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A107756 Numbers k such that Sum_{j=1..k} Catalan(j) == 1 (mod 3). 3
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 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
LINKS
Y. More, Problem 11165, Amer. Math. Monthly, 112 (2005), 568.
FORMULA
Equivalently, numbers k such that base-3 expansion of k+1 contains a 2.
MATHEMATICA
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 *)
CROSSREFS
Equals A074940 - 1.
Sequence in context: A123438 A039024 A065028 * A080723 A288737 A139448
KEYWORD
nonn,easy
AUTHOR
N. J. A. Sloane, Jun 11 2005
STATUS
approved

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Last modified March 28 18:04 EDT 2024. Contains 371254 sequences. (Running on oeis4.)