Let T, T ′ be weak contractions (in the sense of Sz.-Nagy and Foiaş), m, m the minimal functions of their C0 parts and let d be the greatest common inner divisor of m, m . It is proved that the space I(T, T ) of all operators intertwining T, T ′ is reflexive if and only if the model operator S(d) is reflexive. Here S(d) means the compression of the unilateral shift onto the space H⊖dH. In parti...

We use the theory of vertex operator algebras and intertwining operators to obtain systems of q-difference equations satisfied by the graded dimensions of the principal subspaces of certain level k standard modules for ŝl(3). As a consequence we establish new formulas for the graded dimensions of the principal subspaces corresponding to the highest weights iΛ1 + (k − i)Λ2, where 1 ≤ i ≤ k and Λ...

In particular, the fact that ideas regarding Jacquet modules and the double coset method (regarding intertwining operators) can be developed in this generality is useful in applications. Since we are interested in such things as Jacquet modules, we more generally consider the notions of isotype and co-isotype. Spherical representations and admissibility, at the end, may seem mysterious at this ...

This is the sixth part in a series of papers in which we introduce and develop a natural, general tensor category theory for suitable module categories for a vertex (operator) algebra. In this paper (Part VI), we construct the appropriate natural associativity isomorphisms between triple tensor product functors. In fact, we establish a “logarithmic operator product expansion” theorem for logari...