Abstract. Each subset E ⊂ N is realized as the set of essential values of the multiplicity function for the Koopman operator of an ergodic conservative infinite measure preserving transformation.
IntroductionLet T be an ergodic conservative invertible measure preserving transformation of a σ-finite standard measure space (X, B, µ). Consider an associated unitary (Koopman) operator U T in the Hilbert space L 2 (X, µ):In the case of finite measure µ, the operator U T is usually considered only in the orthocomplement to the subspace of constant functions. A general question of the spectral theory of dynamical systems is (0-1) to find out which unitary operators can be realized as Koopman operators.The crux of the problem is related to the multiplicative property In the present paper we consider (0-2) in the class of infinite measure preserving conservative ergodic (i.e. type II ∞ ) transformations. It turns out that (0-2) can be solved completely in this class.