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A039689
Numbers k such that phi(k) + 1 is not a prime.
4
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
1,1
LINKS
EXAMPLE
phi(20)+1 = 8+1 = 9 is not prime.
MATHEMATICA
Select[Range[200], !PrimeQ[EulerPhi[#]+1]&] (* Harvey P. Dale, Aug 31 2018 *)
PROG
(PARI) isok(k) = !isprime(eulerphi(k)+1); \\ Michel Marcus, Jun 28 2021
CROSSREFS
Cf. A000010, A007614, A039649, A039698 (complement).
Positions of zeros in A296079.
Cf. also A263029.
Sequence in context: A031168 A108856 A241750 * A332515 A063530 A160661
KEYWORD
nonn,easy
EXTENSIONS
Name edited by Antti Karttunen, Dec 05 2017
STATUS
approved