The Cosmic Linear Anisotropy Solving System (CLASS) II: Approximation
  schemes
  Boltzmann codes are used extensively by several groups for constraining
cosmological parameters with Cosmic Microwave Background and Large Scale
Structure data. This activity is computationally expensive, since a typical
project requires from 10'000 to 100'000 Boltzmann code executions. The newly
released code CLASS (Cosmic Linear Anisotropy Solving System) incorporates
improved approximation schemes leading to a simultaneous gain in speed and
precision. We describe here the three approximations used by CLASS for basic
LambdaCDM models, namely: a baryon-photon tight-coupling approximation which
can be set to first order, second order or to a compromise between the two; an
ultra-relativistic fluid approximation which had not been implemented in public
distributions before; and finally a radiation streaming approximation taking
reionisation into account.
