Notations
- 矩阵拉直或矩阵列化
设$\boldsymbol{A}=(a_{ij})\in \mathbb{C}^{m\times n}$,记$\boldsymbol{a}_i=(a_{1i},\cdots,a_{mi})^T\ (i=1,\cdots, n)$,令
\begin{align}
\text{vec}(\boldsymbol{A})=
\left[
\begin{matrix}
\boldsymbol{a}_1\\
\vdots\\
\boldsymbol{a}_n
\end{matrix}
\right]
\end{align}