How to correctly represent a nested sum Suppose I have a matrix:
$$
        A = \begin{pmatrix}
        1 & 4 & 7 \\
        2 & 5 & 8 \\
        3 & 6 & 9 \\
        \end{pmatrix}
$$
For which i want to sum the elements.
My first inclination is to write:
$$ \sum_j \sum_i A_{i,j} $$
But I am unsure whether this is normally read:
$$ \sum_j \left( \sum_i A_{i,j} \right) $$
Or:
$$ \left(\sum_j \sum_i \right) A_{i,j}  $$
Where i intend the latter.

The latter notation is meaningless. The first one is correct. However, without the parentheses, it means the same thing. The last one is the big no-no. In other words: $$ \sum_j \sum_i A_{i,j} \equiv \sum_j \left( \sum_i A_{i,j} \right), $$ but you should not write $$ \left( \sum_j \sum_i \right) A_{i,j}. $$
