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Iterated pentagonal numbers starting with 2.
1

%I #23 Sep 07 2024 19:50:46

%S 2,5,35,1820,4967690,37016913420305,2055377818749516111922729385,

%T 6336866966711278058881411525857304983854251425952352645

%N Iterated pentagonal numbers starting with 2.

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/PentagonalNumber.html">Pentagonal Number</a>

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

%t NestList[PolygonalNumber[5,#]&,2,8] (* _Harvey P. Dale_, Sep 07 2024 *)

%o (PARI) a(n)=if(n==1,2,a(n-1)*(3*a(n-1)-1)/2); \\ _Joerg Arndt_, Mar 05 2023

%Y Cf. A007501, A000326.

%K easy,nonn

%O 0,1

%A _Jonathan Vos Post_, Nov 14 2004

%E Edited by _Giovanni Resta_, Jun 17 2016

%E Edited by _Joerg Arndt_, Mar 05 2023