A positive even value of 
for which 
, where 
is the totient function,
has no solution. The first few are 14, 26, 34, 38, 50, ... (OEIS A005277).
Nontotient
See also
Noncototient, 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 A005277/M4927 in "The On-Line Encyclopedia of Integer Sequences."Referenced on Wolfram|Alpha
NontotientCite this as:
Weisstein, Eric W. "Nontotient." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Nontotient.html