A128401 Numbers k such that k^2 divides 21^k-1.

1, 2, 4, 5, 10, 20, 22, 44, 52, 68, 110, 220, 260, 340, 506, 572, 748, 820, 884, 1012, 2530, 2756, 2860, 3740, 3916, 4108, 4420, 5060, 9020, 9724, 10660, 13156, 13780, 13940, 17204, 19580, 20540, 23782, 29084, 30316, 34060, 45188, 46852, 47564

Offset: 1

Author

Alexander Adamchuk, Mar 01 2007

Keywords

• nonn
• less

Links

• Daniel Starodubtsev, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A127103, A127104, A127105, A127106, A127107, A127102, A127101, A127100, A127092, A128405, A128393, A128394, A128395, A128396, A128397, A128398, A128399, A128400, A128402, A128403, A128404.

Programs

Mathematica

Select[Range[50000],Divisible[21^#-1,#^2]&] (* Harvey P. Dale, Jun 17 2011 *)