Stratification theory of matrix pairs under equivalence and contragredient equivalence

Résumé

We develop the theory of perturbations of matrix pencils basing on their miniversal deformations. Several applications of this theory are given. All possible Kronecker pencils that are canonical forms of pencils in an arbitrary small neighbourhood of a given pencil were described by A. Pokrzywa (Linear Algebra Appl., 1986). His proof is very abstract and unconstructive. Even more abstract proof of Pokrzywa’s theorem was given by K. Bongartz (Advances in Mathematics, 1996); he uses the representation theory of finite dimensional algebras. The main purpose of this thesis is to give a direct, constructive, and rather elementary proof of Pokrzywa’s theorem. We first show that it is sufficient to prove Pokrzywa’s theorem only for pencils that are direct sums of at most two indecomposable Kronecker pencils. Then we prove Pokrzywa’s theorem for such pencils. The latter problem is very simplified due to the following observation: it is sufficient to find Kronecker's canonical forms of only those pencils that are obtained by miniversal perturbations of a given pencil. We use miniversal deformations of matrix pencils that are given by M. I. García-Planas and V. V. Sergeichuk (Linear Algebra Appl., 1999) because their deformations have many zero entries unlike the miniversal deformations given by A. Edelman, E. Elmroth, and B. Kagstrom (SIAM J. Matrix Anal. Appl., 1997). Thus, we give not only all possible Kronecker’s canonical forms, but also the corresponding deformations of a given pencil, which is important for applications of this theory. P. Van Dooren (Linear Algebra Appl., 1979) constructed an algorithm for computing all singular summands of Kronecker’s canonical form of a matrix pencil. His algorithm uses only unitary transformations, which improves its numerical stability. We extend Van Dooren’s algorithm both to square complex matrices under consimilarity transformations and to pairs of complex matrices under mixed equivalence. We describe all pairs (A, B) of m-by-n and n-by-m complex matrices for which the product CD is a versal deformation of AB, in which (C, D) is the miniversal deformation of (A, B) under contragredient equivalence given by M. I. García-Planas and V. V. Sergeichuk (Linear Algebra Appl., 1999). We find all canonical matrix pairs (A, B) under contragredient equivalence, for which the first order induced perturbations are nonzero for all nonzero miniversal deformations of (A, B). This problem arises in the theory of differential matrix equations dx= ABx. A complex matrix pencil is called structurally stable if there exists its neighbourhood in which all pencils are strictly equivalent to it. We describe all complex matrix pencils that are structurally stable. We show that there are no pairs of complex matrices that are structurally stable with respect to contragredient equivalence.