Cosmic abundances of SIMP dark matter

Thermal production of light dark matter with sub-GeV scale mass can be attributed to 3 -> 2 self-annihilation processes. We consider the thermal average for annihilation cross sections of dark matter at 3 -> 2 and general higher-order interactions. A correct thermal average for initial dark matter particles is important, in particular, for annihilation cross sections with overall velocity dependence and/or resonance poles. We apply our general results to benchmark models for SIMP dark matter and discuss the effects of the resonance pole in determining the relic density.