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 A140295 #13 Dec 18 2023 08:53:57 %S A140295 1,-2,4,4,4,16,28,52,112,220,436,880,1756,3508,7024,14044,28084,56176, %T A140295 112348,224692,449392,898780,1797556,3595120,7190236,14380468, %U A140295 28760944,57521884,115043764,230087536,460175068,920350132,1840700272,3681400540 %N A140295 a(n) = a(n-1) + a(n-2) + 2a(n-3). %H A140295 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (1,1,2). %F A140295 a(n+1) - 2a(n) = period 3: repeat -4, 8, -4. %F A140295 G.f.: (1-3x+5x^2)/((1-2x)(1+x+x^2)). a(n)=(4*k(n)+3*2^n)/7 where k(n) is the 3-period sequence 1,-5,4,... reminiscent to A130815. [From _R. J. Mathar_, Oct 30 2008] %K A140295 sign,easy %O A140295 0,2 %A A140295 _Paul Curtz_, May 25 2008