code: def k(self, e, n, τ, σ):
          
          d = self._dichalcogenide
          at, Δ, λ = d.at, d.Δ, d.λ
          sqrt = numpy.sqrt
  
          x = 2 * e * Δ**-1
          y = (λ * Δ**-1) * (1 - x)
  
          return (at**-1) * (Δ / 2) * sqrt(x**2 + 2 * τ * σ * y - 1)
docs: Inverse energy-momentum relation.
  
          :param e: Energy :math:`E`.
          :type e: float
  
          :param n: Band index :math:`n = ±1`.
          :type n: int
  
          :param τ: Valley index :math:`τ = ±1`.
          :type τ: int
  
          :param σ: Spin index :math:`σ = ±1`.
          :type σ: int
  
          :return: :math:`k(E)`
          :rtype: float