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 A139784 #8 Jun 13 2015 00:52:37
%S A139784 0,0,0,0,1,2,4,8,19,38,76,152,315,630,1260,2520,5083,10166,20332,
%T A139784 40664,81499,162998,325996,651992,1304667,2609334,5218668,10437336,
%U A139784 20877403,41754806,83509612,167019224,334049371,668098742,1336197484,2672394968
%N A139784 a(4n+1)=2a(4n), a(4n+2)=2a(4n+1), a(4n+3)=2a(4n+2), a(4n+4)=2a(4n+3)+A007583(n).
%C A139784 a(0), a(4), a(8) and a(12) are the first terms of A111420. - _R. J. Mathar_, May 18 2008
%H A139784 <a href="/index/Rec#order_09">Index entries for linear recurrences with constant coefficients</a>, signature (2, 0, 0, 5, -10, 0, 0, -4, 8).
%F A139784 a(4n+s) = 2^s*(-1/45+7*16^n/90-4^n/18), s=0,1,2,3. - _R. J. Mathar_, May 18 2008
%F A139784 a(0)=0, a(1)=0, a(2)=0, a(3)=0, a(4)=1, a(5)=2, a(6)=4, a(7)=8, a(8)=19, a(n)=2*a(n-1)+5*a(n-4)-10*a(n-5)-4*a(n-8)+8*a(n-9). - _Harvey P. Dale_, Jan 21 2013
%t A139784 LinearRecurrence[{2,0,0,5,-10,0,0,-4,8},{0,0,0,0,1,2,4,8,19},40] (* _Harvey P. Dale_, Jan 21 2013 *)
%Y A139784 Cf. a(0), a(4), a(8), a(12) in A111420.
%K A139784 nonn
%O A139784 0,6
%A A139784 _Paul Curtz_, May 16 2008
%E A139784 Edited and extended by _R. J. Mathar_, May 18 2008