\input {amstex}  y=\begin{pmatrix}y_1\\\vdots\\y_n\end{pmatrix}=A \begin{pmatrix}x_1\\\vdots\\x_n\end{pmatrix}=\begin{pmatrix}\sum\limits_{k=1}^{d}a_{1k}x_k\\\vdots\\\sum\limits_{k=1}^{d}a_{dk}x_k\end{pmatrix},