@article{10_4064_ap90_1_5,
     author = {W. M. Mikulski},
     title = {On the {Helmholtz} operator of variational calculus
 in fibered-fibered manifolds},
     journal = {Annales Polonici Mathematici},
     pages = {59--76},
     publisher = {mathdoc},
     volume = {90},
     number = {1},
     year = {2007},
     doi = {10.4064/ap90-1-5},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/ap90-1-5/}
}