ИСТИНА

A description of the algebra of outer derivations of a group algebra of a finitely presented discrete group is given in terms of the Cayley complex of the groupoid of the adjoint action of the group. This task is a smooth version of Johnson’s prob- lem concerning the derivations of a group algebra. It is shown that the algebra of outer derivations is isomorphic to the group of the one-dimensional cohomology with compact supports of the Cayley complex over the field of complex numbers. On the other hand the group of outer derivation is isomorphic to the one di- mensional Hochschild cohomology of the group algebra. Thus the whole Hochschild cohomology group can be described in terms of the cohomology of the classifying space of the groupoid of the adjoint action of the group under the suitable assumption of the finiteness of the supports of cohomology groups. The report presents the results partly obtained jointly with A. Arutyunov, and also with the help of A.I. Shtern.