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 A180438 #9 Sep 08 2022 08:45:54 %S A180438 0,0,6,18,36,78,150,306,420,792,1338,2082,3228,4830,7050,9954,13920, %T A180438 18738,24666,32610 %N A180438 a(n) counts the distinct cubical (on alphabet of 3 symbols) billiard words with length n, acting as prefix to just k = 2 such words of length n+1 (that is, a subset of "special"). %C A180438 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 A180438 Cf. A005598, A180238, A180239, A180437, A180439. %K A180438 nonn,more %O A180438 0,3 %A A180438 _Fred Lunnon_, Sep 05 2010