Potter, Wielandt, and Drazin on the matrix equation $AB=\omega BA$, with   some new answers to old questions
  In this partly historical and partly research oriented note, we display a page of an unpublished mathematical diary of Helmut Wielandt's for 1951. There he gives a new proof of a theorem due to H. S. A. Potter on the matrix equation $AB = \omega BA$, which is related to the $q$-binomial theorem, and asks some further questions, which we answer. We also describe results by M. P. Drazin and others on this equation. 
Olga Holtz, Volker Mehrmann, Hans Schneider