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An analytical multigroup method of nuclear reactor criticality calculation, based on characteristic function representation of neutron group fluxes, is proposed. The technique, applied to a bare or reflected core with N - group treatment of the flux, leads to one or two systems of only N homogenious linear equations in N unknowns. Few - group calculartion can be easily and rapidly carried out on a microcomputer, so that the technique is very suitable for parametric studies. A three - group test calculation has been carried out for the Teheran Research Reactor, the results of which agrees well with the existing exper - imental data.