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A066821 Numbers n such that phi(n^2-1)/(n-1) is an integer. 1
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 (list; graph; refs; listen; history; text; internal format)
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) { n=0; for (m=2, 10^10, if (eulerphi(m^2 - 1) % (m - 1) == 0, write("b066821.txt", n++, " ", m); if (n==1000, return)) ) } \\ 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

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Last modified March 21 22:19 EDT 2019. Contains 321382 sequences. (Running on oeis4.)