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Important classes of locally compact groups(such as groups with Kazhdans property and groups with the Haagerup property)are characterized by their actions by affine isometries on Hilbert spaces.Such an action is described by a space of 1-cohomology with coefficients in a unitary group representation.Using harmonic cocycles,the space of(reduced)1-cohomology can be turned into a Hilbert module over the von Neumann algebra given by the commutant of the representation.We will be interested in the associated von Neumann dimension in the case of a factor representation and its relation to a notion of irreducibility for actions by affine isometries.