@article{700068,
     title = {Relaxation lengths and nonnegative solutions in neutron transport},
     booktitle = {Proceedings of Equadiff III},
     series = {GDML\_Books},
     publisher = {Univ. J. E. Purkyn\v e - P\v r\'\i rodov\v edeck\'a fakulta},
     address = {Brno},
     year = {1973},
     pages = {227-234},
     mrnumber = {MR0347293},
     zbl = {0327.35066},
     url = {http://dml.mathdoc.fr/item/700068}
}

Marek, Ivo. Relaxation lengths and nonnegative solutions in neutron transport, dans Proceedings of Equadiff III, GDML_Books,  (1973), pp. 227-234. http://gdmltest.u-ga.fr/item/700068/