code: def trig(self, name):
          
          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]
docs: 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