A107757 Numbers k such that Sum_{j=1..k} Catalan(j) == 2 (mod 3).

3, 9, 11, 27, 29, 35, 39, 81, 83, 89, 93, 107, 111, 117, 119, 243, 245, 251, 255, 269, 273, 279, 281, 323, 327, 333, 335, 351, 353, 359, 363, 729, 731, 737, 741, 755, 759, 765, 767, 809, 813, 819, 821, 837, 839, 845, 849, 971, 975, 981, 983, 999, 1001, 1007, 1011

Offset

1,1

Links

Table of n, a(n) for n=1..55.

Y. More, Problem 11165, Amer. Math. Monthly, 112 (2005), 568.

Maple

c:=n->binomial(2*n, n)/(n+1): s:=0: for n from 1 to 1500 do s:=s+c(n): a[n]:=s mod 3: od: A:=[seq(a[n], n=1..1500)]: p:=proc(n) if A[n]=2 then n else fi end: seq(p(n), n=1..1500); # Emeric Deutsch, Jun 12 2005

Mathematica

s0 = s2 = {}; s = 0; Do[s = Mod[s + (2 n)!/n!/(n + 1)!, 3]; Switch[ Mod[s, 3], 0, AppendTo[s0, n], 2, AppendTo[s2, n]], {n, 1055}]; s2 (* Robert G. Wilson v, Jun 14 2005 *)

Crossrefs

Cf. A000108, A107755, A107756.

Equals A074939 - 1.

Sequence in context: A032915 A019080 A060141 * A191180 A191128 A057261

Adjacent sequences: A107754 A107755 A107756 * A107758 A107759 A107760

Keyword

nonn,easy

Author

N. J. A. Sloane, Jun 11 2005

Extensions

More terms from Emeric Deutsch, Jun 12 2005

Status

approved