@article{JRAM_1919__149_149479,
     author = {Alfred Haar},
     title = {\"Uber die {Variation} der {Doppelintegrale.}},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {1--18},
     publisher = {mathdoc},
     volume = {149},
     year = {1919},
     zbl = {47.0476.02},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1919__149_149479/}
}