A050909 Iterated triangular numbers with seed 9.

9, 45, 1035, 536130, 143717956515, 10327425512495574450870, 53327858858072239299138189326373798809103885

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..10

Crossrefs

Cf. A000217, A007501, A013589, A050542, A050536, A050548.

Programs

Haskell

a050909 n = a050909_list !! n
a050909_list = iterate a000217 9  -- Reinhard Zumkeller, Apr 10 2014

Mathematica

NestList[Binomial[#+1,2]&,9,10] (* Harvey P. Dale, Oct 12 2011 *)

PARI

a(n)=if(n, binomial(a(n-1)+1,2), 9) \\ Charles R Greathouse IV, Feb 07 2017

Formula

a(n) = binomial(a(n-1)+1, 2), a(0)=9.

a(n) ~ 2 * c^(2^n), where c = 4.770128350983276481977311510917830191429308415174884373259... . - Vaclav Kotesovec, Dec 17 2014