A217628 a(n) = 3^((n-1)*(n+2)/2).

1, 9, 243, 19683, 4782969, 3486784401, 7625597484987, 50031545098999707, 984770902183611232881, 58149737003040059690390169, 10301051460877537453973547267843, 5474401089420219382077155933569751763, 8727963568087712425891397479476727340041449

Offset: 1

Author

Vincenzo Librandi, Oct 24 2012

Keywords

• nonn

Links

• Vincenzo Librandi, Table of n, a(n) for n = 1..60

Crossrefs

Cf. A036442.

Programs

Magma

I:=[1]; [n le 1 select I[n] else Self(n-1)*3^n: n in [1..20]]

Mathematica

Table[3^((n-1) * (n+2)/2), {n, 1, 30}]

Maxima

A217628[n]:=3^((n-1)*(n+2)/2)$
makelist(A217628[n],n,1,30); /* Martin Ettl, Oct 29 2012 */

Formula

a(1) = 1, a(n) = a(n-1) * 3^n.

G.f.: G(0)/(2*x^3) - 1/(3*x)- 1/(3*x^2)- 1/(2*x^3), where G(k)= 1 + 3^(k-1)*x/(1 - 1/(1 + 1/G(k+1) )); (continued fraction). - Sergei N. Gladkovskii, Jul 28 2013