This is a matrix where a is placed on every cell starting with .

Example

0 & 0 & 0& 0& 0& 0& \dots\\ 1 & 0 & 0& 0& 0& 0& \dots\\ 0 & 1 & 0& 0& 0& 0& \dots\\ 0 & 0 & 1& 0& 0& 0& \dots\\ 0 & 0 & 0& 1& 0& 0& \dots\\ \vdots \end{array}\right]$$