A371441 a(n) = a(n-1)*3^n + 1 where a(0)=1.

1, 4, 37, 1000, 81001, 19683244, 14349084877, 31381448626000, 205893684435186001, 4052605390737766057684, 239302295717674347940182517, 42391683779498857714559512339000, 22528678819460652442683221796950499001, 35917990801478965784376042224979510418771324

Offset: 0

Author

Alexandre Herrera, Mar 23 2024

Keywords

• nonn
• easy

Crossrefs

Cf. A073587, A178185.

Programs

Mathematica

Table[Sum[3^(k*(2*n + 1 - k)/2), {k, 0, n}], {n, 0, 15}] (* Vaclav Kotesovec, Apr 10 2024 *)
Block[{n = 0}, NestList[#*3^++n + 1 &, 1, 15]] (* Paolo Xausa, Apr 19 2024 *)

Python

l = [1]
for i in range(1,14):
    l.append(l[-1]*pow(3,i) + 1)
print(l)

Formula

a(n) = Sum_{k=0..n} 3^(k*(2*n + 1 - k)/2). - Vaclav Kotesovec, Apr 10 2024