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%I #5 Nov 21 2013 12:50:00
%S 3,7,21,441,194481,37822859361,1430568690241985328321,
%T 2046526777500669368329342638102622164679041,
%U 4188271851027274266670020743752976432915773724299403513508038881183728210190544679681
%N a(n) = product_{i=1..n-1} a(i), with a(1) = 3 and a(2) = 7.
%C Except for first two terms a(n) = 21^2^n
%C Except for first three terms, a(n) = (a(n-1))^2 .
%t Join[{3,7},NestList[#^2&,21,7]] (* _Harvey P. Dale_, Jan 18 2012 *)
%Y Cf. A175138.
%K nonn
%O 1,1
%A _Giovanni Teofilatto_, Mar 15 2010