\[X=\left( \begin{matrix} x_{11} & x_{12} & \cdots & x_{1d}\\ x_{21} & x_{22} & \cdots & x_{2d}\\ \vdots & \vdots & \ddots & \vdots\\ x_{m1} & x_{m2} & \cdots & x_{md}\\ \end{matrix} \right) =\left( \begin{matrix} x_1^T \\ x_2^T \\ \vdots\\ x_m^T \\ \end{matrix} \right) \]
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