could any one tell me which of the following is/are compact subset?
$S=\{A\in M_n(\mathbb{C}): \rho (A)\le 1\}$
$S=\{A\in M_n(\mathbb{C}): A=A^*,\rho (A)\le 1\}$
$S=\{A\in M_n(\mathbb{C}): AA^*=A^*A=I\}$
I pointed out that only $2$ is compact by the map $Ap:\mapsto A-A^*$ gives $S=p^{-1}(O)$ hence closed and clearly this set is bounded.
$3$ is just $O_n(\mathbb{C})$ I can prove that it is not bounded so not compact