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 A087440 #15 May 03 2018 13:56:38
%S A087440 1,4,13,46,172,664,2608,10336,41152,164224,656128,2622976,10488832,
%T A087440 41949184,167784448,671113216,2684403712,10737516544,42949869568,
%U A087440 171799085056,687195553792,2748780642304,10995119423488,43980471402496
%N A087440 Expansion of (1-2x-3x^2)/((1-2x)(1-4x)).
%C A087440 Binomial transform is A087439. Second binomial transform of A084221 (with extra leading 1).
%H A087440 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (6,-8).
%F A087440 a(n) = 5*4^n/8 + 3*2^n/4 - 3*0^n/8.
%F A087440 a(n) = 6*a(n-1) - 8*a(n-2), n>2. - _Harvey P. Dale_, Jan 18 2012
%F A087440 a(n) = A000217(2^n) + floor(A000217(2^(n-1))). - _J. M. Bergot_, May 03 2018
%t A087440 CoefficientList[Series[(1-2x-3x^2)/((1-2x)(1-4x)),{x,0,30}],x] (* or *) Join[{1},LinearRecurrence[{6,-8},{4,13},30]] (* _Harvey P. Dale_, Jan 18 2012 *)
%Y A087440 Cf. A000217, A084221, A087439.
%K A087440 easy,nonn
%O A087440 0,2
%A A087440 _Paul Barry_, Sep 03 2003