known that d = δ, and for 0 ≤ i ≤ d the dimensions of Vi, [Formula: see text], Vd-i, [Formula: see text] coincide. Denote this common dimension by ρi and call A, A*sharp whenever ρ0 = 1. Let T denote the 𝔽-subalgebra of End 𝔽(V) generated by A, A*. We show: (i) the center Z(T) is a field whose dimension over 𝔽 is ρ0; (ii) the field Z(T) is isomorphic to each of E0TE0, EdTEd, [Formula: see text], [Formula: see text], where Ei (resp. [Formula: see text]) is the primitive idempotent of A (resp. A*) associated with Vi (resp. [Formula: see text]); (iii) with respect to the Z(T)-vector space V the pair A, A* is a sharp tridiagonal pair.