I am reading David Fraleigh, A first course in Abstract Algebra, and they mention in one of the questions about
$<R^*,•>$
my questions is, what does $R^*$ denote?
Edit: some context
The question is to show $<U,•>$ is not isomorphic to either $<R,+>$ or $<R^*,•>$
where they are all groups. I am not asking for how to do it, I'm just not sure what the notation means
If $R$ is a ring then $R^*$ or $R^\times$ is its set (group) of units. $R^* = \{ x \in R : xy = 1 \text{ for some } y\}$. When it's written as $\langle R^*, \cdot \rangle$ it is reminding you that the group operation is the multiplication operator of $R$ not the addition operator. If one is being extra formal, we should also include $1$ in that list: $\langle R^*, 1, \cdot \rangle$.