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 A026621 #15 Jun 18 2024 11:09:10
%S A026621 1,1,3,5,10,17,34,60,120,217,434,798,1596,2970,5940,11154,22308,42185,
%T A026621 84370,160446,320892,613054,1226108,2351440,4702880,9048522,18097044,
%U A026621 34916300,69832600,135059220,270118440,523521630,1047043260,2033066025,4066132050,7908332190
%N A026621 a(n) = A026615(n, floor(n/2)).
%H A026621 G. C. Greubel, <a href="/A026621/b026621.txt">Table of n, a(n) for n = 0..1000</a>
%F A026621 a(n) = 2*( (49*n^2 - 287*n + 360)*a(n-1) + 2*(n-3)*(7*n-8)*(7*n-17)*a(n-2) )/((n+1)*(7*n-24)*(7*n-15)) for n > 2. - _G. C. Greubel_, Jun 13 2024
%t A026621 T[n_, k_]:= T[n, k]= If[k==0 || k==n, 1, If[k==1 || k==n-1, 2*n-1, T[n -1,k-1] +T[n-1,k]]]; (* T = A026615 *)
%t A026621 Table[T[n, Floor[n/2]], {n,0,40}] (* _G. C. Greubel_, Jun 13 2024 *)
%o A026621 (Magma)
%o A026621 I:=[1,3]; [1] cat [n le 2 select I[n] else 2*((49*n^2-287*n+360 )*Self(n-1) + 2*(n-3)*(7*n-8)*(7*n-17)*Self(n-2) )/((n+1)*(7*n-24)*(7*n-15)) : n in [1..40]]; // _G. C. Greubel_, Jun 13 2024
%o A026621 (SageMath)
%o A026621 @CachedFunction
%o A026621 def T(n, k): # T = A026615
%o A026621 if k==0 or k==n: return 1
%o A026621 elif k==1 or k==n-1: return 2*n-1
%o A026621 else: return T(n-1, k-1) + T(n-1, k)
%o A026621 def A026621(n): return T(n, int(n//2))
%o A026621 [A026621(n) for n in range(41)] # _G. C. Greubel_, Jun 13 2024
%Y A026621 Cf. A026615, A026616, A026617, A026618, A026619, A026620, A026622.
%Y A026621 Cf. A026623, A026624, A026625, A026956, A026957, A026958, A026959.
%Y A026621 Cf. A026960.
%K A026621 nonn
%O A026621 0,3
%A A026621 _Clark Kimberling_