A087965 Numbers k such that 2^k - 1 is divisible by (k-1).

2, 4, 16, 36, 120, 256, 456, 1296, 2556, 2704, 3816, 3856, 4356, 5544, 6480, 8008, 9216, 10440, 10620, 11952, 16212, 22896, 23436, 26320, 26796, 27840, 28680, 35208, 43056, 44100, 47520, 47880, 49680, 51120, 57240, 61920, 62568, 63168, 63936

Offset: 1

Author

Labos Elemer, Sep 22 2003

Keywords

• nonn

Comments

Solutions to 2^x == 1 (mod x-1).

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000 (terms 1..300 from Harvey P. Dale)

Crossrefs

Cf. A006521.

Programs

Mathematica

Join[{2},Select[Range[2,64000],PowerMod[2,#,#-1]==1&]] (* Harvey P. Dale, Jun 01 2021 *)

PARI

is(k) = !(k % 2) && Mod(2, k-1)^k == 1; \\ Amiram Eldar, Jul 10 2024