code: def Δk(self):
          
          Δc, Δv = self.Δc, self.Δv
          cos = self._energy.trig('cos θ')
          return lambda e: (1 / 2) * ((Δc + Δv) + (Δc - Δv) * cos(e))
docs: The superconducting parameter :math:`Δ_\\mathbf{k}`
          as a function of the energy :math:`E`.
  
          :return: :math:`Δ(E)`
          :rtype: func