where the 2m exponent ensures that all terms of kr are even. The leading order k 0 r term, M0, represents the degree to which bands are inverted at k0 (i.e. when kr = 0), where M0 < 0 represents bands that are inverted and M0 > 0 represents normally-ordered bands. This can be understood from the eigenfunctions of the Hamiltonian in Eq. ( ). On the other hand, the odd functions A0,y and Ai,x have leading order term k 1 r , meaning that squaring the functions would make the leading order term k 2 r . In other words, we can express