In the vicinity of high symmetry points k 0 = k 0 + G in a D -dimensional momentum space, where G is a reciprocal lattice vector, the curvature function typically displays a Lorentzian shape F ( k 0 + δ δ k , M ) = F ( k 0 , M ) 1 + ξ ξ 2 δ δ k 2 , where 1 / ξ ξ defines the width of the multidimensional peak. Approaching the critical point M → → M c the peak gradually diverges, flipping sign across the transition: lim M → → M c + F ( k 0 , M ) = − − lim M → → M c − − F ( k 0 , M ) = ± ± ∞ ∞ , lim M → → M c ξ ξ = ∞ ∞ , This behavior is displayed in the video on the side for the case D = 1 .