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sech(arctan(tanh(x)))=1-1/2!*x^2+21/4!*x^4-1101/6!*x^6+109001/8!*x^8...
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%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)