@article{1077488977,
author = {Birkhoff, G. D. and Hestenes, M. R.},
title = {Natural isoperimetric conditions in the calculus of variations},
journal = {Duke Math. J.},
volume = {1},
number = {1},
year = {1935},
pages = { 198-286},
language = {en},
url = {http://dml.mathdoc.fr/item/1077488977}
}
Birkhoff, G. D.; Hestenes, M. R. Natural isoperimetric conditions in the calculus of variations. Duke Math. J., Tome 1 (1935) no. 1, pp. 198-286. http://gdmltest.u-ga.fr/item/1077488977/