A097605 - OEIS

A097605

Numbers n such that phi(n) divides n^2 - 1, where phi(n) (A000010) is number of positive integers <= n and coprime to n.

1, 2, 3, 5, 7, 11, 13, 15, 17, 19, 23, 29, 31, 35, 37, 41, 43, 47, 53, 59, 61, 65, 67, 71, 73, 79, 83, 89, 91, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 255, 257

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OFFSET

1,2

COMMENTS

All primes are in the sequence. 2 is the only even integer in the sequence.

LINKS

Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

EXAMPLE

phi(35) = 24, 24 divides 35^2 -1 = 1224.

MATHEMATICA

Select[ Range[ 262], Mod[ #^2 - 1, EulerPhi[ # ]] == 0 &] (* Robert G. Wilson v, Sep 01 2004 *)

PROG