Some remarks about the old problem of recombination
  Context: In the seminal works of Zeldovich et al. (1968) and Peebles (1968),
a procedure was outlined to obtain the equation of evolution of the hydrogen
fraction without an explicit use of the radiative transfer equation. This
procedure is based, explicitly or implicitly, on the concept of escape
probability and, using the Sobolev approximation for this problem Sobolev
(1960), has extensively been used since then in developing refined
approximations.
  Aims: To derive in a simple, rigorous and general manner the above mentioned
procedure and to obtain exact analytical expressions for the spectral density
of radiation generated at one photon and two photon recombination transitions.
These expressions are used to estimate the implications of several interesting
effects.
  Methods: Some slight re-elaborations of basic principles of transport theory.
  Results: We have obtained the expressions searched for and used them in
several explicit computations. We have found that the relative change in the
electronic fraction due to the absorption by the two photon line, 1s->2s, of
photons that have escaped from the line 2p-> 1s is 0.67%, that is a result 12%
higher than the previous ones obtained using some approximations (Kholupenko &
Ivanchik 2006; Hirata 2008). The photons generated by the transition 2s->1s and
later absorbed by the same transition (in combination with a photon more
energetic than its original partner) imply a 0.05% maximum variation of the
electronic fraction. This problem has not been treated previously analytically,
although a numerical estimate have been recently carried out in Chluba & Thomas
2011.
