1. C. B. Allendoerfer and C. O. Oakley, Principles of Mathematics, McGraw-Hill, New York, 1955 (11.2.2, p. 288), 1963 (10.2.2, p. 324), 1969 (12.5.6, p. 465).
2. Eugene P. Northrop, Riddles in Mathematics: A Book of Paradoxes, Van Nostrand, New York, 1944; p. 138.