[en] The stability of a nuclear reactor without and with feedback by a control system has been investigated. The main objective was to demonstrate whether self-excited oszillations (i.e. limit cycles) can exist or not. Measurements indicated the existence of such limit cycles. which always may occur under the influence of a non-linear element. As they were expected to occur in the low frequency region, for the transfer functions of the reactor and of the non-linear controller simplified approximations were derived restricted to low frequencies. Applying these approximations, for a description in the parameter plane a distinct insight into the stability of the reactor operated without and with an automatic control system was obtained. By means of this method, developed by Siljak, the influence of two free system parameters on the stability could be simply analyzed. For the reactor without control system the influence of the reactivity coefficients for the fuel and the moderator have been investigated in this way. For the reactor with controller this has been done with respect to the fuel reactivity coefficient and to the controller adjustment. In the latter case the occurence of stable limit cycles could be demonstrated unequivocally. The partical significance of such investigations for surveillance purposes and for neutron noise measurements is discussed. (orig.)