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 A026642 #11 Jul 02 2024 02:17:20
%S A026642 1,7,28,112,439,1711,6652,25846,100450,390670,1520764,5925718,
%T A026642 23112931,90239407,352654084,1379410438,5400188206,21157958962,
%U A026642 82959736504,325514137048,1278093308806,5021436970822,19740128055928
%N A026642 a(n) = A026637(2*n-1, n-2).
%H A026642 G. C. Greubel, <a href="/A026642/b026642.txt">Table of n, a(n) for n = 2..1000</a>
%F A026642 a(n) = ( (7*n^2 - 11*n + 6)*a(n-1) + 2*(n-1)*(2*n-1)*a(n-2) )/(2*(n-2)*(n+1)), n >= 4. - _G. C. Greubel_, Jul 01 2024
%t A026642 a[n_]:= a[n]= If[n<4, 7^(n-2), ((7*n^2-11*n+6)*a[n-1] + 2*(n-1)*(2*n- 1)*a[n-2])/(2*(n-2)*(n+1))];
%t A026642 Table[a[n], {n,2,40}] (* _G. C. Greubel_, Jul 01 2024 *)
%o A026642 (Magma)
%o A026642 [n le 2 select 7^(n-1) else ((7*n^2+3*n+2)*Self(n-1) + 2*n*(2*n+1)*Self(n-2))/(2*(n-1)*(n+2)): n in [1..40]]; // _G. C. Greubel_, Jul 01 2024
%o A026642 (SageMath)
%o A026642 @CachedFunction
%o A026642 def a(n): # a = A026642
%o A026642 if n<4: return 7^(n-2)
%o A026642 else: return ((7*n^2-11*n+6)*a(n-1) + 2*(n-1)*(2*n-1)*a(n-2))/(2*(n-2)*(n+1))
%o A026642 [a(n) for n in range(2,41)] # _G. C. Greubel_, Jul 01 2024
%Y A026642 Cf. A026637, A026638, A026639, A026640, A026641, A026643, A026644.
%Y A026642 Cf. A026645, A026646, A026647, A026966, A026967, A026968, A026969.
%Y A026642 Cf. A026970.
%K A026642 nonn
%O A026642 2,2
%A A026642 _Clark Kimberling_