%I #12 Apr 14 2020 05:44:05
%S 1,2,4,5,10,20,22,44,52,68,110,220,260,340,506,572,748,820,884,1012,
%T 2530,2756,2860,3740,3916,4108,4420,5060,9020,9724,10660,13156,13780,
%U 13940,17204,19580,20540,23782,29084,30316,34060,45188,46852,47564
%N Numbers k such that k^2 divides 21^k-1.
%H Daniel Starodubtsev, <a href="/A128401/b128401.txt">Table of n, a(n) for n = 1..1000</a>
%t Select[Range[50000],Divisible[21^#-1,#^2]&] (* _Harvey P. Dale_, Jun 17 2011 *)
%Y Cf. A127103, A127104, A127105, A127106, A127107, A127102, A127101, A127100, A127092, A128405, A128393, A128394, A128395, A128396, A128397, A128398, A128399, A128400, A128402, A128403, A128404.
%K nonn,less
%O 1,2
%A _Alexander Adamchuk_, Mar 01 2007