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A078892
Numbers n such that phi(n) - 1 is prime, where phi is Euler's totient function (A000010).
8
5, 7, 8, 9, 10, 12, 13, 14, 15, 16, 18, 19, 20, 21, 24, 25, 26, 27, 28, 30, 31, 33, 35, 36, 38, 39, 42, 43, 44, 45, 49, 50, 51, 52, 54, 56, 61, 62, 64, 65, 66, 68, 69, 70, 72, 73, 77, 78, 80, 81, 84, 86, 90, 91, 92, 93, 95, 96, 98, 99, 102, 103, 104, 105, 109, 111, 112, 117
OFFSET
1,1
COMMENTS
For all primes p: p is in the sequence iff p is the greater member of a twin prime pair (A006512), see A078893.
Union of A006512 and A078893. - Ray Chandler, May 26 2008
LINKS
MATHEMATICA
Select[Range[200], PrimeQ[EulerPhi[#] - 1]&] (* Vincenzo Librandi, Aug 13 2013 *)
PROG
(PARI) is(n)=isprime(eulerphi(n)-1) \\ Charles R Greathouse IV, Feb 21 2013
(Magma) [n: n in [1..200] | IsPrime(EulerPhi(n)-1)]; // Vincenzo Librandi, Aug 13 2013
KEYWORD
nonn,easy
AUTHOR
Reinhard Zumkeller, Dec 12 2002
STATUS
approved