Similar to Fig. 4, but for the first transverse overtone. Only a limited range is plotted for the quantity Λ_i because Λ_i takes up too diverse a range. Panel c overviews how the overtone behaves on the k_y − k_z plane. Trapped 3D modes are forbidden where $\sqrt{k_y^2+k_z^2}\le k_z^{(1/2)}$. When allowed, the overtone is further classified into three regimes based on the angle information (see text for more details).