Warping due to band inversion can be understood from the curvatures of the band edges. We can build a general model of the valence and conduction band edges expanded around the kpoint where band inversion occurs, noting that topological insulators preserve crystal inversion symmetry (i.e. centrosymmetric structures) and time-reversal symmetry (e.g. non-magnetic materials). We arrive at a condition which must be satisfied for band warping to occur in all topological insulators (see Sections 5.1 and 5.2 for more details and the origin of the subscripts):