@article{MZ_1956__64_169566,
     author = {John de Groot},
     title = {A {System} of {Continuous,} {Mutually} {Non-differentiable} {Functions.}},
     journal = {Mathematische Zeitschrift},
     pages = {192--194},
     publisher = {mathdoc},
     volume = {64},
     year = {1956},
     zbl = {0070.05603},
     url = {http://geodesic.mathdoc.fr/item/MZ_1956__64_169566/}
}