Notation of matrices, conditional elements and rows/columns I have a question on how to notate the certain matrices.
Lets say I have a $N\times M$ matrix called $A$.


*

*What is the notation of the $i$th row of $A$?

*What is the notation of the sum of all elements in $A$ such that each element is larger than $c$?

*What is the notation of the number of all elements in $A$ such that each element is larger than $c$?


Well, I don't think that there is a widely spread commonly accepted notation for these...
You could do it like that:
Let $A=[a_{i,j}]\in \Bbb R^{N\times M}$ and denote by $a_1,\ldots,a_N\in\Bbb R^M$ the rows of $A$.  (this is basically how Horn and Johnson do it in their book Matrix Analysis).  
For the rest, let $$S:=\{a_{i,j}\mid 1\leq i \leq N, 1 \leq j \leq M \text{ and } a_{i,j}\geq c\}.$$
and consider 
$$ \gamma = \sum_{x\in S}x \qquad \text{and}\qquad \Gamma = |S|$$
where $|S|$ denotes the cardinality of $S$.
