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 A067319 Numbers n such that phi(n)^phi(n)+1 is prime. 0
 1, 2, 3, 4, 5, 6, 8, 10, 12 (list; graph; refs; listen; history; text; internal format)
 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 LINKS 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 Cf. A063439, A000010. Sequence in context: A253012 A036409 A005423 * A086049 A328208 A173643 Adjacent sequences:  A067316 A067317 A067318 * A067320 A067321 A067322 KEYWORD nonn AUTHOR Labos Elemer, Jan 15 2002 STATUS approved

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Last modified October 7 16:02 EDT 2022. Contains 357275 sequences. (Running on oeis4.)