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 A009725 #15 Oct 29 2020 17:30:02
%S A009725 0,1,2,2,8,16,96,272,2176,7936,79360,353792,4245504,22368256,
%T A009725 313155584,1903757312,30460116992,209865342976,3777576173568,
%U A009725 29088885112832,581777702256640,4951498053124096,108932957168730112,1015423886506852352,24370173276164456448
%N A009725 Expansion of e.g.f.: tan(x)*(1+x).
%F A009725 E.g.f.: x*(1+x)*Q(0), where Q(k) = 1 - x^2/(x^2 - (2*k+1)*(2*k+3)/Q(k+1) ); (continued fraction). - _Sergei N. Gladkovskii_, Dec 16 2013
%t A009725 With[{nn=30},CoefficientList[Series[Tan[x]*(1+x),{x,0,nn}],x] Range[0,nn]!] (* _Harvey P. Dale_, Oct 29 2020 *)
%Y A009725 a(2n) = A009752(n), a(2n+1) = A000182(n+1).
%K A009725 nonn,easy
%O A009725 0,3
%A A009725 _R. H. Hardin_
%E A009725 Definition clarified and prior Mathematica program replaced by _Harvey P. Dale_, Oct 29 2020