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

Author

Paul Barry, May 16 2006

Keywords

• easy
• nonn

Comments

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

Formula

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