OFFSET
1,2
LINKS
Andrew Howroyd, Table of n, a(n) for n = 1..200
EXAMPLE
The a(3) = 18 trees:
(123) ((1)(12)) ((1)(1)(1))
((1)(23)) ((2)(12)) ((1)((1)(1)))
((2)(13)) ((1)(2)(2))
((3)(12)) ((1)(1)(2))
((1)(2)(3)) ((1)((2)(2)))
((1)((2)(3))) ((1)((1)(2)))
((2)((1)(3))) ((2)((1)(2)))
((3)((1)(2))) ((2)((1)(1)))
PROG
(PARI)
EulerT(v)={Vec(exp(x*Ser(dirmul(v, vector(#v, n, 1/n))))-1, -#v)}
R(n, k)={my(v=[]); for(n=1, n, v=concat(v, EulerT(concat(v, [binomial(k, n)]))[n])); v}
seq(n)={sum(k=1, n, R(n, k)*sum(r=k, n, binomial(r, k)*(-1)^(r-k)))}
CROSSREFS
KEYWORD
nonn
AUTHOR
Andrew Howroyd, Dec 29 2019
STATUS
approved