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A107756
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Numbers k such that Sum_{j=1..k} Catalan(j) == 1 (mod 3).
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3
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1, 4, 5, 6, 7, 10, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 28, 31, 32, 33, 34, 37, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 82, 85, 86, 87, 88, 91, 94, 95
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OFFSET
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1,2
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LINKS
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FORMULA
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Equivalently, numbers k such that base-3 expansion of k+1 contains a 2.
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MATHEMATICA
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Select[ Range[ 100], Mod[ Sum[(2 n)!/n!/(n + 1)!, {n, #}], 3] == 1 &] (* Robert G. Wilson v, Jun 14 2005 *)
Position[Accumulate[CatalanNumber[Range[100]]], _?(Mod[#, 3]==1&)]//Flatten (* Harvey P. Dale, Aug 26 2022 *)
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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