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 A180439 #14 Jan 16 2024 14:07:49 %S A180439 1,3,3,3,9,15,33,63,153,219,261,351,585,879,933,1233,1401,1899,2301, %T A180439 3111 %N A180439 a(n) counts the distinct cubical (on alphabet of 3 symbols) billiard words with length n, acting as prefix to just k = 3 such words of length n+1 (that is, a subset of "special"). %C A180439 By symmetry under reversal, a(n) also counts length n cubical billiard words acting as suffix to just k length n+1 cubical billiard words. Computation: _Fred Lunnon_ for 0 <= n <= 19 (Magma). The program in A180437 counts k-special words for k = 1,...,m, where m = 3 denotes the size of the alphabet. %Y A180439 Cf. A005598, A180238, A180239, A180437, A180438. %K A180439 nonn,more %O A180439 0,2 %A A180439 _Fred Lunnon_, Sep 05 2010