A308570 a(n) = sigma_{2*n}(n).

1, 17, 730, 65793, 9765626, 2177317874, 678223072850, 281479271743489, 150094635684419611, 100000095367432689202, 81402749386839761113322, 79496851942053939878082786, 91733330193268616658399616010, 123476696151234472370970011268514

Offset: 1

Author

Seiichi Manyama, Jun 08 2019

Keywords

• nonn

Links

• Seiichi Manyama, Table of n, a(n) for n = 1..214

Crossrefs

Diagonal of A308504.

Column k=2 of A308569.

Cf. A073705.

Programs

Mathematica

Table[DivisorSigma[2 n, n], {n, 1, 20}] (* Vaclav Kotesovec, Jun 08 2019 *)

PARI

{a(n) = sigma(n, 2*n)}

PARI

N=20; x='x+O('x^N); Vec(x*deriv(-log(prod(k=1, N, (1-(k^2*x)^k)^(1/k)))))

Formula

L.g.f.: -log(Product_{k>=1} (1 - (k^2*x)^k)^(1/k)) = Sum_{k>=1} a(k)*x^k/k.

a(n) ~ n^(2*n). - Vaclav Kotesovec, Jun 08 2019