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A012211 sech(arctan(arctan(x)))=1-1/2!*x^2+21/4!*x^4-1149/6!*x^6+119305/8!*x^8... 0
1, -1, 21, -1149, 119305, -20082841, 4984310493, -1711500674389, 776816116717457, -450301381427975089, 324553968807761095973, -284661632752853556677805, 298518404617173649259722905 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

LINKS

Table of n, a(n) for n=0..12.

MATHEMATICA

With[{nn=30}, Take[CoefficientList[Series[Sech[ArcTan[ArcTan[x]]], {x, 0, nn}], x] Range[0, nn]!, {1, -1, 2}]] (* Ray Chandler, Nov 28 2016 *)

CROSSREFS

Sequence in context: A270505 A086098 A086875 * A297071 A036059 A036519

Adjacent sequences:  A012208 A012209 A012210 * A012212 A012213 A012214

KEYWORD

sign

AUTHOR

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

STATUS

approved

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Last modified October 22 22:22 EDT 2020. Contains 337962 sequences. (Running on oeis4.)