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 A013299 #20 Aug 03 2014 14:29:45
%S A013299 1,6,135,6300,496125,58939650,9833098275,2191376187000,
%T A013299 628651043645625,225615874552818750,99022807341232149375,
%U A013299 52176017395434685252500,32501310835906189355203125
%N A013299 -sinh(log(x+1)-arctanh(x)) = 1/2!*x^2 + 6/4!*x^4 + 135/6!*x^6 + 6300/8!*x^8 + ...
%C A013299 Number of degree-2n permutations without odd cycles and with odd number of even cycles, offset 1. E.g.f.: x^2/(2*sqrt(1-x^2)). - _Vladeta Jovovic_, Aug 10 2007
%F A013299 a(n) ~ (2*n)^(2*n+2)/exp(2*n). - _Vaclav Kotesovec_, Oct 24 2013
%t A013299 nn = 30; Select[Range[0, nn]! CoefficientList[Series[Sinh[Log[1/(1 - x^2)^(1/2)]], {x, 0, nn}], x], # > 0 &] (* _Geoffrey Critzer_, Jan 15 2012 *)
%t A013299 With[{nn=30},Take[-CoefficientList[Series[Sinh[Log[x+1]-ArcTanh[x]], {x,0,nn}], x] Range[0,nn]!, {3,-1,2}]] (* _Harvey P. Dale_, Oct 30 2013 *)
%Y A013299 Cf. A013302.
%K A013299 nonn
%O A013299 0,2
%A A013299 Patrick Demichel (patrick.demichel(AT)hp.com)