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A081868
Numbers k such that Sum_{j=1..k} (binomial(2*j,j) mod 3) is even.
1
1, 2, 3, 10, 11, 28, 29, 36, 37, 38, 39, 82, 83, 90, 91, 92, 93, 108, 109, 110, 111, 118, 119, 244, 245, 252, 253, 254, 255, 270, 271, 272, 273, 280, 281, 324, 325, 326, 327, 334, 335, 352, 353, 360, 361, 362, 363, 730, 731, 738, 739, 740, 741, 756, 757, 758
OFFSET
1,2
LINKS
MAPLE
N:= 10000: # to get all terms <= N
alpha:= 2:
beta:= 0:
t:= 0:
A[1]:= 1:
count:= 1:
for i from 2 to N do
d:= padic:-ordp(4 - 2/i, 3);
beta:= beta + d;
alpha:= alpha * (4-2/i)/3^d mod 3;
if beta = 0 then
t:= t + alpha mod 2;
fi;
if t = 0 then
count:= count+1;
A[count]:= i;
fi
od:
seq(A[i], i=1..count); # Robert Israel, May 05 2014
MATHEMATICA
Select[Range[800], EvenQ[Sum[Mod[Binomial[2j, j], 3], {j, #}]]&] (* Harvey P. Dale, Jul 23 2023 *)
PROG
(PARI) isok(n) = ! (sum(k=1, n, binomial(2*k, k) % 3) % 2); \\ Michel Marcus, Dec 04 2013
CROSSREFS
Sequence in context: A281366 A324921 A307034 * A212103 A193652 A345369
KEYWORD
nonn
AUTHOR
Benoit Cloitre, Apr 12 2003
EXTENSIONS
More terms from Michel Marcus, Dec 04 2013
STATUS
approved