A050548 - OEIS (original) (raw)

A050548

Iterated triangular numbers with seed 7.

9

7, 28, 406, 82621, 3413156131, 5824817388998022646, 16964248807586870937449001943463431981, 143892868802856286225154411591351342616163027795335641150249224655238508171

FORMULA

a(n) = binomial(a(n-1)+1, 2), a(0)=7.

a(n) ~ 2 * c^(2^n), where c = 3.77579046114281148578572146955902030830005575599864345238... . - Vaclav Kotesovec, Dec 17 2014

MATHEMATICA

NestList[Binomial[#+1, 2]&, 7, 10] (* Harvey P. Dale, May 08 2011 *)

t = {7}; Do[AppendTo[t, t[[-1]]*(1 + t[[-1]])/2], {9}] (* T. D. Noe, Mar 03 2014 *)

PROG

(PARI) a(n)=my(k=7); for(i=1, n, k=binomial(k+1, 2)); k \\ Charles R Greathouse IV, Mar 03 2014

(Haskell)

a050548 n = a050548_list !! n

a050548_list = iterate a000217 7

(Python)

from itertools import accumulate

def f(an, _): return an*(an+1)//2

AUTHOR

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