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 A038520 #25 Aug 02 2019 08:41:41
%S A038520 0,1,0,3,4,6,20,28,64,136,240,528,1024,2016,4160,8128,16384,32896,
%T A038520 65280,131328,262144,523776,1049600,2096128,4194304,8390656,16773120,
%U A038520 33558528,67108864,134209536,268451840,536854528,1073741824
%N A038520 Number of elements of GF(2^n) with trace 1 and subtrace 0.
%H A038520 Colin Barker, <a href="/A038520/b038520.txt">Table of n, a(n) for n = 0..1000</a>
%H A038520 F. Ruskey, <a href="http://combos.org/TSpoly">Number of irreducible polynomials over GF(2) with given trace and subtrace</a>
%H A038520 F. Ruskey, <a href="http://combos.org/TSGF2">Number of elements of GF(2^n) with given trace and subtrace</a>
%H A038520 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (0,2,4).
%F A038520 a(n) = C(n, r+0)+C(n, r+4)+C(n, r+8)+... where r = 1 if n odd, r = 3 if n even.
%F A038520 a(n) = 2*a(n-2) + 4*a(n-3), n > 3. - _Paul Curtz_, Feb 06 2008
%F A038520 From _Colin Barker_, Aug 02 2019: (Start)
%F A038520 G.f.: x*(1 + x^2) / ((1 - 2*x)*(1 + 2*x + 2*x^2)).
%F A038520 a(n) = (2^n + i*((-1-i)^n - (-1+i)^n)) / 4 for n>0, where i=sqrt(-1).
%F A038520 (End)
%t A038520 LinearRecurrence[{0,2,4},{0,1,0,3},40] (* _Harvey P. Dale_, Oct 15 2017 *)
%o A038520 (PARI) concat(0, Vec(x*(1 + x^2) / ((1 - 2*x)*(1 + 2*x + 2*x^2)) + O(x^40))) \\ _Colin Barker_, Aug 02 2019
%Y A038520 Cf. A038504, A000749, A038518, A038519, A038521.
%K A038520 easy,nonn
%O A038520 0,4
%A A038520 _Frank Ruskey_