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

def k(self, e, n, τ, σ):
        """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
        """
        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)