A050542 Iterated triangular numbers with seed 5.

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

Offset

0,1

Links

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

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

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 *)

Prog

(Haskell)
a050542 n = a050542_list !! n
a050542_list = iterate a000217 5  -- Reinhard Zumkeller, Apr 10 2014
(PARI) a(n)=if(n, binomial(a(n-1)+1, 2), 5) \\ Charles R Greathouse IV, Feb 08 2017

Crossrefs

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

Sequence in context: A303230 A173736 A244298 * A091096 A116957 A124209

Adjacent sequences: A050539 A050540 A050541 * A050543 A050544 A050545

Keyword

easy,nice,nonn

Author

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

Status

approved