Interpolation of the Energy Dependence of Partial-Wave Amplitudes

Two methods are described for interpolating the energy dependence of phase shifts. Both methods are based on normed analytic approximation theory. The first method interpolates the D function of the N/D method; the second interpolates the logarithm of this function. As an illustration, the methods are applied to the ¹S₀ and ¹P₁ n-p phase shifts below 400 MeV.