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 A026640 #9 Jul 02 2024 02:17:23
%S A026640 1,8,38,161,662,2672,10676,42398,167756,662252,2610758,10283861,
%T A026640 40490702,159394424,627456188,2470223186,9726696572,38308366784,
%U A026640 150916209308,594704861546,2344206594332,9243186573248,36456892635848
%N A026640 a(n) = A026637(2*n, n-2).
%H A026640 G. C. Greubel, <a href="/A026640/b026640.txt">Table of n, a(n) for n = 2..1000</a>
%F A026640 a(n) = ((7*n^2 - 4*n + 4)*a(n-1) + 2*n*(2*n-1)*a(n-2))/(2*(n-2)*(n+2)), n >= 5. - _G. C. Greubel_, Jul 01 2024
%t A026640 a[n_]:= a[n]= If[n<5, 4^(n-1) -3^(n-1) +1 -Boole[n==2], ((7*n^2 -4*n + 4)*a[n-1] +2*n*(2*n-1)*a[n-2])/(2*(n-2)*(n+2))];
%t A026640 Table[a[n], {n,2,40}] (* _G. C. Greubel_, Jul 01 2024 *)
%o A026640 (Magma)
%o A026640 [1] cat [n le 2 select 4^(n+1) -3^(n+1) +1 else ((7*n^2+24*n+24 )*Self(n-1) + 2*(2*n+3)*(n+2)*Self(n-2))/(2*n*(n+4)): n in [1..40]]; // _G. C. Greubel_, Jul 01 2024
%o A026640 (SageMath)
%o A026640 @CachedFunction
%o A026640 def a(n): # a = A026640
%o A026640 if n<5: return 4^(n-1) -3^(n-1) +1 -int(n==2)
%o A026640 else: return ((7*n^2-4*n+4)*a(n-1) + 2*n*(2*n-1)*a(n-2))/(2*(n-2)*(n+2))
%o A026640 [a(n) for n in range(2,41)] # _G. C. Greubel_, Jul 01 2024
%Y A026640 Cf. A026637, A026638, A026639, A026641, A026642, A026643, A026644.
%Y A026640 Cf. A026645, A026646, A026647, A026966, A026967, A026968, A026969.
%Y A026640 Cf. A026970.
%K A026640 nonn
%O A026640 2,2
%A A026640 _Clark Kimberling_