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 A138495 #23 Jan 02 2024 08:59:37
%S A138495 0,1,1,1,3,9,15,25,55,121,231,441,903,1849,3655,7225,14535,29241,
%T A138495 58311,116281,232903,466489,932295,1863225,3727815,7458361,14913991,
%U A138495 29822521,59650503,119311929,238612935,477204025,954429895,1908903481,3817763271,7635439161,15270965703
%N A138495 First differences of A138477.
%H A138495 G. C. Greubel, <a href="/A138495/b138495.txt">Table of n, a(n) for n = 0..1000</a>
%H A138495 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (1, 0, 2, 4).
%F A138495 From _R. J. Mathar_, May 19 2008: (Start)
%F A138495 O.g.f.: x/((1-2*x)*(1+x)*(1+2*x^2)).
%F A138495 a(n) = a(n-1) + 2*a(n-3) + 4*a(n-4). (End)
%F A138495 E.g.f.: (2*exp(2*x) - exp(-x) - cos(sqrt(2)*x) + 2*sqrt(2)*sin(sqrt(2)*x) )/9. - _G. C. Greubel_, May 24 2019
%t A138495 LinearRecurrence[{1,0,2,4}, {0,1,1,1}, 40] (* _G. C. Greubel_, May 24 2019 *)
%o A138495 (PARI) concat(0, Vec(x/((1-2*x)*(1+x)*(1+2*x^2)) + O(x^40))) \\ _Michel Marcus_, May 24 2019
%o A138495 (Magma) R<x>:=PowerSeriesRing(Integers(), 40); [0] cat Coefficients(R!( x/((1-2*x)*(1+x)*(1+2*x^2)) )); // _G. C. Greubel_, May 24 2019
%o A138495 (Sage) (x/((1-2*x)*(1+x)*(1+2*x^2))).series(x, 40).coefficients(x, sparse=False) # _G. C. Greubel_, May 24 2019
%o A138495 (GAP) a:=[0,1,1,1];; for n in [5..40] do a[n]:=a[n-1]+2*a[n-3]+4*a[n-4]; od; a; # _G. C. Greubel_, May 24 2019
%Y A138495 Cf. A138477.
%K A138495 nonn
%O A138495 0,5
%A A138495 _Paul Curtz_, May 18 2008
%E A138495 a(13) corrected by _Georg Fischer_, May 24 2019
%E A138495 More terms from _Michel Marcus_, May 24 2019