This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A099137 #24 Feb 16 2025 08:32:54 %S A099137 2,5,35,1820,4967690,37016913420305,2055377818749516111922729385, %T A099137 6336866966711278058881411525857304983854251425952352645 %N A099137 Iterated pentagonal numbers starting with 2. %H A099137 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/PentagonalNumber.html">Pentagonal Number</a> %F A099137 Let P(n) = n*(3*n-1)/2, a(1) = 2 and a(n) = P(a(n-1)) for n >= 2. %t A099137 NestList[PolygonalNumber[5,#]&,2,8] (* _Harvey P. Dale_, Sep 07 2024 *) %o A099137 (PARI) a(n)=if(n==1,2,a(n-1)*(3*a(n-1)-1)/2); \\ _Joerg Arndt_, Mar 05 2023 %Y A099137 Cf. A007501, A000326. %K A099137 easy,nonn %O A099137 0,1 %A A099137 _Jonathan Vos Post_, Nov 14 2004 %E A099137 Edited by _Giovanni Resta_, Jun 17 2016 %E A099137 Edited by _Joerg Arndt_, Mar 05 2023