  Radiative transfer (RT) problems in which the source function includes a
scattering-like integral are typical two-points boundary problems. Their
solution via differential equations implies to make hypotheses on the solution
itself, namely the specific intensity I(tau;n) of the radiation field. On the
contrary, integral methods require to make hypotheses on the source function
S(tau). It looks of course more reasonable to make hypotheses on the latter
because one can expect that the run of S(tau) with depth be smoother than that
of I(tau;n).
  In previous works we assumed a piece-wise parabolic approximation for the
source function, which warrants the continuity of S(tau) and its first
derivative at each depth point. Here we impose the continuity of the second
derivative S"(tau). In other words, we adopt a cubic spline representation to
the source function, which highly stabilize the numerical processes.
