Über Orthogonalsysteme
Compositio Mathematica, Volume 7 (1940), p. 372-375 
@article{CM_1940__7__372_0,
     author = {Sidon, S.},
     title = {\"Uber Orthogonalsysteme},
     journal = {Compositio Mathematica},
     publisher = {Johnson Reprint Corporation},
     volume = {7},
     year = {1940},
     pages = {372-375},
     zbl = {0023.02503},
     mrnumber = {1995},
     language = {de},
     url = {http://www.numdam.org/item/CM_1940__7__372_0}
}

Sidon, S. Über Orthogonalsysteme. Compositio Mathematica, Volume 7 (1940) pp. 372-375. http://www.numdam.org/item/CM_1940__7__372_0/