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 A026631 #8 Jun 20 2024 17:43:18
%S A026631 1,7,30,119,467,1820,7076,27493,106848,415538,1617504,6302416,
%T A026631 24581249,95968478,375029576,1466881997,5742440324,22498218218,
%U A026631 88212326756,346114729562,1358944775654,5338963361408,20987909276600
%N A026631 a(n) = A026626(2*n-1, n-2).
%H A026631 G. C. Greubel, <a href="/A026631/b026631.txt">Table of n, a(n) for n = 2..1000</a>
%F A026631 a(n) = ((357*n^4 -1982*n^3 +3819*n^2 -3082*n +840)*a(n-1) +2*(2*n-5)*(51*n^3 -152*n^2 +133*n -24)*a(n-2))/(2*(n+1)*(51*n^3 -305*n^2 +590*n -360)), for n >= 5, with a(2) = 1, a(3) = 7, and a(4) = 30. - _G. C. Greubel_, Jun 20 2024
%t A026631 a[n_]:= a[n]= If[n<5, 23*n-62, ((357*n^4 -1982*n^3 +3819*n^2 -3082*n + 840)*a[n-1] +2*(2*n-5)*(51*n^3 -152*n^2 +133*n -24)*a[n-2] )/(2*(n + 1)*(51*n^3 -305*n^2 +590*n -360))] +17*Boole[n==2];
%t A026631 Table[a[n], {n, 2, 40}] (* _G. C. Greubel_, Jun 20 2024 *)
%o A026631 (Magma)
%o A026631 [1] cat [n le 2 select 23*(n+2)-62 else ((357*n^4 +874*n^3 +495*n^2 - 166*n -192)*Self(n-1) + 2*(2*n-1)*(51*n^3 +154*n^2 +137*n + 42 )*Self(n-2))/(2*(n+3)*(51*n^3 +n^2 -18*n +8)): n in [1..40]]; // _G. C. Greubel_, Jun 20 2024
%o A026631 (SageMath)
%o A026631 @CachedFunction
%o A026631 def a(n): # a = A026631
%o A026631 if (n==2): return 1
%o A026631 elif (n<5): return 23*n - 62
%o A026631 else: return ((357*n^4 -1982*n^3 +3819*n^2 -3082*n +840)*a(n-1) +2*(2*n-5)*(51*n^3 -152*n^2 +133*n -24)*a(n-2))/(2*(n+1)*(51*n^3 -305*n^2 +590*n -360))
%o A026631 [a(n) for n in range(2,41)] # _G. C. Greubel_, Jun 20 2024
%Y A026631 Cf. A026626, A026627, A026628, A026629, A026630, A026632, A026633.
%Y A026631 Cf. A026634, A026635, A026636, A026961, A026962, A026963, A026964.
%Y A026631 Cf. A026965.
%K A026631 nonn
%O A026631 2,2
%A A026631 _Clark Kimberling_