@article{MS2_1978__42_166595,
     author = {Jaak Peetre},
     title = {The {Euler} derivative. {An} intrinsic approach to the calculus of variations.},
     journal = {Mathematica Scandinavica},
     pages = {313--333},
     year = {1978},
     volume = {42},
     zbl = {0404.58003},
     url = {http://geodesic.mathdoc.fr/item/MS2_1978__42_166595/}
}