See also Dedekind Eta Function, Euler Product, Totient Function
References
Cox, D. A. Primes of the Form : Fermat, Class Field Theory and Complex Multiplication.
New York: Wiley, p. 228, 1997.
Guy, R. K. Unsolved Problems in Number Theory, 2nd ed. New York: Springer-Verlag, p. 96, 1994.
Sloane, N. J. A. Sequence
A001615/M2315
in ``An On-Line Version of the Encyclopedia of Integer Sequences.''
http://www.research.att.com/~njas/sequences/eisonline.html and Sloane, N. J. A. and Plouffe, S.
The Encyclopedia of Integer Sequences. San Diego: Academic Press, 1995.