A128401 - OEIS

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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

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,2

LINKS

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

MATHEMATICA

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

CROSSREFS

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

Sequence in context: A249399 A118551 A161706 * A018467 A035524 A018283

Adjacent sequences: A128398 A128399 A128400 * A128402 A128403 A128404

KEYWORD

nonn,less

AUTHOR

Alexander Adamchuk, Mar 01 2007

STATUS

approved