“…Actually, the solution of the latter below threshold presents a quite simple task thanks to the Sturmian representation of the Coulomb Green's function. The Sturmians, or Sturm-Liouville functions [13,14,15] S ℓν (b, r), where b is a free parameter (Re b > 0) are enumerated by the nodal quantum number ν = 0, 1, 2, ... and result as solution of the differential equation…”