A141515 a(n) = phi(A067774(n)) where phi is Euler totient function.

1, 6, 12, 18, 22, 30, 36, 42, 46, 52, 60, 66, 72, 78, 82, 88, 96, 102, 108, 112, 126, 130, 138, 150, 156, 162, 166, 172, 180, 192, 198, 210, 222, 228, 232, 240, 250, 256, 262, 270, 276, 282, 292, 306, 312, 316, 330, 336, 348, 352, 358, 366, 372, 378, 382, 388

Offset

1,2

Comments

Count of numbers smaller than and coprime to p for primes p such that p + 2 is composite.

Subsequence of A006093.

Links

Vincenzo Librandi, Table of n, a(n) for n = 1..6970

Mathematica

EulerPhi[#]&@Select[Prime@Range@80, !PrimeQ[# + 2] &] (* Vincenzo Librandi, Feb 09 2018 *)

Prog

(PARI) {forprime(p=2, 400, if(!isprime(p+2), print1(eulerphi(p), ", ")))} \\ Klaus Brockhaus, Aug 31 2008
(Magma) [EulerPhi(p): p in PrimesUpTo(400) | not IsPrime(p+2)]; // Vincenzo Librandi, Feb 09 2018

Crossrefs

Cf A067774 (primes p such that p+2 is composite), A000010 (Euler totient function), A006093 (primes minus 1), A141426, A141427.

Sequence in context: A315732 A315733 A315734 * A315735 A315736 A315737

Adjacent sequences: A141512 A141513 A141514 * A141516 A141517 A141518

Keyword

nonn

Author

Giovanni Teofilatto, Aug 11 2008

Extensions

Edited and a(1) = 1, a(12) = 66 inserted by Klaus Brockhaus, Aug 31 2008

Status

approved