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A201823
Terms of A001764 not divisible by 3, where A001764 enumerates ternary trees.
0
1, 1, 55, 300830572, 1414282077098335379544565517191
OFFSET
0,3
COMMENTS
The number of digits for the terms begin:
[1, 1, 2, 9, 31, 97, 298, 902, 2715, 8155, 24478, 73446, 220354, ...].
FORMULA
a(n) = A001764( (3^n-1)/2 ) = binomial( 3*(3^n-1)/2, (3^n-1)/2 ) / 3^n.
a(n) == 1 (mod 3).
EXAMPLE
Sequence A001764 begins: [1, 1, 3, 12, 55, 273, 1428, 7752, 43263, 246675, ...],
where A001764(n) == 1 (mod 3) at positions: [0, 1, 4, 13, 40, 121, 364, 1093, ...].
PROG
(PARI) a(n)=binomial(3*(3^n-1)/2, (3^n-1)/2)/3^n
CROSSREFS
Sequence in context: A172856 A093255 A172896 * A115410 A250489 A308097
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Dec 05 2011
STATUS
approved