OFFSET
0,3
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..400
FORMULA
a(n) = Sum_{k=0..n} A257740(n,k).
EXAMPLE
a(0) = 1: {}.
a(1) = 1: {a}.
a(2) = 5: {a,a}, {aa}, {ab}, {ba}, {a,b}.
MATHEMATICA
A[n_, k_] := A[n, k] = If[n == 0, 1, Sum[DivisorSum[j, #*k^#&]*A[n - j, k], {j, 1, n}]/n];
T[n_, k_] := Sum[A[n, k - i]*(-1)^i*Binomial[k, i], {i, 0, k}];
a[n_] := Sum[T[n, k], {k, 0, n}];
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, May 06 2015
STATUS
approved