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 A305693 #17 Jun 08 2018 13:51:07
%S A305693 1,2,54,852,12550,183356,2698108,40090728,600970566,9074671980,
%T A305693 137844584020,2104090834456,32247569822364,495918392331992,
%U A305693 7648690018326840,118264579157865424,1832624131015069254,28453041434367110220,442512540108817131364
%N A305693 a(n) = binomial(4*n, 2*n) - 4*n*binomial(2*n-2, n-1).
%H A305693 Seiichi Manyama, <a href="/A305693/b305693.txt">Table of n, a(n) for n = 0..500</a>
%H A305693 J. H. Conway and N. J. A. Sloane, <a href="https://doi.org/10.1098/rspa.1997.0126">Low-Dimensional Lattices VII: Coordination Sequences</a>, Proc. Royal Soc. London, A453 (1997), 2369-2389 (<a href="http://neilsloane.com/doc/Me220.pdf">pdf</a>).
%F A305693 a(n) = A000984(2*n) - 4*n*A000984(n-1) for n > 0.
%F A305693 G.f.: sqrt(1 + sqrt(1 - 16*x))/sqrt(2*(1 - 16*x)) - 4*x*(1 - 2*x)/(1 - 4*x)^(3/2). - _Ilya Gutkovskiy_, Jun 08 2018
%o A305693 (PARI) {a(n) = binomial(4*n, 2*n)-4*n*binomial(2*n-2, n-1)}
%Y A305693 Cf. A000984, A108558.
%K A305693 nonn
%O A305693 0,2
%A A305693 _Seiichi Manyama_, Jun 08 2018