Wielandt wrote a paper titled "Remarks on diagonable matrices".


According to Mathematische Werke - Mathematical Works : Linear Algebra and Analysis
by Helmut Wielandt, Hans Schneider, Bertram Huppert (Editor) page 260 this paper from Wielandt remained unpublished (at least from the 1950s to the 1980s).


Does anyone have a copy of it or an idea of the proof on non defective pencils?


The main theorem states that for $A,B \in \mathcal{M}\_n(\mathbb C)$, if in the pencil $\lambda A+ \mu B$ all matrices are diagnosable ($\forall \lambda. \mu \in \mathbb{C}$), then $AB=BA$.


Motzkin and Taussky proved that result (MR0086781 (19,242c)), using algebraic geometry, Kato proved it differently (MR1335452 (96a:47025)), using theory of complex functions in one variable.
Wielandt seemed to have given another proof, hence my request.


Thanks



Unpublished work of Wielandt
Now that the server is back up, I am posting this as a real answer.


With some work, you might be able to find the proof in Wielandt's notebooks, which were TeXxed and put online [here](http://www3.math.tu-berlin.de/numerik/Wielandt/index_en.html).


The TeX source files are also published, so you can download them and use an automated search tool. Nevertheless, there's lots of material there, so it is not an easy task if you have no idea which period to look at.


