1. L. E. Dickson, History of the Theory of Numbers, Vol. 2, chapters 4–9, 21–24, Publication No. 256, Carnegie Institution of Washington, Washington, D. C, 1920; reprint, Stechert, New York, 1934.
2. L. E. Dickson, ibid., pp. 165–170.
3. L. E. Dickson, ibid., p. 252.
4. J. L. Selfridge & B. W. Pollack, Notices Amer. Math. Soc., no. 11, 1964, p. 97.