@article{BSMA_1882_2_6_1_111_0,
     author = {Weierstrass, C.},
     title = {Recherches sur les fonctions $2r$ fois p\'eriodiques de $r$ variables. {(Lettres} de {M.} {C.} {Weierstrass} \`a {M.} {C.-W.} {Borchardt)}},
     journal = {Bulletin des Sciences Math\'ematiques et Astronomiques},
     pages = {111--120},
     publisher = {Gauthier-Villars},
     volume = {2e s{\'e}rie, 6},
     number = {1},
     year = {1882},
     language = {fr},
     url = {http://www.numdam.org/item/BSMA_1882_2_6_1_111_0/}
}