What does the notation $A_a^n$ mean?

Given a set of matrices

$$M = \left\{\begin{bmatrix}1&a\\0&3\end{bmatrix} \mid a \in \mathbb R\right\},$$

what does the notation $A_a^n, n \in \mathbb N$ mean?

• Stock question: in what book/paper did you see these? – J. M. is a poor mathematician Apr 19 '11 at 16:27
• A Romanian high-school textbook. – Paul Manta Apr 19 '11 at 16:29
• Your markup was mostly right, but you tried to use curly braces without escaping them. Curly braces are grouping delimiters in $\TeX$, so you need to escape them with a backslash if you mean them literally. Also, it's better to use \mid for the vertical "such that" bar instead of just |, since it's treated as an operator and gives you the appropriate spacing. – joriki Apr 19 '11 at 16:36
• Addendum: bmatrix gives bracketed matrices, which is what you seem to want. – J. M. is a poor mathematician Apr 19 '11 at 16:37

$A_a$ seems to be a shorthand for a typical matrix in $M$. Then $A_a^n$ is the $n$-th power of $A_a$.