\[X=\left(
\begin{matrix}
x_{11} & x_{12} & \cdots & x_{1d}\\
x_{21} & x_{22} & \cdots & x_{2d}\\
\vdots & \vdots & \ddots & \vdots\\
x_{m1} & x_{m2} & \cdots & x_{md}\\
\end{matrix}
\right)
=\left(
\begin{matrix}
x_1^T \\
x_2^T \\
\vdots\\
x_m^T \\
\end{matrix}
\right)
\]
参考：
https://www.cnblogs.com/cmt/p/3279312.html
https://www.cnblogs.com/Sinte-Beuve/p/6160905.html
https://www.cnblogs.com/houkai/p/3399646.html
