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%I #9 Mar 30 2012 18:37:33
%S 1,1,55,300830572,1414282077098335379544565517191
%N Terms of A001764 not divisible by 3, where A001764 enumerates ternary trees.
%C The number of digits for the terms begin:
%C [1, 1, 2, 9, 31, 97, 298, 902, 2715, 8155, 24478, 73446, 220354, ...].
%F a(n) = A001764( (3^n-1)/2 ) = binomial( 3*(3^n-1)/2, (3^n-1)/2 ) / 3^n.
%F a(n) == 1 (mod 3).
%e Sequence A001764 begins: [1, 1, 3, 12, 55, 273, 1428, 7752, 43263, 246675, ...],
%e where A001764(n) == 1 (mod 3) at positions: [0, 1, 4, 13, 40, 121, 364, 1093, ...].
%o (PARI) a(n)=binomial(3*(3^n-1)/2,(3^n-1)/2)/3^n
%Y Cf. A038003, A001764.
%K nonn
%O 0,3
%A _Paul D. Hanna_, Dec 05 2011
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