@article{CM_1938__5__433_0,
     author = {Sidon, S.},
     title = {Nachtrag zu meiner Arbeit ``\"Uber unvollst\"andige Orthogonalsysteme''},
     journal = {Compositio Mathematica},
     publisher = {Johnson Reprint Corporation},
     volume = {5},
     year = {1938},
     pages = {433-434},
     zbl = {0018.30202},
     language = {de},
     url = {http://www.numdam.org/item/CM_1938__5__433_0}
}

Sidon, S. Nachtrag zu meiner Arbeit “Über unvollständige Orthogonalsysteme”. Compositio Mathematica, Volume 5 (1938) , pp. 433-434. http://www.numdam.org/item/CM_1938__5__433_0/