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 A261056 #6 Feb 24 2017 11:17:45
%S A261056 2,8,27,84,248,708,1973,5400,14574,38896,102863,269964,703972,1825612,
%T A261056 4711785,12110064,31010266,79148184,201420163,511233156,1294489296,
%U A261056 3270662036,8247316765,20758752648,52163239622,130875524096,327893289783,820410399804,2050189683644
%N A261056 Expansion of ( 2-x^2 ) / (x^2+2*x-1)^2 .
%C A261056 2nd column of A210637.
%H A261056 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (4,-2,-4,-1)
%F A261056 a(n) = 2*A006645(n+2) -A006645(n).
%t A261056 CoefficientList[Series[(2-x^2)/(x^2+2*x-1)^2,{x,0,40}],x] (* or *) LinearRecurrence[{4,-2,-4,-1},{2,8,27,84},40] (* _Harvey P. Dale_, Feb 24 2017 *)
%Y A261056 Cf. A210637.
%K A261056 nonn,easy
%O A261056 0,1
%A A261056 _R. J. Mathar_, Aug 08 2015