This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A012467 #16 Aug 25 2025 10:29:22
%S A012467 1,0,-12,480,-21168,741120,61942848,-29951061504,8015861467392,
%T A012467 -1745546459848704,215844160641143808,91751419512905687040,
%U A012467 -109687232118939992862720,71900040403600384794624000,-32879657977585141660017868800,2569990403264790442179575808000
%N A012467 Expansion of e.g.f. sech(arctan(x)^2), even powers only.
%e A012467 1 - 12/4!*x^4 + 480/6!*x^6 - 21168/8!*x^8 + ...
%t A012467 With[{nn=30},Take[CoefficientList[Series[Sech[ArcTan[x]^2],{x,0,nn}],x] Range[0,nn]!,{1,-1,2}]] (* _Harvey P. Dale_, Apr 30 2012 *)
%K A012467 sign,changed
%O A012467 0,3
%A A012467 Patrick Demichel (patrick.demichel(AT)hp.com)
%E A012467 Definition corrected by _Harvey P. Dale_, Apr 30 2012
%E A012467 Name edited by _Jason Yuen_, Aug 25 2025