In the April 2020 issue of Crux Mathematicorum, there is a question in the Olympiad corner, OC479, which involves the following definition of a function,
$$f : \mathbb{Q}_{+}^{*} \rightarrow \mathbb{Q}$$
What is the set that is given as the domain, that is, $\mathbb{Q}_+^*$? I know that $\mathbb{Q}_+$ is the set of of positive rationals, but I've never seen this notation before.
What set does $\mathbb{Q}_{+}^{*}$ denote
notation
Best Answer
More context would be nice, but my guess is that by $\mathbb{Q}^{*}_{+}$ they meant the multiplicative group of positive rationals (i.e. all non-zero positive rationals).
For example, the multiplicative group of integers modulo $n$ is denoted this way, and the "multiplication" cross is sometimes replaced by an asterisk.