A039698 Numbers k such that phi(k) + 1 is prime.

1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 17, 18, 19, 21, 22, 23, 26, 27, 28, 29, 31, 32, 34, 36, 37, 38, 40, 41, 42, 43, 46, 47, 48, 49, 53, 54, 55, 57, 58, 59, 60, 61, 62, 63, 67, 71, 73, 74, 75, 76, 77, 79, 82, 83, 86, 88, 89, 91, 93, 94, 95, 97, 98, 99, 100, 101, 103

Offset: 1

Author

Olivier Gérard

Keywords

• nonn
• easy

Comments

Positive integers k for which values of A039649(k) are primes. - Vladimir Shevelev, May 10 2008

For every prime p, the numbers p and 2p are terms of this sequence. - Vladimir Shevelev, May 10 2008

Union of A000040 and A066071. - Ray Chandler, May 26 2008

Examples

			phi(10)+1 = 4+1 = 5, a prime number, so 10 is a term.
		

Links

• Antti Karttunen, Table of n, a(n) for n = 1..26197 (first 1000 terms from Vincenzo Librandi)

Crossrefs

Cf. A000010, A000040, A006093, A039649, A066071, A007614.

Cf. A039689 (complement), A296079 (characteristic function).

Cf. also A065512, A078892, A263028, A248792.

Programs

Magma

[n: n in [1..200] | IsPrime(EulerPhi(n)+1)]; // Vincenzo Librandi, Aug 13 2013

Mathematica

Select[Range[300], PrimeQ[EulerPhi[#] + 1]&] (* Vincenzo Librandi, Aug 13 2013 *)

Extensions

Edited by N. J. A. Sloane, May 21 2008 at the suggestion of R. J. Mathar