@article{10_4153_CJM_1998_027_6,
     author = {Bolle, Philippe},
     title = {Morse {Index} of {Approximating} {Periodic} {Solutions} for the {Billiard} {Problem.} {Application} to {Existence} {Results}},
     journal = {Canadian journal of mathematics},
     pages = {497--524},
     year = {1998},
     volume = {50},
     number = {3},
     doi = {10.4153/CJM-1998-027-6},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CJM-1998-027-6/}
}