OFFSET
0,4
COMMENTS
Permuting the symbols does not change the structure.
LINKS
Andrew Howroyd, Table of n, a(n) for n = 0..500
EXAMPLE
The a(3) = 2 word structures are 111, 112.
The a(4) = 2 word structures are 1111, 1112.
The a(5) = 3 word structures are 11111, 11112, 11122.
The a(6) = 7 word structures are 111111, 111112, 111122, 111221, 111211, 112111, 111223.
PROG
(PARI)
P(n) = {Vec(-1 + prod(k=1, n, 1 + y*x^k + O(x*x^n)))}
R(u, k) = {k*[subst(serlaplace(p)/y, y, k-1) | p<-u]}
seq(n)={my(u=P(n)); concat([1], sum(k=1, n, R(u, k)*sum(r=k, n, binomial(r, k)*(-1)^(r-k)/(r!)^2) ))}
CROSSREFS
KEYWORD
nonn
AUTHOR
Andrew Howroyd, May 20 2022
STATUS
approved