A positive value of 
for which 
has no solution, where 
is the totient function.
The first few are 10, 26, 34, 50, 52, ... (OEIS A005278).
Noncototient
See also
Nontotient, Totient FunctionExplore with Wolfram|Alpha
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 "The On-Line Encyclopedia of Integer Sequences."Referenced on Wolfram|Alpha
NoncototientCite this as:
Weisstein, Eric W. "Noncototient." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Noncototient.html