%I #9 Sep 23 2024 16:15:24
%S 1,1,1,-2,-5,7,19,-26,-71,97,265,-362,-989,1351,3691,-5042,-13775,
%T 18817,51409,-70226,-191861,262087,716035,-978122,-2672279,3650401,
%U 9973081,-13623482,-37220045,50843527,138907099,-189750626,-518408351,708158977,1934726305
%N Expansion of (1+x+5x^2+2x^3) / (1-4x^2+x^4).
%H Colin Barker, <a href="/A108413/b108413.txt">Table of n, a(n) for n = 0..1000</a>
%H <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (0,-4,0,-1).
%F a(0)=a(1)=a(2)=1, a(n)*a(n+3) - a(n+1)*a(n+2) = -3.
%F a(n) = -4*a(n-2) - a(n-4) for n>3. - _Colin Barker_, Sep 07 2017
%t a[ n_] := If[n<0, a[2-n], SeriesCoefficient[(1 + x + 5*x^2 + 2*x^3) / (1 + 4*x^2 + x^4), {x, 0, n}]]; (* _Michael Somos_, Sep 23 2024 *)
%o (PARI) Vec((1 + x + 5*x^2 + 2*x^3) / (1 + 4*x^2 + x^4) + O(x^40)) \\ _Colin Barker_, Sep 07 2017
%o (PARI) {a(n) = if(n<0, n=2-n); polcoeff( (1 + x + 5*x^2 + 2*x^3) / (1 + 4*x^2 + x^4) + x*O(x^n), n)}; /* _Michael Somos_, Sep 23 2024 */
%Y Unsigned values are in A002531.
%K sign,easy
%O 0,4
%A _Ralf Stephan_, Jun 05 2005