A099137 Iterated pentagonal numbers starting with 2.

2, 5, 35, 1820, 4967690, 37016913420305, 2055377818749516111922729385, 6336866966711278058881411525857304983854251425952352645

Offset: 0

Author

Jonathan Vos Post, Nov 14 2004

Keywords

• easy
• nonn

Links

• Eric Weisstein's World of Mathematics, Pentagonal Number

Crossrefs

Cf. A007501, A000326.

Programs

Mathematica

NestList[PolygonalNumber[5,#]&,2,8] (* Harvey P. Dale, Sep 07 2024 *)

PARI

a(n)=if(n==1,2,a(n-1)*(3*a(n-1)-1)/2); \\ Joerg Arndt, Mar 05 2023

Formula

Let P(n) = n*(3*n-1)/2, a(1) = 2 and a(n) = P(a(n-1)) for n >= 2.

Extensions

Edited by Giovanni Resta, Jun 17 2016

Edited by Joerg Arndt, Mar 05 2023