The Jordan forms of AB and BA
Ross A. Lippert, Gilbert Strang · Electronic Journal of Linear Algebra · 2009
The relationship between the Jordan forms of the matrix products AB and BA for some given A and B was first described by Harley Flanders in 1951. Their non-zero eigenvalues and non-singular Jordan structures are the same, but their singular Jordan block sizes can differ by 1. We present an elementary proof that owes its simplicity to a novel use of the Weyr characteristic.