Abstract. We give a classification of unitary representations of certain Polish, not necessarily locally compact, groups: the groups of all measurable functions with values in the circle and the groups of all continuous functions on compact, second countable, zero-dimensional spaces with values in the circle. In the proofs of our classification results, certain structure theorems and factorization theorems for linear operators are used.
IntroductionWe study unitary representations of the groups of measurable and continuous functions with values in the circle. A description of unitary representations of such groups is of interest especially in view of recent considerable activity around topological and measurable dynamics of these groups; see the remarks below. The reader may consult [14] for background information on dynamics of Polish non-locally compact groups. (Recall that a topological group is Polish if its topology is metrizable by a complete separable metric.)For a Borel probability measure µ on a standard Borel space and a topological group H, let L 0 (µ, H) be the topological group of all µ-equivalence classes of µ-measurable functions with values in H. The multiplication on L 0 (µ, H) is implemented pointwise and the topology is the convergence in measure topology. Groups of this form were perhaps first systematically considered in [3] to provide an embedding of each topological group into a connected group. Recently, the more particular Polish groups L 0 (µ, T) generated substantial interest in the context of extreme amenability