def __landau_stuart(self, amplitude, index):
return (self.__properties[index] - numpy.absolute(amplitude) ** <NUM_LIT:2>) * amplitude<EOL>
!
        @brief Calculate Landau-Stuart state.

        @param[in] amplitude (double): Current amplitude of oscillator.
        @param[in] index (uint): Oscillator index whose state is calculated. 

        @return (double) Landau-Stuart state.