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A081868 Numbers n such that sum(k=1,n,C(2k,k) 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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

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

PROG

(PARI) isok(n) = ! (sum(k=1, n, binomial(2*k, k) % 3) % 2); \\ Michel Marcus, Dec 04 2013

CROSSREFS

Sequence in context: A047473 A270474 A008509 * A212103 A193652 A092986

Adjacent sequences:  A081865 A081866 A081867 * A081869 A081870 A081871

KEYWORD

nonn

AUTHOR

Benoit Cloitre, Apr 12 2003

EXTENSIONS

More terms from Michel Marcus, Dec 04 2013

STATUS

approved

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Last modified December 9 20:00 EST 2016. Contains 278986 sequences.