A119367 Number of rooted planar n-trees whose number of leaves is equal to 2 modulo 3.

0, 0, 1, 3, 6, 11, 30, 126, 519, 1836, 5877, 18866, 65472, 242463, 905425, 3307371, 11889414, 42930441, 157641714, 586908936, 2194106568, 8189323686, 30541703733, 114219616809, 429214875498, 1619584557885, 6125465491551

Offset

0,4

Comments

a(n)+A119365(n)+A119366(n)=A000108(n).

Links

Table of n, a(n) for n=0..26.

Formula

a(n)=sum{k=0..n, if(mod(n-k,3)=2, (1/n)*C(n,k)*C(n,k+1), 0)}.

Crossrefs

Sequence in context: A360888 A101958 A153982 * A079801 A280259 A119104

Adjacent sequences: A119364 A119365 A119366 * A119368 A119369 A119370

Keyword

easy,nonn

Author

Paul Barry, May 16 2006

Status

approved