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 A117856 #22 Sep 13 2024 00:39:27
%S A117856 3,3,12,12,48,48,192,192,768,768,3072,3072,12288,12288,49152,49152,
%T A117856 196608,196608,786432,786432,3145728,3145728,12582912,12582912,
%U A117856 50331648,50331648,201326592,201326592,805306368,805306368,3221225472,3221225472,12884901888
%N A117856 Number of palindromes of length n (in base 4).
%H A117856 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (0,4).
%F A117856 a(n) = 3*4^floor((n-1)/2).
%F A117856 From _Colin Barker_, Feb 15 2013: (Start)
%F A117856 a(n) = 4*a(n-2).
%F A117856 G.f.: -3*x*(x+1) / ((2*x-1)*(2*x+1)). (End)
%t A117856 NestList[4#&,{3,3},20]//Flatten (* _Harvey P. Dale_, Dec 19 2016 *)
%Y A117856 Cf. A050683.
%K A117856 nonn,base,easy
%O A117856 1,1
%A A117856 _Martin Renner_, May 02 2006
%E A117856 More terms from _Colin Barker_, Feb 15 2013