A050542 Iterated triangular numbers with seed 5.

5, 15, 120, 7260, 26357430, 347357071281165, 60328467484514345085777519195, 1819761994515052295911016533077539330911500847167185483610

Offset: 0

Author

Klaus Strassburger (strass(AT)ddfi.uni-duesseldorf.de), Dec 29 1999

Keywords

• easy
• nice
• nonn

Links

• Reinhard Zumkeller, Table of n, a(n) for n = 0..11

Crossrefs

Cf. A000217, A007501, A013589, A050536, A050548, A050909.

Programs

Haskell

a050542 n = a050542_list !! n
a050542_list = iterate a000217 5  -- Reinhard Zumkeller, Apr 10 2014

Mathematica

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 *)
NestList[Binomial[#+1,2]&,5,9] (* Harvey P. Dale, Sep 24 2011 *)

PARI

a(n)=if(n, binomial(a(n-1)+1, 2), 5) \\ Charles R Greathouse IV, Feb 08 2017

Formula

a(n) = binomial(a(n-1)+1, 2), a(0)=5.

a(n) ~ 2 * c^(2^n), where c = 2.786070277619247476352890606543375642991890742669788965024... . - Vaclav Kotesovec, Dec 17 2014