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A201823
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Terms of A001764 not divisible by 3, where A001764 enumerates ternary trees.
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0
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OFFSET
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0,3
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COMMENTS
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The number of digits for the terms begin:
[1, 1, 2, 9, 31, 97, 298, 902, 2715, 8155, 24478, 73446, 220354, ...].
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LINKS
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FORMULA
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a(n) = A001764( (3^n-1)/2 ) = binomial( 3*(3^n-1)/2, (3^n-1)/2 ) / 3^n.
a(n) == 1 (mod 3).
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EXAMPLE
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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, ...].
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PROG
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(PARI) a(n)=binomial(3*(3^n-1)/2, (3^n-1)/2)/3^n
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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