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