Modelling CH$_3$OH masers: Sobolev approximation and accelerated lambda
  iteration method
  A simple one-dimensional model of CH$_3$OH maser is considered. Two
techniques are used for the calculation of molecule level populations: the
accelerated lambda iteration (ALI) method and the large velocity gradient
(LVG), or Sobolev, approximation. The LVG approximation gives accurate results
provided that the characteristic dimensions of the medium are larger than 5-10
lengths of the resonance region. We presume that this condition can be
satisfied only for the largest observed maser spot distributions. Factors
controlling the pumping of class I and class II methanol masers are considered.
