# Flanders’ theorem for many matrices under commutativity assumptions – Fernando De Terán (Universidad Carlos III de Madrid)

#### Abstract

Given two matrices $A \in C^{m \times n}$ and $B \in C^{n \times m}$ , it is known [1] that theJordan canonical form of AB and BA can only differ in the sizes of theJordan blocks associated with the eigenvalue zero, and the difference in thesize of any two corresponding blocks is, at most, 1. Moreover, this change is exhaustive, in the sense that given two ordered lists of numbers such thatthe corresponding elements differ at most by 1, then there are two matricesA, B such that the fist list is the list of sizes of Jordan blocks associated with0 in AB and the second one is the list of sizes of Jordan blocks associatedwith 0 in BA. The motivation of this work is the question: What happens with productsof more than two matrices? First, we will see that, if we do not imposeany restriction, the products ABC and CBA may have completely differenteigenvalues. Therefore, some conditions must be imposed to the factors inorder to be able to relate the Jordan canonical forms of any two products. By imposing some natural commutativity assumptions, we analyze the relationship between the Jordan canonical form of products, in different orders, of k square matrices $A_1, …, A_k$. In particular, we study permuted products of $A_1, …, A_k$ under the assumption that the graph of non-commutativity relations is a forest. Under this condition, we show that the Jordan structure of all nonzero eigenvalues is the same for all permuted products. For the eigenvalue zero, we obtain an upper bound on the difference between the sizes of Jordan blocks for any two permuted products, and we show that this bound is attainable. For k = 3 we show that, moreover, the bound is exhaustive. This is joint work with Ross A. Lippert, Yuji Nakatsukasa, and Vanni Noferini.

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