@book{PPN379914212, title = {Abhandlungen \"uber Variations-Rechnung 2}, series = {GDML\_Books}, year = {1921}, language = {ger}, url = {http://dml.mathdoc.fr/item/PPN379914212} }
(éd.). Abhandlungen über Variations-Rechnung 2. GDML_Books (1921), http://gdmltest.u-ga.fr/item/PPN379914212/