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 A050542 #24 Aug 13 2025 23:44:26
%S A050542 5,15,120,7260,26357430,347357071281165,60328467484514345085777519195,
%T A050542 1819761994515052295911016533077539330911500847167185483610
%N A050542 Iterated triangular numbers with seed 5.
%H A050542 Reinhard Zumkeller, <a href="/A050542/b050542.txt">Table of n, a(n) for n = 0..11</a>
%F A050542 a(n) = binomial(a(n-1)+1, 2), a(0)=5.
%F A050542 a(n) ~ 2 * c^(2^n), where c = 2.786070277619247476352890606543375642991890742669788965024... . - _Vaclav Kotesovec_, Dec 17 2014
%t A050542 a=5;lst={a};Do[b=(a+1)*a/2;AppendTo[lst,b];a=b,{n,1,9}];lst (* _Vladimir Joseph Stephan Orlovsky_, Mar 16 2010 *)
%t A050542 NestList[Binomial[#+1,2]&,5,9] (* _Harvey P. Dale_, Sep 24 2011 *)
%o A050542 (Haskell)
%o A050542 a050542 n = a050542_list !! n
%o A050542 a050542_list = iterate a000217 5 -- _Reinhard Zumkeller_, Apr 10 2014
%o A050542 (PARI) a(n)=if(n, binomial(a(n-1)+1, 2), 5) \\ _Charles R Greathouse IV_, Feb 08 2017
%Y A050542 Cf. A000217, A007501, A013589, A050536, A050548, A050909.
%K A050542 easy,nice,nonn
%O A050542 0,1
%A A050542 Klaus Strassburger (strass(AT)ddfi.uni-duesseldorf.de), Dec 29 1999