Invariant subspaces generated by a single function in the polydisc
  In this study, we partially answer the question left open in Rudin's book "Function theory in polydiscs" on the structure of invariant subspaces of the Hardy space $H^2(U^n)$ on the polydisc $U^n$. We completely describe all invariant subspaces generated by a single function in the polydisc. Then, using our results, we give the unitary equivalence of this type of invariant subspace and a characterization of outer functions in $H^2(U^n)$. 
Beyaz Basak Koca and Nazim Sadik
math.CV