A201823 Terms of A001764 not divisible by 3, where A001764 enumerates ternary trees.

1, 1, 55, 300830572, 1414282077098335379544565517191

Offset: 0

Author

Paul D. Hanna, Dec 05 2011

Keywords

• nonn

Comments

The number of digits for the terms begin:

[1, 1, 2, 9, 31, 97, 298, 902, 2715, 8155, 24478, 73446, 220354, ...].

Examples

			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, ...].
		

Crossrefs

Cf. A038003, A001764.

Programs

PARI

a(n)=binomial(3*(3^n-1)/2,(3^n-1)/2)/3^n

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).