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 A026850 #21 Sep 08 2022 08:44:49
%S A026850 1,4,15,57,221,872,3489,14113,57575,236457,976271,4047871,16840879,
%T A026850 70259892,293790127,1230783085,5164196117,21696512073,91254256589,
%U A026850 384165925259,1618551762085,6823801074549,28785680471185,121490461772347
%N A026850 a(n) = T(2n,n+1), T given by A026736.
%H A026850 G. C. Greubel, <a href="/A026850/b026850.txt">Table of n, a(n) for n = 1..1000</a>
%F A026850 G.f.: (x * C(x)^3)/(1 - x/sqrt(1 - 4 * x)) where C(x) is the g.f. for Catalan numbers A000108. - _David Callan_, Jan 16 2016
%F A026850 a(n) ~ (3 - sqrt(5))^3 * (2 + sqrt(5))^(n+1) / (8*sqrt(5)). - _Vaclav Kotesovec_, Jul 18 2019
%t A026850 CoefficientList[ Series[(1-Sqrt[1-4x])^3/(8x^3(1-x/Sqrt[1-4x])), {x, 0, 30}], x] (* _David Callan_, Jan 16 2016 *)
%o A026850 (PARI) my(x='x+O('x^30)); Vec(sqrt(1-4*x)*(1-sqrt(1-4*x))^3/(8*x^2*(sqrt(1-4*x) -x)) ) \\ _G. C. Greubel_, Jul 17 2019
%o A026850 (Magma) R<x>:=PowerSeriesRing(Rationals(), 30); Coefficients(R!( Sqrt(1-4*x)*(1-Sqrt(1-4*x))^3/(8*x^2*(Sqrt(1-4*x) -x)) )); // _G. C. Greubel_, Jul 17 2019
%o A026850 (Sage) a=(sqrt(1-4*x)*(1-sqrt(1-4*x))^3/(8*x^2*(sqrt(1-4*x) -x))).series(x, 30).coefficients(x, sparse=False); a[1:] # _G. C. Greubel_, Jul 17 2019
%Y A026850 Cf. A000108, A026736.
%K A026850 nonn
%O A026850 1,2
%A A026850 _Clark Kimberling_