A012645 - OEIS

%I #6 May 15 2022 16:46:23

%S 1,-1,21,-765,53193,-6008025,993328413,-226691205013,68261145008657,

%T -26216583047151921,12507068079360896165,-7255577391165958142893,

%U 5029681350011635244424921,-4106028806747521109741788425

%N sech(arcsinh(x)*cos(x))=1-1/2!*x^2+21/4!*x^4-765/6!*x^6+53193/8!*x^8...

%t With[{nn=30},Take[CoefficientList[Series[Sech[ArcSinh[x]Cos[x]],{x,0,nn}],x] Range[ 0,nn]!,{1,-1,2}]] (* _Harvey P. Dale_, May 15 2022 *)

%K sign

%O 0,3

%A Patrick Demichel (patrick.demichel(AT)hp.com)