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A098271
Nonprime numbers n such that phi(n) divides n^2 - 1, where phi(n) (A000010) is Euler's totient function.
2
1, 15, 35, 65, 91, 255, 377, 559, 1295, 2001, 2059, 2431, 4081, 7743, 8911, 10745, 12851, 22791, 26335, 39689, 51271, 61699, 65535, 73151, 95201, 97681, 112897, 114191, 139231, 150177, 184255, 216545, 229503, 286937, 294785, 296191, 334369
OFFSET
1,2
COMMENTS
Number of terms < 10^k: 1, 5, 8, 15, 26, 46, 71, 105, 160, 228, ..., . Robert G. Wilson v, Jan 10 2018
LINKS
Robert G. Wilson v, Table of n, a(n) for n = 1..229 (first 99 terms from G. C. Greubel)
MATHEMATICA
Select[ Range[ 335000], Mod[ #^2 - 1, EulerPhi[ # ]] == 0 && !PrimeQ[ # ] &]
PROG
(PARI) isok(n) = !isprime(n) && !((n^2-1) % eulerphi(n)); \\ Michel Marcus, Jan 11 2018
CROSSREFS
Sequence in context: A146656 A229108 A160497 * A234542 A242235 A082663
KEYWORD
nonn
AUTHOR
STATUS
approved