TITLE: What does $\mathbb{R}_{++}$ stand for?
QUESTION [9 upvotes]: What does $\mathbb{R}_{++}$ mean?
I know $\mathbb{R}_+$ means all non-negative real numbers, but I have no clue what $\mathbb{R}_{++}$ means.

REPLY [7 votes]: From my Course: Probabilistic Methods in Finance, we denoted $R+, R++$ like so:
$$R+ =\{x\in R : x\ge0\}$$ 
$$R++ =\{x\in R : x≫0\}.$$