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 A026620 #14 Jun 14 2024 08:46:59
%S A026620 1,9,37,147,576,2244,8723,33891,131716,512278,1994202,7770734,
%T A026620 30310320,118343970,462501135,1809134115,7082699580,27750808470,
%U A026620 108812919270,426966196410,1676471166240,6586744582080,25894139638302,101852815940622,400840469986376,1578280410414204
%N A026620 a(n) = A026615(2*n-1, n-2).
%H A026620 G. C. Greubel, <a href="/A026620/b026620.txt">Table of n, a(n) for n = 2..1000</a>
%F A026620 From _G. C. Greubel_, Jun 13 2024: (Start)
%F A026620 a(n) = (7*n^2 - 11*n + 6)*binomial(2*n, n)/(4*(n+1)*(2*n-1)) - [n=2].
%F A026620 G.f.: ( (2 - 7*x + 3*x^2) - (2 - 3*x + x^2 + 2*x^3)*sqrt(1-4*x) )/(2*x*sqrt(1-4*x)).
%F A026620 E.g.f.: (1/2)*exp(2*x)*( 3*(-1 + x)*BesselI(0, 2*x) + (4 - 3*x)*BesselI(1, 2*x) ) + (1/2)*(3 - x - x^2). (End)
%t A026620 Table[(7*n^2-11*n+6)*Binomial[2*n,n]/(4*(n+1)*(2*n-1))-Boole[n==2], {n,2,40}] (* _G. C. Greubel_, Jun 13 2024 *)
%o A026620 (Magma) [n eq 2 select 1 else (7*n^2-11*n+6)*Catalan(n)/(4*(2*n-1)): n in [2..40]]; // _G. C. Greubel_, Jun 13 2024
%o A026620 (SageMath) [(7*n^2-11*n+6)*binomial(2*n,n)/(4*(n+1)*(2*n-1)) -int(n==2) for n in range(2,41)] # _G. C. Greubel_, Jun 13 2024
%Y A026620 Cf. A026615, A026616, A026617, A026618, A026619, A026621, A026622.
%Y A026620 Cf. A026623, A026624, A026625, A026956, A026957, A026958, A026959.
%Y A026620 Cf. A026960.
%K A026620 nonn
%O A026620 2,2
%A A026620 _Clark Kimberling_