Annihilation Cross Sections for Relic Densities in the Low Velocity   Limit
  A simple method to calculate the total annihilation cross section of massive cold relics is presented. Exact expressions for $\arel$, $\brel$, and $\crel$ of $\sigma \vrel=\arel+\brel \vrel^2+\crel \vrel^4$ are obtained from simple derivatives of the squared annihilation amplitude. Three examples are given. In one example, the computed annihilation cross section of massive Dirac neutrinos confirms recent correct calculations. 
James D. Wells