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 A167912 #19 Feb 16 2025 08:33:11
%S A167912 1,217,913083596083,18744974860247264575032720770000376335095039
%N A167912 a(n) = (1/(3^n)^2) * Sum_{k=0..(3^n-1)} binomial(2k,k).
%C A167912 Note that a(n) mod 27 = a(n) mod 9 = a(n) mod 3 = 1.
%C A167912 The Maple program yields the first seven terms; easily adjustable for obtaining more terms. However, a(4) has 44 digits, a(5) has 140 digits, a(6) has 432 digits and a(7) has 1308 digits. - _Emeric Deutsch_, Nov 22 2009
%H A167912 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/CentralBinomialCoefficient.html">Central Binomial Coefficient</a>.
%H A167912 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/BinomialSums.html">Binomial Sums</a>.
%p A167912 a := proc (n) options operator, arrow: (sum(binomial(2*k, k), k = 0 .. 3^n-1))/3^(2*n) end proc: seq(a(n), n = 1 .. 7); # _Emeric Deutsch_, Nov 22 2009
%t A167912 Table[(1/3^n)^2 * Sum[Binomial[2 k, k], {k, 0, 3^n - 1}], {n, 1, 5}] (* _G. C. Greubel_, Jul 01 2016 *)
%Y A167912 Cf. A006134, A083096, A066796, A083097, A081601, A010060, A122485.
%K A167912 nonn
%O A167912 1,2
%A A167912 _Alexander Adamchuk_, Nov 15 2009
%E A167912 a(4) from _Emeric Deutsch_, Nov 22 2009