where ζ2 and α2 together represent band-to-band interactions, and f2 represents the gap opening due to SOC, where f2 is positive (see Section 5.2). M0 represents the degree to which bands are inverted: M0 > 0 for normal insulators, and M0 < 0 for topological insulators with inverted bands. M0 is more negative if the bands are more inverted. Suppose band inversion occurs at some k-point k0 in the Brillouin zone. We can identify two rules pertaining to whether band inversion leads to warping of the bulk bands at k0: (i) the bands must be sufficiently inverted, and (ii) band-to-band coupling must be strong and allowed by symmetry. The two rules are illustrated in Figure .