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 A138341 #13 Sep 23 2021 14:09:15
%S A138341 1,-2,-7,-7,2,13,13,-2,-19,-19,2,25,25,-2,-31,-31,2,37,37,-2,-43,-43,
%T A138341 2,49,49,-2,-55,-55,2,61,61,-2,-67,-67,2,73,73,-2,-79,-79,2,85,85,-2,
%U A138341 -91,-91,2,97,97,-2,-103,-103,2,109,109,-2,-115,-115,2,121,121,-2
%N A138341 Expansion of (1-4x-x^3)/(1-x+x^2)^2.
%C A138341 Hankel transform of A079309.
%H A138341 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (2,-3,2,-1).
%F A138341 a(n) = (2n+1)*cos(Pi*n/3) - (2n+5)*sin(Pi*n/3)/sqrt(3).
%F A138341 a(n) = 2*a(n-1) - 3*a(n-2) + 2*a(n-3) - a(n-4) for n > 3. - _Jinyuan Wang_, Apr 09 2020
%t A138341 CoefficientList[Series[(1-4x-x^3)/(1-x+x^2)^2,{x,0,100}],x] (* or *) LinearRecurrence[{2,-3,2,-1},{1,-2,-7,-7},100] (* _Harvey P. Dale_, Sep 23 2021 *)
%o A138341 (PARI) Vec((1-4*x-x^3)/(1-x+x^2)^2 + O(x^62)) \\ _Jinyuan Wang_, Apr 09 2020
%Y A138341 Cf. A079309, A138342.
%K A138341 sign,easy
%O A138341 0,2
%A A138341 _Paul Barry_, Mar 15 2008
%E A138341 More terms from _Jinyuan Wang_, Apr 09 2020