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 A121981 #15 Mar 31 2012 10:25:30
%S A121981 1,1,3,73,5056783
%N A121981 Number of finite maximal bifix codes of degree n on a two-letter alphabet.
%C A121981 A bifix (sometimes biprefix) code is a set of nonempty words X such that no word of X is a proper prefix or a proper suffix of another. The degree of a finite maximal bifix code X is the maximal number of parses that a word can have with respect to X.
%C A121981 It is known that there are finitely many finite maximal bifix codes of each degree.
%D A121981 J. Berstel and D. Perrin, Theory of Codes, Academic Press, 1985, Chapter III.
%e A121981 On the alphabet {a,b}, for n=3 the a(3)=3 codes are:
%e A121981 {aaa,aab,aba,abb,baa,bab,bba,bbb},
%e A121981 {aaa,aaba,aabb,ab,baa,baba,babb,bba,bbb},
%e A121981 {aaa,aab,abaa,abab,abb,ba,bbaa,bbab,bbb}
%K A121981 nonn,hard
%O A121981 1,3
%A A121981 _Alessandro De Luca_, Feb 09 2011