@article{EXMA_2005__14_3_55277,
     author = {Kamphorst, Sylvie Oliffson and Pinto-de-Carvalho, S\^onia},
     title = {The first {Birkhoff} coefficient and the stability of 2-periodic orbits on billiards.},
     journal = {Experimental mathematics},
     pages = {299--306},
     publisher = {mathdoc},
     volume = {14},
     number = {3},
     year = {2005},
     zbl = {1159.37423},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/EXMA_2005__14_3_55277/}
}