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 A009252 #22 May 28 2025 20:10:44
%S A009252 1,2,20,456,18192,1111840,96035136,11101474944,1651123634432,
%T A009252 306656507699712,69472549405824000,18838618322988648448,
%U A009252 6019938761233443262464,2237523930630521828745216,956638251655236579066724352,465986443313532664723710443520,256462409164406988067105270726656
%N A009252 E.g.f. exp(x*tan(x)) (even powers only).
%H A009252 Vaclav Kotesovec, <a href="/A009252/b009252.txt">Table of n, a(n) for n = 0..238</a>
%F A009252 a(n)=sum(k=1..n, (binomial(2*n,k)*sum(j=k..2*n-k, binomial(j-1,k-1)*j!*(-1)^(n+j)*2^(2*n-k-j)*stirling2(2*n-k,j)))), n>0, a(0)=1. - _Vladimir Kruchinin_, Jun 06 2011
%F A009252 a(n) ~ n^(2*n-1/4) * 2^(4*n+1/4) * exp(2*sqrt(2*n)-2*n-1/2) / Pi^(2*n) * (1 - (Pi^2-1)/(12*sqrt(2*n))). - _Vaclav Kotesovec_, Jan 20 2015
%t A009252 Exp[ Tan[ x ]*x ] (* Even Part *)
%t A009252 With[{nn=40},Take[CoefficientList[Series[Exp[Tan[x]*x],{x,0,nn}],x]*Range[0,nn]!,{1,-1,2}]] (* _Vaclav Kotesovec_, Jan 20 2015 *)
%o A009252 (Maxima)
%o A009252 a(n):=sum((binomial(2*n,k)*sum(binomial(j-1,k-1)*j!*(-1)^(n+j)*2^(2*n-k-j)*stirling2(2*n-k,j),j,k,2*n-k)),k,1,n); /* _Vladimir Kruchinin_, Jun 06 2011 */
%Y A009252 Cf. A024263.
%K A009252 nonn
%O A009252 0,2
%A A009252 _R. H. Hardin_
%E A009252 Extended and signs tested Mar 15 1997 by _Olivier Gérard_.