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 A138269 #13 Dec 04 2022 13:45:50
%S A138269 1,3,12,53,231,1000,4329,18747,81188,351597,1522639,6594000,28556241,
%T A138269 123666803,535556412,2319302053,10044062391,43497219000,188370799289,
%U A138269 815766130347,3532789487188,15299239691997
%N A138269 a(n+1) is the Hankel transform of C(n)+C(n+2), where C(n) = A000108(n).
%H A138269 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (5,-4,5,-1).
%F A138269 G.f.: (1-x)^2/(1-5*x+4*x^2-5*x^3+x^4).
%F A138269 a(n) = Re(P(2n,i)*CONJ(P(2n+1,i))) where i=sqrt(-1) and P(n,x) = Sum_{k=0..floor(n/2)} binomial(n-k,k) x^k.
%F A138269 a(n) = 5*a(n-1)-4*a(n-2)+5*a(n-3)-a(n-4). - _Wesley Ivan Hurt_, Mar 07 2022
%t A138269 LinearRecurrence[{5,-4,5,-1},{1,3,12,53},30] (* _Harvey P. Dale_, Dec 04 2022 *)
%Y A138269 Cf. A000108, A092886, A138268.
%K A138269 easy,nonn
%O A138269 0,2
%A A138269 _Paul Barry_, Mar 10 2008