Reducibility of Matrix Weights

Abstract

In this paper we discuss the notion of reducibility for matrix weights and introduce a real vector space ${\mathcal{C}}_{\mathbb{R}}$ which encodes all information about the reducibility of $W$. In particular a weight $W$ reduces if and only if there is a non-scalar matrix $T$ such that $TW=W{T}^{\ast }$. Also, we prove that reducibility can be studied by looking at the commutant of the monic orthogonal polynomials or by looking at the coefficients of the corresponding three term recursion relation. A matrix weight may not be expressible as direct sum of irreducible weights, but it is always equivalent to a direct sum of irreducible weights. We also establish that the decompositions of two equivalent weights as sums of irreducible weights have the same number of terms and that, up to a permutation, they are equivalent. We consider the algebra of right-hand-side matrix differential operators $\mathcal{D}(W)$ of a reducible weight $W$, giving its general structure. Finally, we make a change of emphasis by considering reducibility of polynomials, instead of reducibility of matrix weights.

References in corpus (4)

• Some examples of orthogonal matrix polynomials satisfying odd order differential equations
• Integrable systems from the classical reflection equation
• Matrix Valued Classical Pairs Related to Compact Gelfand Pairs of Rank One
• Orthogonal vs. Non-Orthogonal Reducibility of Matrix-Valued Measures

Cited by in corpus (5)

• $2\times 2$ Hypergeometric operators with diagonal eigenvalues
• Darboux transformations and the algebra $\mathcal{D}(W)$
• Structure of operator algebras for matrix orthogonal polynomials
• On a new equivalence relation for matrix-valued orthogonal polynomials
• Structural formulas for matrix-valued orthogonal polynomials related to $2\times 2$ hypergeometric operators