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 A128213 #14 Nov 06 2016 03:36:40
%S A128213 1,1,1,-1,-4,-4,1,7,7,-1,-10,-10,1,13,13,-1,-16,-16,1,19,19,-1,-22,
%T A128213 -22,1,25,25,-1,-28,-28,1,31,31,-1,-34,-34,1,37,37,-1,-40,-40,1,43,43,
%U A128213 -1,-46,-46,1,49,49,-1,-52,-52,1,55,55,-1,-58,-58,1,61,61,-1
%N A128213 Expansion of (1-x+2x^2-2x^3)/(1-x+x^2)^2.
%C A128213 a(n+1) is the Hankel transform of {1,0,1,3,9,28,90,297,1001,3432,11934,...}, cf. A000245.
%C A128213 Binomial transform of A128214.
%C A128213 a(n+2) is the Hankel transform of A014138. - _Paul Barry_, Mar 15 2008
%H A128213 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (2,-3,2,-1).
%F A128213 a(n) = cos(Pi*n/3) + (2n/sqrt(3)-1/sqrt(3))*sin(Pi*n/3).
%F A128213 a(n) = y(n,n), where y(m+1,n) = y(m,n) - y(m-1,n), with y(0,n)=1 and y(1,n)=n. - _Benedict W. J. Irwin_, Nov 05 2016
%t A128213 Table[DifferenceRoot[Function[{y, m}, {y[1 + m] == y[m] - y[m - 1], y[0] == 1, y[1] == n}]][n], {n, 0, 100}] (* _Benedict W. J. Irwin_, Nov 05 2016 *)
%o A128213 (PARI) Vec((1-x+2*x^2-2*x^3)/(1-x+x^2)^2 + O(x^100)) \\ _Michel Marcus_, May 31 2014
%K A128213 easy,sign
%O A128213 0,5
%A A128213 _Paul Barry_, Feb 19 2007