$\mathbb{R}_*$ Notation

Question

What does the notation $\mathbb{R}_*$ denote? I am seeing it used for showing domain of matrices, $M\in \mathbb{R}_*^{a \times b}$, which is different from $N \in \mathbb{R}^{a \times b}$. But I do not get what it means.

Answer 1

It seems likely to me that this means one of

1. $a\times b$ matrices with real values that are not all $0$, or
2. $a\times b$ matrices with nonzero real values, or
3. $a\times b$ matrices with values that are all either real or $\pm \infty$.

Hopefully one of these seems more likely than the others to you.