%I #6 Oct 01 2017 12:13:39
%S 1,-1,21,-1101,109001,-17491001,4139085981,-1355308485701,
%T 586612833746961,-324261948714025201,222851423174495787941,
%U -186365710421614910029501,186332939048243649892744921
%N sech(arctan(tanh(x)))=1-1/2!*x^2+21/4!*x^4-1101/6!*x^6+109001/8!*x^8...
%t With[{nn=30},Take[CoefficientList[Series[Sech[ArcTan[Tanh[x]]],{x,0,nn}],x] Range[0,nn]!,{1,-1,2}]] (* _Harvey P. Dale_, Oct 01 2017 *)
%K sign
%O 0,3
%A Patrick Demichel (patrick.demichel(AT)hp.com)