I am wondering what is a proper way to refer to a single or multipe elements taken from a bigger matrix. Lets say there is the matrix $A$. If I want to pick a single element, I would write something like this:
$b = A_{12}$, $b = (A)_{12}$,
and for multiple elements, this:
$b = A_{1:2,3}$, $b = A_{[1:2,3]}$
Is there some better or more appropriate notation? Do you have ideas how to write this in a better way?