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 A172018 #11 Mar 16 2022 02:54:06
%S A172018 1,4,182,27410,8890310,5051270688,4459786293372,5659222645997646,
%T A172018 9770821427711370950,22041005972637205198568,
%U A172018 62967534725721252354766676,222256499446324679350316816644,950020052553444052606973276792092,4836606673194788521307702032786510240,28920975283745982162014025622769293094712
%N A172018 a(n) = A142458(2*n-1, n)/n.
%H A172018 G. C. Greubel, <a href="/A172018/b172018.txt">Table of n, a(n) for n = 1..190</a>
%F A172018 a(n) = A142458(2*n-1, n)/n.
%t A172018 T[n_, k_, m_]:= T[n, k, m]= If[k==1 || k==n, 1, (m*n-m*k+1)*T[n-1, k-1, m] + (m*k-m+1)*T[n-1, k, m]];
%t A172018 A172018[n_]:= T[2*n-1, n, 3]/n;
%t A172018 Table[A172018[n], {n, 30}] (* modified by _G. C. Greubel_, Mar 16 2022 *)
%o A172018 (Sage)
%o A172018 @CachedFunction
%o A172018 def T(n,k,m):
%o A172018 if (k==1 or k==n): return 1
%o A172018 else: return (m*(n-k)+1)*T(n-1,k-1,m) + (m*k-m+1)*T(n-1,k,m)
%o A172018 def A172018(n): return T(2*n-1, n, 3)/n
%o A172018 [A172018(n) for n in (1..30)] # _G. C. Greubel_, Mar 16 2022
%Y A172018 Cf. A142458, A172010.
%K A172018 nonn
%O A172018 1,2
%A A172018 _Roger L. Bagula_, Nov 19 2010
%E A172018 Offset changed and more terms added by _G. C. Greubel_, Mar 16 2022