\pmatrix - Used to create matrix enclosed in parentheses.
{ \pmatrix { <math> & <math> ... \cr <repeat as needed> }}
\pmatrix command is used to create matrix enclosed in parentheses; alignment occurs at the ampersands; a double-backslash can be used in place of the \cr; the final \\ or \cr is optional
A = \pmatrix{ a_{11} & a_{12} & \ldots & a_{1n} \cr a_{21} & a_{22} & \ldots & a_{2n} \cr \vdots & \vdots & \ddots & \vdots \cr a_{m1} & a_{m2} & \ldots & a_{mn} \cr }