OFFSET
0,3
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..214
FORMULA
a(n) = A319501(2n,n).
EXAMPLE
a(0) = 1: {}.
a(1) = 1: {aa}.
a(2) = 38: {aaab}, {aaba}, {aabb}, {abaa}, {abab}, {abba}, {abbb}, {baaa}, {baab}, {baba}, {babb}, {bbaa}, {bbab}, {bbba}, {a,aab}, {a,aba}, {a,abb}, {a,baa}, {a,bab}, {a,bba}, {a,bbb}, {aa,ab}, {aa,ba}, {aa,bb}, {aaa,b}, {aab,b}, {ab,ba}, {ab,bb}, {aba,b}, {abb,b}, {b,baa}, {b,bab}, {b,bba}, {ba,bb}, {a,aa,b}, {a,ab,b}, {a,b,ba}, {a,b,bb}.
MAPLE
h:= proc(n, i, k) option remember; `if`(n=0, 1, `if`(i<1, 0,
add(h(n-i*j, i-1, k)*binomial(k^i, j), j=0..n/i)))
end:
a:= n-> add((-1)^i*binomial(n, i)*h(2*n$2, n-i), i=0..n):
seq(a(n), n=0..15);
MATHEMATICA
h[n_, i_, k_] := h[n, i, k] = If[n == 0, 1, If[i < 1, 0,
Sum[h[n-i*j, i-1, k]*Binomial[k^i, j], {j, 0, n/i}]]];
a[n_] := Sum[(-1)^i*Binomial[n, i]*h[2n, 2n, n-i], {i, 0, n}];
Table[a[n], {n, 0, 15}] (* Jean-François Alcover, May 10 2022, after Alois P. Heinz *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Sep 21 2018
STATUS
approved