Novel approach to root functions of matrix polynomials with applications in differential equations and meromorphic matrix functions

TL;DR

This paper introduces a new method for deriving root functions of matrix polynomials and applies it to solve linear differential systems and represent matrix Nevanlinna functions within Pontryagin spaces.

Contribution

It presents a simple, elementary transformation-based approach to root functions of matrix polynomials and develops an algorithm for Krein-Langer representations of matrix Nevanlinna functions.

Findings

New method for root functions of matrix polynomials

Elementary transformations simplify solving differential systems

Algorithm for Krein-Langer representation of Nevanlinna functions

Abstract

In the first part of the paper, we address an invertible matrix polynomial $L(z)$ and its inverse $\hat{L}(z) := -L(z)^{-1}$. We present a method for obtaining a canonical set of root functions and Jordan chains of $L(z)$ through elementary transformations of the matrix $L(z)$ alone. This method provides a new and simple approach to deriving a general solution of the system of ordinary linear differential equations $L\left(\frac{d}{dt}\right)u=0$ using only elementary transformations of the corresponding matrix polynomial $L(z)$. In the second part of the paper, given a matrix generalized Nevanlinna function $Q\in N_{\kappa }^{n \times n}$ and a canonical set of root functions of $\hat{Q}(z) := -Q(z)^{-1}$, we provide an algorithm to determine a specific Pontryagin space $(\mathcal{K}, [.,.])$, a specific self-adjoint operator $A:\mathcal{K}\rightarrow \mathcal{K}$ and an operator $\Gamma: \mathbb{C}^{n}\rightarrow \mathcal{K}$ that represent the function $Q$ in a Krein-Langer type representation. We demonstrate the main results through examples of linear systems of ODEs.