$$ Tr(A) = \sum_iA_{i,i} $$

Frobenious范数：
$$ ||A||_F = \sum {Tr(AA^T)} $$

$$ \begin{aligned} Tr(A) = Tr(A^T) \ Tr(ABC) = Tr(CBA) = Tr(BCA) \ a = Tr(a) \end{aligned} $$