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 A012009 #24 Aug 04 2024 06:36:59
%S A012009 1,0,-3,-30,-63,12540,602877,6625710,-1991169183,-241970036520,
%T A012009 -7540177734243,2917041754949850,699983161534169697,
%U A012009 46722975483964508820,-21334067257986056115363,-8882421213380429461235610,-1081286159351846822872767423
%N A012009 Expansion of e.g.f. sech(log(cos(x))) (even exponents only).
%F A012009 E.g.f.: sech(log(cos(x))) = 2*cos(x)/(1+cos^2(x)) is the reciprocal of the e.g.f. of A012007. - _Peter Bala_, Dec 02 2011
%e A012009 1 - (3/4!)*x^4 - (30/6!)*x^6 - (63/8!)*x^8 + (12540/10!)*x^10 ...
%t A012009 With[{nn=40},Take[CoefficientList[Series[Sech[Log[Cos[x]]],{x,0,nn}], x] Range[0,nn]!,{1,-1,2}]] (* _Harvey P. Dale_, Dec 16 2011 *)
%Y A012009 Cf. A012003, A012007, A012008.
%K A012009 sign
%O A012009 0,3
%A A012009 Patrick Demichel (patrick.demichel(AT)hp.com)
%E A012009 More terms from _Harvey P. Dale_, Dec 16 2011
%E A012009 New name from _Michel Marcus_, Aug 04 2024