Strongly irreducible operators, $K_0$-groups, and Jones indices in the hyperfinite type $\mathrm{II}_1$ factor

Abstract

In this paper, we study the strong irreducibility of a class of operators associated with irrational rotation algebras in the hyperfinite type $\mathrm{II}_1$ factor $\mathcal R$. By developing weighted shift models for their relative commutants, we obtain $K_0$ calculations and splitting criteria for direct sums. In particular, we construct strongly irreducible operators in $\mathcal R$ whose relative commutants have $K_0$-groups isomorphic to $\mathbb Z$ and $\mathbb Z^2$. We realize every Jones index of an irreducible subfactor of $\mathcal R$ as $[\mathcal R:W^*(T)]$ for some strongly irreducible operator $T$ relative to $\mathcal R$. Furthermore, we construct strong generators of $\mathcal R$ and strongly irreducible operators in every type $\mathrm{II}_1$ factor $\mathcal M$ with separable predual.