\input amstex [f_1,f_2,\dots,f_m] = [e_1,e_2,\dots,e_n] 
\underbrace{\begin{bmatrix}
\alpha_{11} &  \alpha_{12}  & \cdots & \alpha_{1m}\\
\vdots & \vdots & \ddots & \vdots\\
\alpha_{n1}  &   \alpha_{n2} &\cdots & \alpha_{nm}
\end{bmatrix}}_A