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%I #7 Apr 25 2021 12:21:32
%S 1,13,9841,3812798742493,221713244121518884974124815309574946401
%N a(n) = (3^(3^n) - 1)/2.
%H G. C. Greubel, <a href="/A173506/b173506.txt">Table of n, a(n) for n = 0..6</a>
%F 2*a(n+1) + 1 = ( 2*a(n) + 1 )^3.
%t Table[(3^(3^m) -1)/2, {m, 0, 10}]
%o (Sage) [(3^(3^n) -1)/2 for n in (0..10)] # _G. C. Greubel_, Apr 25 2021
%Y Cf. A051179, A173507.
%K nonn
%O 0,2
%A _Roger L. Bagula_, Feb 20 2010
%E Introduced standard nomenclature in the definition, added recurrence - the Assoc. Editors of the OEIS, Feb 24 2010