Consider a matrix $X \in \mathbb{R}_{n \times m}$.
One compact yet unclear notation to select a row or column from this matrix is:
$$x \in X$$
How do you clearly select a row or column from a matrix?
I know $X = (x_{ij})$ is a standard notation to select elements. Though I haven't seen this used, $x_i \in X_{ij}$ for rows and $x_j \in X_{ij}$ for columns might make sense. This is motivated by a similar notation I have seen, namely $\sum\limits_{i}X_{ij}$ for row sum or $\sum\limits_{j}X_{ij}$ for column sum.
Best Answer
You could for example use elementary vectors. Let $e_j$ be an $m \times 1$ vector of zeros with a one in $j$-th position. Then
$$ x_j = X e_j $$
would be the $j$-th column of $X$.