A078892 Numbers n such that phi(n) - 1 is prime, where phi is Euler's totient function (A000010).

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

Vincenzo Librandi, Table of n, a(n) for n = 1..1000

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

Crossrefs

Cf. A000010, A000040, A008864, A109606, A006512, A039698, A072281, A078893.

Sequence in context: A043602 A243624 A038609 * A370090 A164374 A072281

Adjacent sequences: A078889 A078890 A078891 * A078893 A078894 A078895

Keyword

nonn,easy

Author

Reinhard Zumkeller, Dec 12 2002

Status

approved