@article{Roy_1971,
title={Exact integral equation for pion-pion scattering involving only physical region partial waves},
volume={36},
ISSN={0370-2693},
url={http://dx.doi.org/10.1016/0370-2693(71)90724-6},
DOI={10.1016/0370-2693(71)90724-6},
number={4},
journal={Physics Letters B},
publisher={Elsevier BV},
author={Roy,
S.M.},
year={1971},
month=sep,
pages={353–356} }