A066821 Numbers k such that phi(k^2-1)/(k-1) is an integer.

2, 3, 5, 9, 13, 17, 21, 25, 33, 37, 45, 55, 57, 61, 65, 73, 76, 81, 85, 93, 97, 105, 117, 121, 129, 133, 141, 145, 157, 165, 177, 193, 201, 205, 213, 217, 225, 244, 253, 257, 261, 273, 277, 297, 298, 301, 313, 325, 333, 345, 357, 361, 381, 385, 393, 397, 406

Offset

1,1

Comments

Every number 2*p-1, p a prime, is a term of the sequence. - Vladeta Jovovic, Jan 22 2002

Links

Harry J. Smith, Table of n, a(n) for n = 1..1000

Mathematica

Select[Range[2, 500], Divisible[EulerPhi[#^2-1], #-1]&] (* Harvey P. Dale, Jul 29 2018 *)

Prog

(PARI) isok(k) = {k > 1 && eulerphi(k^2 - 1) % (k - 1) == 0 } \\ Harry J. Smith, Mar 30 2010

Crossrefs

Sequence in context: A033945 A098142 A270812 * A108568 A221708 A256408

Adjacent sequences: A066818 A066819 A066820 * A066822 A066823 A066824

Keyword

nonn

Author

Benoit Cloitre, Jan 19 2002

Extensions

Offset changed from 2 to 1 by Harry J. Smith, Mar 30 2010

Status

approved