code: def __landau_stuart(self, amplitude, index):
          
          
          return (self.__properties[index] - numpy.absolute(amplitude) ** 2) * amplitude;
docs: !
          @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.