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 A093303 #27 Dec 18 2015 12:54:54
%S A093303 0,2,10,56,400,3610,39722,516400,7746016,131682290,2501963530,
%T A093303 52541234152,1208448385520,30211209638026,815702660226730,
%U A093303 23655377146575200,733316691543831232,24199450820946430690,846980778733125074186
%N A093303 a(n) = a(n-1)*(2n-1) + 2n with a(0)=0.
%C A093303 Obviously, a(n) is always an even number. a(2) and a(6) are even semiprimes. - _Altug Alkan_, Dec 07 2015
%H A093303 Vincenzo Librandi, <a href="/A093303/b093303.txt">Table of n, a(n) for n = 0..200</a>
%F A093303 a(n) = n!*C(2*n-1,n)/2^(n-1) * Sum_{k=1..n} 2^k*k/(k!*C(2*k-1,k)), for n>0. - _Vaclav Kotesovec_, Oct 28 2012
%F A093303 From _Altug Alkan_, Dec 07 2015: (Start)
%F A093303 a(A047212(k)) mod 10 = 0.
%F A093303 a(A016861(k)) mod 10 = 2.
%F A093303 a(A016885(k)) mod 10 = 6. (End)
%F A093303 a(n) ~ (sqrt(2) + 2*sqrt(Pi)*exp(1/2)*erf(1/sqrt(2))) * 2^n * n^n / exp(n). - _Vaclav Kotesovec_, Dec 18 2015
%t A093303 Flatten[{0,Table[n!*Binomial[2*n-1,n]/2^(n-1)*Sum[2^k*k/(k!*Binomial[2*k-1,k]), {k,1,n}],{n,1,20}]}] (* _Vaclav Kotesovec_, Oct 28 2012 *)
%o A093303 (PARI) a(n) = if(n==0, 0, n!*binomial(2*n-1,n)/2^(n-1) * sum(k=1, n, 2^k*k/(k!*binomial(2*k-1,k)))) \\ _Altug Alkan_, Dec 07 2015
%o A093303 (PARI) a(n) = if(n==0, 0, a(n-1)*(2*n-1) + 2*n); \\ _Altug Alkan_, Dec 07 2015
%Y A093303 Cf. A005843.
%K A093303 easy,nonn
%O A093303 0,2
%A A093303 Emrehan Halici (emrehan(AT)halici.com.tr), Apr 24 2004
%E A093303 More terms from Pab Ter (pabrlos(AT)yahoo.com), May 24 2004