@article{Lee1964DegenerateSA,
title={Degenerate Systems and Mass Singularities},
author={T. Lee and M. Nauenberg},
journal={Physical Review},
year={1964},
volume={133},
pages={25-38}
}

For a system with degenerate energies, the power series expansions of the s-matrix elements may become singular. An elementary quantum mechanical theorem is proved which shows that under certain general conditions such singularities do not appear in the power series expansions of the transition probabilities, provided these are averaged over an appropriate ensemble of degenerate states. Application of the theorem leads to cancellations of mass singularities and infra-red divergences in quantumâ€¦Â Expand