OFFSET
1,2
COMMENTS
a(n) is the number of labeled incomplete ternary trees on n vertices in which each left or middle child has a larger label than its parent and each right child has a smaller label than its parent. For example, a(2)=3 because we have 2L1, 2M1 and 1R2. Here aLb means a is a left child of b, etc.
LINKS
Alois P. Heinz, Table of n, a(n) for n = 1..100
MAPLE
N:= 8: exp(RootOf(2*_Z*exp(_Z)-x*exp(_Z)-2*x*exp(_Z)^2-x*exp(_Z)^3 -1 +exp(_Z)^2))-1: series(%, x, N+1): convert(%, polynom): seq( i!*coeff(%, x, i), i=1..N);
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Brian Drake, Jul 28 2008
STATUS
approved