@article{10_4064_ap91_2_4,
     author = {Urban Cegrell},
     title = {The gradient lemma},
     journal = {Annales Polonici Mathematici},
     pages = {143--146},
     publisher = {mathdoc},
     volume = {91},
     number = {2-3},
     year = {2007},
     doi = {10.4064/ap91-2-4},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/ap91-2-4/}
}