Table 1.

Stationary points of the lower dispersion surface for $0<\tilde{\alpha }<\mathrm{\infty }$.

point	$({\tilde{k}}_1,{\tilde{k}}_2)$	${\tilde{\omega }}_1$	type
A	$(\pi ,{\displaystyle \frac{\pi }{\sqrt{3}}})$	$\sqrt{{\displaystyle \frac{6}{2+\sqrt{1+3{\tilde{\alpha }}^2}}}}$	saddle point
D	$({\displaystyle \frac{4\pi }{3}},0)$	$\sqrt{{\displaystyle \frac{9}{2(1+\tilde{\alpha })}}}$	maximum