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 A138231 #10 Oct 15 2017 19:15:14
%S A138231 0,1,1,1,2,0,2,-2,0,-4,-4,-4,-8,0,-8,8,0,16,16,16,32,0,32,-32,0,-64,
%T A138231 -64,-64,-128,0,-128,128,0,256,256,256,512,0,512,-512,0,-1024,-1024,
%U A138231 -1024,-2048,0,-2048,2048,0,4096,4096,4096,8192,0,8192,-8192,0,-16384,-16384
%N A138231 A009545 alternated with its first differences.
%C A138231 The unsigned sequence contains 3 copies of the strictly positive powers of 2.
%H A138231 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (0, 2, 0, -2).
%F A138231 a(2n)= A009545(n). a(2n+1)= A009545(n+1)-A009545(n) = (-1)^(n+1)*A009116(n).
%F A138231 a(n) = 2a(n-2)-2a(n-4). a(n) = -4a(n-8), n > 7.
%F A138231 O.g.f.: x(1+x-x^2)/(1-2x^2+2x^4). - _R. J. Mathar_, Jul 08 2008
%t A138231 With[{c=LinearRecurrence[{2,-2},{0,1},40]},Riffle[c,Differences[c]]] (* or *) LinearRecurrence[{0,2,0,-2},{0,1,1,1},80] (* _Harvey P. Dale_, Oct 15 2017 *)
%Y A138231 Cf. A000079.
%K A138231 sign
%O A138231 0,5
%A A138231 _Paul Curtz_, May 05 2008
%E A138231 Edited by _R. J. Mathar_, Jul 08 2008