Top panel, solid lines: roots of the set of Eqs. (34)–(35) along R_nl(q) plotted on the top panel in Fig. 6 by the solid line; dashed lines: Eqs. (36) and (37) along R_nl(q) plotted on the top panel in Fig. 6 by the dashed line. Bottom panel, solid line: obtained from the condition (31) with k_x = k_{x max} and k̃_{x max} along the corresponding solid lines on the top panel of this figure; dashed line: Eq. (40) along R_nl(q) plotted on the top panel in Fig. 6 by the dashed line.