"Problems and Solutions: Problems: 10306-10313"
Seung-Jin Bang, John Calvin Williams, I. Martin Isaacs, Robert Connelly, John H. Hubbard, Walter Whiteley, Walter Rudin, E. Rodney Canfield, Solomon W. Golomb, Hongyuan Zha, O. Krafft and M. Schaefer
Journal: Amer. Math. Monthly, (1993), (Vol. 100), No. 5, (Pgs. 498--499) (doi:
10.2307/2324311)