How to proof the following equation: $$Diag(U^*PU)_{qq}=\sum_{l,m=0}U_{lq}P_{lm}U_{mq}$$
I could work out the $$(U^*PU)_{qq}$$ part, but could not understant the diag means? Thank you!
Diag of Matrix Linear Algebra
0
$\begingroup$
linear-algebra
-
0Diag stands for diagonal. These are the diagonal entries of $U^*PU$. – 2017-01-24
-
0Thanks, but why it's the diagonal entries of $$U^*PU$$ – 2017-01-24
-
0The $qq$ matrix elements are on the diagonal. – 2017-01-24
-
0Ohhh, I see. Thank you! – 2017-01-25