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 A165202 #27 Sep 08 2022 08:45:47
%S A165202 1,3,3,-1,-6,-6,1,9,9,-1,-12,-12,1,15,15,-1,-18,-18,1,21,21,-1,-24,
%T A165202 -24,1,27,27,-1,-30,-30,1,33,33,-1,-36,-36,1,39,39,-1,-42,-42,1,45,45,
%U A165202 -1,-48,-48,1,51,51,-1,-54,-54,1,57,57,-1,-60,-60,1
%N A165202 Expansion of (1+x)/(1 - x + x^2)^2.
%H A165202 G. C. Greubel, <a href="/A165202/b165202.txt">Table of n, a(n) for n = 0..1000</a>
%H A165202 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (2,-3,2,-1).
%F A165202 a(n) = cos(Pi*n/3) + sin(Pi*n/3)*(2n/3 + 1)*sqrt(3).
%F A165202 a(n) = A099254(n) + A099254(n-1). - _R. J. Mathar_, May 02 2013
%t A165202 LinearRecurrence[{2,-3,2,-1}, {1,3,3,-1}, 70] (* _G. C. Greubel_, Jul 18 2019 *)
%t A165202 (-1)^Quotient[#-1,3]{1,1+#,#}[[Mod[#,3,1]]]&/@Range[0, 10] (* _Federico Provvedi_, Jul 18 2021 *)
%o A165202 (PARI) my(x='x+O('x^70)); Vec((1+x)/(1-x+x^2)^2) \\ _G. C. Greubel_, Jul 18 2019
%o A165202 (Magma) R<x>:=PowerSeriesRing(Integers(), 70); Coefficients(R!( (1+x)/(1-x+x^2)^2 )); // _G. C. Greubel_, Jul 18 2019
%o A165202 (Sage) ((1+x)/(1-x+x^2)^2).series(x, 70).coefficients(x, sparse=False) # _G. C. Greubel_, Jul 18 2019
%o A165202 (GAP) a:=[1,3,3,-1];; for n in [5..70] do a[n]:=2*a[n-1]-3*a[n-2]+ 2*a[n-3] -a[n-4]; od; a; # _G. C. Greubel_, Jul 18 2019
%Y A165202 Cf. A100050 (first differences).
%Y A165202 Hankel transform of A165201.
%K A165202 easy,sign
%O A165202 0,2
%A A165202 _Paul Barry_, Sep 07 2009