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 A109247 #18 Mar 22 2020 10:01:03
%S A109247 1,0,-3,-3,0,0,3,3,0,0,-3,-3,0,0,3,3,0,0,-3,-3,0,0,3,3,0,0,-3,-3,0,0,
%T A109247 3,3,0,0,-3,-3,0,0,3,3,0,0,-3,-3,0,0,3,3,0,0,-3,-3,0,0,3,3,0,0,-3,-3,
%U A109247 0,0,3,3,0,0,-3,-3,0,0,3,3,0,0,-3,-3,0,0,3,3,0,0,-3,-3
%N A109247 Expansion of (1 - 3*x^2 - 3*x^3 + x^4)/(1 + x^4).
%C A109247 Row sums of Riordan array (1-x-2x^2,x(1-x)), A109246.
%C A109247 After the initial terms, cyclic with period 8: [0,0,-3,-3,0,0,3,3]. - _Antti Karttunen_, Aug 12 2017
%H A109247 Antti Karttunen, <a href="/A109247/b109247.txt">Table of n, a(n) for n = 0..8191</a>
%H A109247 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (0,0,0,-1).
%F A109247 a(0)=1, a(1)=0, a(2)=-3, a(3)=-3, a(4)=0, a(n)=-a(n-4) - _Harvey P. Dale_, Mar 24 2012
%F A109247 For n > 0, a(n) = -3 * A132380(n). - _Antti Karttunen_, Aug 12 2017
%t A109247 CoefficientList[Series[(1-3x^2-3x^3+x^4)/(1+x^4),{x,0,90}],x] (* or *) Join[{1},LinearRecurrence[{0,0,0,-1},{0,-3,-3,0},90]] (* _Harvey P. Dale_, Mar 24 2012 *)
%o A109247 (Scheme) (define (A109247 n) (case n ((0) 1) ((1 4) 0) ((2 3) -3) (else (- (A109247 (- n 4)))))) ;; (After _Harvey P. Dale_'s Mar 24 2012 recurrence) - _Antti Karttunen_, Aug 12 2017
%o A109247 (PARI) Vec((1-3*x^2-3*x^3+x^4)/(1+x^4) + O(x^80)) \\ _Jinyuan Wang_, Mar 22 2020
%Y A109247 Cf. A109246, A132380.
%K A109247 sign,easy
%O A109247 0,3
%A A109247 _Paul Barry_, Jun 23 2005