

A107756


Numbers n such that Sum_{k=1..n} Catalan(k) == 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
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OFFSET

1,2


LINKS

Table of n, a(n) for n=1..73.
Y. More, Problem 11165, Amer. Math. Monthly, 112 (2005), 568.


FORMULA

Equivalently, numbers n such that base 3 expansion of n+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 *)


CROSSREFS

Cf. A000108, A107755, A107757.
Equals A074940  1.
Sequence in context: A123438 A039024 A065028 * A080723 A288737 A139448
Adjacent sequences: A107753 A107754 A107755 * A107757 A107758 A107759


KEYWORD

nonn,easy


AUTHOR

N. J. A. Sloane, Jun 11 2005


STATUS

approved



