text: enum boundary_condition { freefree, clampedclamped, clampedfree };
  double frequency_equation(double a,
                            double b,
                            double gamma2,
                            bool is_sub_critical,
                            boundary_condition bc,
                            double* F_a,
                            double* F_b);
  
  
  double euler_wave_number( unsigned id, boundary_condition bc );
  
  