The twenty-first William Lowell Putnam mathematical competition (December 3, 1960), afternoon session, problem 1 // The William Lowell Putnam mathematical competition problems and solutions: 1938-1964 / A. M. Gleason, R. E. Greenwood, L. M. Kelly. — MAA, 1980. — P. 59. — ISBN 0-88385-428-7.