The DL(P) vector space of pencils for singular matrix polynomials

Abstract

Given a possibly singular matrix polynomial P(z), we study how the eigenvalues, eigenvectors, root polynomials, minimal indices, and minimal bases of the pencils in the vector space DL(P) introduced in Mackey, Mackey, Mehl, and Mehrmann [SIAM J. Matrix Anal. Appl. 28(4), 971-1004, 2006] are related to those of P(z). If P(z) is regular, it is known that those pencils in DL(P) satisfying the generic assumptions in the so-called eigenvalue exclusion theorem are strong linearizations for P(z). This property and the block-symmetric structure of the pencils in DL(P) have made these linearizations among the most influential for the theoretical and numerical treatment of structured regular matrix polynomials. However, it is also known that, if P(z) is singular, then none of the pencils in DL(P) is a linearization for P(z). In this paper, we prove that despite this fact a generalization of the eigenvalue exclusion theorem holds for any singular matrix polynomial P(z) and that such a generalization allows us to recover all the relevant quantities of P(z) from any pencil in DL(P) satisfying the eigenvalue exclusion hypothesis. Our proof of this general theorem relies heavily on the representation of the pencils in DL(P) via Bézoutians by Nakatsukasa, Noferini and Townsend [SIAM J. Matrix Anal. Appl. 38(1), 181-209, 2015].