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 A026306 #18 Jan 01 2025 09:49:52
%S A026306 0,2,12,69,392,2235,12804,73710,426192,2473704,14405800,84137130,
%T A026306 492652824,2891110235,16999928820,100136858625,590778928800,
%U A026306 3490370847876,20647839813048,122287764072938,725023671281520,4302720916638417
%N A026306 a(n) = T(2n,n+1), where T is the array in A026300.
%F A026306 G.f.: A(x) = (1/B(x))'-1, where B(x) is the g.f. of A006605.
%F A026306 a(n) = n*(Sum_{j=0..2*n+1} binomial(j,-3*n+2*j-1)*binomial(2*n+1,j))/(2*n+1). - _Vladimir Kruchinin_, May 15 2014
%e A026306 G.f. = 2*x + 12*x^2 + 69*x^3 + 392*x^4 + 2235*x^5 + 12804*x^6 + 73710*x^7 + ...
%o A026306 (Maxima)
%o A026306 a(n):=(n*sum(binomial(j,-3*n+2*j-1)*binomial(2*n+1,j),j,0,2*n+1))/(2*n+1); /* _Vladimir Kruchinin_, May 15 2014 */
%K A026306 nonn
%O A026306 0,2
%A A026306 _Clark Kimberling_