This site is supported by donations to The OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

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