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 A094559 #8 Aug 19 2019 10:10:37 %S A094559 0,0,4,16,76,304,1264,5056,20404,81616,327184,1308736,5237776, %T A094559 20951104,83815744,335262976,1341097036,5364388144,21457733104, %U A094559 85830932416,343324451056,1373297804224,5493194102464,21972776409856,87891117178864 %N A094559 Number of words of length n over an alphabet of size 4 that are not "bifix-free". %F A094559 Equals 4^n - A019309(n). %F A094559 a(0)=a(1)=0, a(2n)=4^n + 4a(2n-1) - a(n), a(2n+1)=4a(2n). - _Emeric Deutsch_, Feb 04 2006 %p A094559 a:=proc(n) if n=0 or n=1 then 0 elif n mod 2 = 0 then 4*a(n-1)-a(n/2)+4^(n/2) else 4*a(n-1) fi end: seq(a(n),n=0..28); # _Emeric Deutsch_, Feb 04 2006 %Y A094559 See A019308, A019309 and A003000 for much more information. Cf. A094578. %K A094559 nonn,easy %O A094559 0,3 %A A094559 _N. J. A. Sloane_, Jun 06 2004 %E A094559 More terms from _Emeric Deutsch_, Feb 04 2006