Muthuramalingam, Pl. “Spectral Properties of Vaguely Elliptic Pseudo Differential Operators With Momentum Dependent Long Range Potentials Using Time Dependent Scattering Theory - II”. Mathematica Scandinavica, vol. 58, June 1986, pp. 255-74, https://doi.org/10.7146/math.scand.a-12145.