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%I #7 Sep 10 2019 14:46:06
%S 1,-12,2128,3482688,-28510476032,-29203453996032,18429307563035250688,
%T -709977989050867131727872,-146234048852276431866584563712,
%U 20573909798214617779391304930951168
%N sech(tanh(x)*tan(x))=1-12/4!*x^4+2128/8!*x^8+3482688/12!*x^12...
%t With[{nn=40},Take[CoefficientList[Series[Sech[Tanh[x]Tan[x]],{x,0,nn}],x] Range[0,nn]!,{1,-2,4}]] (* _Harvey P. Dale_, Sep 10 2019 *)
%K sign
%O 0,2
%A Patrick Demichel (patrick.demichel(AT)hp.com)
%E More terms from _Harvey P. Dale_, Sep 10 2019