OFFSET
0,4
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..1000
FORMULA
a(n) = A360634(n,2).
EXAMPLE
a(2) = 1: {aa}.
a(3) = 6: {aab}, {aba}, {baa}, {a,ab}, {a,ba}, {aa,b}.
a(4) = 16: {aabb}, {abab}, {abba}, {baab}, {baba}, {bbaa}, {a,abb}, {a,bab}, {a,bba}, {aa,bb}, {aab,b}, {ab,ba}, {aba,b}, {b,baa}, {a,ab,b}, {a,b,ba}.
MAPLE
g:= proc(n, i, j) option remember; convert(series(`if`(j=0, 1,
`if`(i<0, 0, add(g(n, i-1, j-k)*x^(i*k)*binomial(
binomial(n, i), k), k=0..j))), x, 3), polynom)
end:
b:= proc(n, i) option remember; convert(series(`if`(n=0, 1, `if`(i<1, 0,
add(b(n-i*j, i-1)*g(i$2, j), j=0..n/i))), x, 3), polynom)
end:
a:= n-> coeff(b(n$2), x, 2):
seq(a(n), n=0..45);
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Feb 15 2023
STATUS
approved