This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A173506 #7 Apr 25 2021 12:21:32
%S A173506 1,13,9841,3812798742493,221713244121518884974124815309574946401
%N A173506 a(n) = (3^(3^n) - 1)/2.
%H A173506 G. C. Greubel, <a href="/A173506/b173506.txt">Table of n, a(n) for n = 0..6</a>
%F A173506 2*a(n+1) + 1 = ( 2*a(n) + 1 )^3.
%t A173506 Table[(3^(3^m) -1)/2, {m, 0, 10}]
%o A173506 (Sage) [(3^(3^n) -1)/2 for n in (0..10)] # _G. C. Greubel_, Apr 25 2021
%Y A173506 Cf. A051179, A173507.
%K A173506 nonn
%O A173506 0,2
%A A173506 _Roger L. Bagula_, Feb 20 2010
%E A173506 Introduced standard nomenclature in the definition, added recurrence - the Assoc. Editors of the OEIS, Feb 24 2010