# trace

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

Frobenious范数：

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

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