$R$ = $\left\{ {\left(\dfrac{a}{3^{n}}\right)|\; n \in N_{0}, a ∈ \mathbb{Z} }\right\}$. Is $R$ a PID? What are the invertible elements of $R$? What are the irreducible elements of $R$?
My thoughts: I think this is a PID, but I don't know how to begin proving this. I think the invertible elements are the elements such that $a=3^{m}$ for some $m ∈ N_{0}$?