Assume, we know that $A,B\in M_{n}(\mathbb R)$. Can we say that:
$$\mathrm{rank}(AB-BA)\leq [n/2]$$
Thanks for any hint.
Assume, we know that $A,B\in M_{n}(\mathbb R)$. Can we say that:
$$\mathrm{rank}(AB-BA)\leq [n/2]$$
Thanks for any hint.
For any $n\ge2$, consider the following $n\times n$ permutation matrix and the diagonal matrix $$ A=\begin{pmatrix}&&&1\\1\\&\ddots\\&&1&\end{pmatrix}, \ B=\begin{pmatrix}1\\&2\\&&\ddots\\&&&n\end{pmatrix}. $$ Then $$ AB-BA=\begin{pmatrix}&&&n-1\\-1\\&\ddots\\&&-1&\end{pmatrix} $$ has full rank with determinant $n-1$.
Hint: try almost any $2 \times 2$ example.