@article{JMPA_1928_9_7__409_0,
     author = {Hedrick, E. R. and Ingold, Louis},
     title = {Conjugate {Functions} in three {Dimensions}},
     journal = {Journal de Math\'ematiques Pures et Appliqu\'ees},
     pages = {409--416},
     publisher = {Gauthier-Villars},
     volume = {Ser. 9, 7},
     year = {1928},
     language = {en},
     url = {http://www.numdam.org/item/JMPA_1928_9_7__409_0/}
}