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%I #4 Mar 30 2012 17:38:57
%S 3,6,21,231,26796,359026206,64449908476890321,
%T 2076895351339769460477611370186681,
%U 2156747150208372213435450937462082366919951682912789656986079991221
%N Least nontrivial n-tuply triangular number.
%F a(n)=A007501(n). [From _R. J. Mathar_, Sep 04 2008]
%t f[n_] := n(n + 1)/2; Table[ Nest[f, 2, n], {n, 10}]
%Y Cf. A000217, A002817, A064322, A066370.
%K nonn
%O 1,1
%A _Robert G. Wilson v_, Jul 02 2004