Dependence of ϖ_f and ϖ_o on q. The lines show the approximated dependence valid for small q and determined by Eqs. (59) and (67) for the fundamental mode, and by Eqs. (68) and (70) for the first overtone. The solid, dotted and dashed lines correspond to for ζ = 3, 6, and 10. The lower three lines correspond to the fundamental mode, and the upper three lines to the overtones. The circles show the numerical results.