A066902 Integers k such that phi(prime(k)+1) = phi(prime(k)-1).

3, 5, 20, 156, 254, 377, 593, 1800, 5903, 5981, 7925, 18669, 19240, 41274, 48296, 135700, 146866, 228028, 234303, 251216, 407377, 654288, 802222, 886223, 938654, 1063412, 1072766, 1212140, 1238668, 1515063, 1609346, 2080991, 2097725, 2363130, 2408674, 2916514

Offset: 1

Author

Benoit Cloitre, Jan 26 2002

Keywords

• nonn

Comments

Integers k such that A008331(k) = A008330(k).

Links

• Giovanni Resta, Table of n, a(n) for n = 1..675

Crossrefs

Cf. A000010, A008864, A006093, A008330, A008331, A067890.

Programs

Mathematica

Select[Range[1000000],EulerPhi[Prime[#]-1]==EulerPhi[Prime[#]+1]&] (* Harvey P. Dale, Feb 25 2012 *)

PARI

isok(k) = my(p=prime(k)); eulerphi(p+1) == eulerphi(p-1); \\ Michel Marcus, Apr 06 2020

Formula

a(n) = primepi(A067890(n)). - Giovanni Resta, Apr 06 2020

Extensions

More terms from Harvey P. Dale, Feb 25 2012

More terms from Jinyuan Wang, Apr 05 2020