A039689 Numbers k such that phi(k) + 1 is not a prime.

15, 16, 20, 24, 25, 30, 33, 35, 39, 44, 45, 50, 51, 52, 56, 64, 65, 66, 68, 69, 70, 72, 78, 80, 81, 84, 85, 87, 90, 92, 96, 102, 104, 105, 112, 116, 120, 121, 123, 128, 129, 130, 136, 138, 140, 141, 143, 144, 147, 155, 156, 159, 160, 161, 162, 164, 165, 168, 170

Offset: 1

Author

Olivier Gérard

Keywords

• nonn
• easy

Examples

			phi(20)+1 = 8+1 = 9 is not prime.
		

Links

• Antti Karttunen, Table of n, a(n) for n = 1..39340

Crossrefs

Cf. A000010, A007614, A039649, A039698 (complement).

Positions of zeros in A296079.

Cf. also A263029.

Programs

Mathematica

Select[Range[200],!PrimeQ[EulerPhi[#]+1]&] (* Harvey P. Dale, Aug 31 2018 *)

PARI

isok(k) = !isprime(eulerphi(k)+1); \\ Michel Marcus, Jun 28 2021

Extensions

Name edited by Antti Karttunen, Dec 05 2017