OFFSET
1,2
FORMULA
G.f.: x * (1/(1 - x) + Sum_{i>=1} Sum_{j>=1} a(i) * a(j) * x^(i*j)).
a(n) ~ c * 2^n, where c = 1.27442410710035207761153205319824525254716841098942446508584158048310907298... - Vaclav Kotesovec, Dec 16 2020
MAPLE
a:= proc(n) option remember; uses numtheory;
1+add(a(d)*a((n-1)/d), d=divisors(n-1))
end:
seq(a(n), n=1..35); # Alois P. Heinz, Dec 15 2020
MATHEMATICA
a[1] = 1; a[n_] := a[n] = 1 + Sum[a[(n - 1)/d] a[d], {d, Divisors[n - 1]}]; Table[a[n], {n, 1, 35}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Dec 15 2020
STATUS
approved