%I #15 Jul 10 2024 02:58:12

%S 2,4,16,36,120,256,456,1296,2556,2704,3816,3856,4356,5544,6480,8008,

%T 9216,10440,10620,11952,16212,22896,23436,26320,26796,27840,28680,

%U 35208,43056,44100,47520,47880,49680,51120,57240,61920,62568,63168,63936

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

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

%H Amiram Eldar, <a href="/A087965/b087965.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..300 from Harvey P. Dale)

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

%o (PARI) is(k) = !(k % 2) && Mod(2, k-1)^k == 1; \\ _Amiram Eldar_, Jul 10 2024

%Y Cf. A006521.

%K nonn

%O 1,1

%A _Labos Elemer_, Sep 22 2003