OFFSET
0,3
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..238
FORMULA
a(n) = A226874(2n,n).
EXAMPLE
a(2) = 10: aaab, aaba, aabb, abaa, abab, abba, baaa, baab, baba, bbaa.
MAPLE
b:= proc(n, i, t) option remember;
`if`(t=1, 1/n!, add(b(n-j, j, t-1)/j!, j=i..n/t))
end:
a:= n-> `if`(n=0, 1, (2*n)!*b(2*n, 1, n)):
seq(a(n), n=0..20);
MATHEMATICA
b[n_, i_, t_] := b[n, i, t] =
If[t == 1, 1/n!, Sum[b[n-j, j, t-1]/j!, {j, i, n/t}]];
a[n_] := If[n == 0, 1, (2n)! b[2n, 1, n]];
Table[a[n], {n, 0, 20}] (* Jean-François Alcover, May 05 2022, after Alois P. Heinz *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Sep 03 2018
STATUS
approved