@article{MAN_1923__89_159000,
     author = {R. Courant},
     title = {Zur {Theorie} der linearen {Integralgleichungen}},
     journal = {Mathematische Annalen},
     pages = {161--178},
     publisher = {mathdoc},
     volume = {89},
     year = {1923},
     zbl = {49.0274.01},
     url = {http://geodesic.mathdoc.fr/item/MAN_1923__89_159000/}
}