OFFSET
1,2
COMMENTS
It is conjectured that the sequence of Fermat primes (A019434) is complete; if so then this sequence is complete:
Suppose that x is a positive integer for which x^x+1 is prime. If x has an odd prime factor p, then x^x + 1 = (x^(x/p))^p + 1 is divisible by x^(x/p) + 1, so it is not prime. So x must be a power of 2. Hence x^x is also a power of 2, so x^x+1 is a Fermat prime.
If there are no Fermat primes beyond the known ones (as is widely believed), then x must be 1, 2, or 4. Letting x=phi(n), it is easy to see that n must be one of the numbers listed. - Dean Hickerson, Feb 11 2002
EXAMPLE
Cases n=1-12 are based on the primes 2, 5, 257.
MATHEMATICA
ephiQ[n_]:=Module[{eph=EulerPhi[n]}, PrimeQ[eph^eph+1]]; Select[ Range[ 20], ephiQ] (* Harvey P. Dale, Feb 23 2021 *)
PROG
(PARI) isok(n) = isprime(eulerphi(n)^eulerphi(n) + 1); \\ Michel Marcus, Oct 07 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
Labos Elemer, Jan 15 2002
STATUS
approved