Hello everybody
There is a nice classical result in linear algebra: if A, B $A, B$ are two matrices in M_n(k), $M_n(k),$ where k $k$ is a filedfield, and B $B$ commutes with every element of M_n(k) $M_n(k)$ which commutes with A, $A$, then $B = f(A) f(A)$ for some polynomial f(x) $f(x)$ in k[x].$k[x].$
I was wondering if anybody knows any (important) theorem which is proved using this result. Thank you.

