def trig(self, name):
        """Trigonometric functions of the energy.

        * ``sin θ``       - :math:`\\sin θ(E)`
        * ``cos θ``       - :math:`\\cos θ(E)`
        * ``sin^2 θ/2``   - :math:`\\sin^2 \\frac{θ(E)}{2}`
        * ``cos^2 θ/2``   - :math:`\\cos^2 \\frac{θ(E)}{2}`

        :param name: One of the above function names.
        :type name: str

        :return: Corresponding function of the energy :math:`E`.
        :rtype: function
        """
        fn = {}
        λ, Δ = self._dichalcogenide.λ, self._dichalcogenide.Δ
        sqrt = numpy.sqrt
        x = lambda e: (2 * e - λ)**(-1)

        fn['sin θ'] = lambda e: sqrt((2 * e - Δ) * (2 * e + Δ - 2 * λ)) * x(e)
        fn['cos θ'] = lambda e: (Δ - λ) * x(e)
        fn['sin^2 θ/2'] = lambda e: (e - Δ / 2) * x(e)
        fn['cos^2 θ/2'] = lambda e: (e + Δ / 2 - λ) * x(e)

        return fn[name]
Trigonometric functions of the energy.

        * ``sin θ``       - :math:`\\sin θ(E)`
        * ``cos θ``       - :math:`\\cos θ(E)`
        * ``sin^2 θ/2``   - :math:`\\sin^2 \\frac{θ(E)}{2}`
        * ``cos^2 θ/2``   - :math:`\\cos^2 \\frac{θ(E)}{2}`

        :param name: One of the above function names.
        :type name: str

        :return: Corresponding function of the energy :math:`E`.
        :rtype: function