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 A050909 #15 Sep 29 2023 18:58:32 %S A050909 9,45,1035,536130,143717956515,10327425512495574450870, %T A050909 53327858858072239299138189326373798809103885 %N A050909 Iterated triangular numbers with seed 9. %H A050909 Reinhard Zumkeller, <a href="/A050909/b050909.txt">Table of n, a(n) for n = 0..10</a> %F A050909 a(n) = binomial(a(n-1)+1, 2), a(0)=9. %F A050909 a(n) ~ 2 * c^(2^n), where c = 4.770128350983276481977311510917830191429308415174884373259... . - _Vaclav Kotesovec_, Dec 17 2014 %t A050909 NestList[Binomial[#+1,2]&,9,10] (* _Harvey P. Dale_, Oct 12 2011 *) %o A050909 (Haskell) %o A050909 a050909 n = a050909_list !! n %o A050909 a050909_list = iterate a000217 9 -- _Reinhard Zumkeller_, Apr 10 2014 %o A050909 (PARI) a(n)=if(n, binomial(a(n-1)+1,2), 9) \\ _Charles R Greathouse IV_, Feb 07 2017 %Y A050909 Cf. A000217, A007501, A013589, A050542, A050536, A050548. %K A050909 easy,nice,nonn %O A050909 0,1 %A A050909 Klaus Strassburger (strass(AT)ddfi.uni-duesseldorf.de), Dec 29 1999