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 A119303 #12 Oct 11 2019 15:15:01
%S A119303 1,-2,-4,1,7,1,-12,-7,18,20,-23,-45,21,88,1,-154,-68,241,223,-327,
%T A119303 -532,345,1082,-140,-1959,-597,3181,2416,-4543,-6194,5308,13153,-3657,
%U A119303 -24655,-4188,41465,25186,-61932,-70839,78211,157957,-69304,-307007,-10442,534268
%N A119303 Expansion of (1 - 3x)/(1 - x + 2x^2 - x^3).
%C A119303 Row sums of number triangle A119302.
%H A119303 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (1,-2,1).
%F A119303 a(n) = a(n-1) - 2a(n-2) + a(n-3);
%F A119303 a(n) = Sum_{k=0..n} (C(2k, n-k) + 3*C(2k, n-k-1))*(-1)^(n-k).
%t A119303 RecurrenceTable[{a[0]==1, a[1]== -2, a[2]== -4, a[n]== a[n-1] - 2*a[n-2] + a[n-3]}, a, {n,30}] (* _G. C. Greubel_, Aug 07 2015 *)
%t A119303 LinearRecurrence[{1,-2,1},{1,-2,-4},50] (* _Harvey P. Dale_, Oct 11 2019 *)
%K A119303 easy,sign
%O A119303 0,2
%A A119303 _Paul Barry_, May 13 2006