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 A220944 #16 Dec 25 2023 18:03:18
%S A220944 1,3,9,11,33,35,105,107,321,323,969,971,2913,2915,8745,8747,26241,
%T A220944 26243,78729,78731,236193,236195,708585,708587,2125761,2125763,
%U A220944 6377289,6377291,19131873,19131875,57395625,57395627,172186881,172186883,516560649,516560651
%N A220944 Expansion of (1+3*x+5*x^2-x^3)/((1-x^2)*(1-3*x^2)).
%H A220944 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (0, 4, 0, -3).
%F A220944 a(n) = a(n-1)+2 if n odd.
%F A220944 a(n) = a(n-1)*3 if n even.
%F A220944 a(2n) = 4*3^n-3 = A036543(n), a(2n+1) = 4*3^n-1 = A171498(n).
%F A220944 a(n) = 4*a(n-2) - 3*a(n-4) with a(0)=1, a(1)=3, a(2)=9, a(3)=11.
%t A220944 LinearRecurrence[{0, 4, 0, -3}, {1, 3, 9, 11}, 40] (* _T. D. Noe_, Apr 17 2013 *)
%Y A220944 Cf. A036543, A171498.
%K A220944 nonn,easy
%O A220944 0,2
%A A220944 _Philippe Deléham_, Apr 13 2013