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A128401
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Numbers k such that k^2 divides 21^k-1.
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16
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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
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OFFSET
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1,2
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LINKS
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MATHEMATICA
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Select[Range[50000], Divisible[21^#-1, #^2]&] (* Harvey P. Dale, Jun 17 2011 *)
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CROSSREFS
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Cf. A127103, A127104, A127105, A127106, A127107, A127102, A127101, A127100, A127092, A128405, A128393, A128394, A128395, A128396, A128397, A128398, A128399, A128400, A128402, A128403, A128404.
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KEYWORD
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nonn,less
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AUTHOR
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STATUS
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approved
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