Property of 10x10 matrix with non negative eigenvalues
Let $A$ be a $10\times 10$ matrix with complex entries such that all its eigenvalues are non-negative real numbers, and at least one eigenvalue is positive. Which of the following statements is always false ?
A. There exists a matrix $B$ such that $AB - BA = B$.
B. There exists a matrix $B$ such that $AB - BA = A$.
C. There exists a matrix $B$ such that $AB + BA = A$.
D. There exists a matrix $B$ such that $AB + BA = B$.
I just need a hint to start thinking.
Solution 1:
B is always false. The trace of $AB-BA$ is always zero, but the trace of $A$, which is the sum of its eigenvalues, is positive.
Solution 2:
Edit: Since I read the question wrong, I will try a different way to at least show which answer you should suspect to be false. Namely, I will show that options A, C, and D are sometimes possible.
For A, let $B$ be the zero matrix. This shows that A holds for any choice of matrix $A$.
For C, Let $B$ be the diagonal matrix with diagonal entries equal to $\frac{1}{2}$. This shows C is true for any matrix $A$.
For D, let $A$ be the matrix $B$ above. This choice of $A$ meets the hypothesis, and for this choice of $A$, D holds for any other matrix $B$, so D is at least sometimes true.