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 A012131 #10 Mar 08 2015 17:52:11
%S A012131 1,1,-3,21,-263,4841,-99723,-199939,501445617,-101818966319,
%T A012131 19731909099757,-4192563651606299,1009030667701246697,
%U A012131 -277080625752723191879,86724157841631252590437,-30813037783471577493355059,12363651257099764677344554977
%N A012131 cosh(arcsin(tanh(x)))=1+1/2!*x^2-3/4!*x^4+21/6!*x^6-263/8!*x^8...
%F A012131 Observe that arcsin(tanh(x)) = int {0..x} 1/cosh(t) so the generating function for this sequence is cosh( int {0..x} 1/cosh(t) ). Note the similarity to the generating function for A000364: cosh( int {0..x} 1/cos(t) ) = 1+x^2/2!+5*x^4/4!+61*x^6/6!+... - Peter Bala, Aug 24 2011.
%t A012131 With[{nn=30},Take[CoefficientList[Series[Cosh[ArcSin[Tanh[x]]],{x,0,nn}],x] Range[0,nn]!,{1,-1,2}]] (* _Harvey P. Dale_, Mar 08 2015 *)
%K A012131 sign
%O A012131 0,3
%A A012131 Patrick Demichel (patrick.demichel(AT)hp.com)
%E A012131 More terms from _Harvey P. Dale_, Mar 08 2015