A Positive value of for which has no solution, where is the Totient Function. The first few are 10, 26, 34, 50, 52, ... (Sloane's A005278).
See also Nontotient, Totient Function
References
Guy, R. K. Unsolved Problems in Number Theory, 2nd ed. New York: Springer-Verlag, p. 91, 1994.
Sloane, N. J. A. Sequence
A005278/M4688
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.