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 A180239 a(n) counts the distinct billiard words with length n on alphabet of 4 symbols. 5
 1, 4, 16, 64, 244, 856, 2776, 8356, 23032, 59200, 142624, 324484, 696256, 1422436, 2779900, 5219452, 9455596 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Computation: Fred Lunnon for n <= 16 (MAGMA). LINKS Fred Lunnon, MAGMA Program Jean-Pierre Borel, A geometrical Characterization of factors of multidimensional Billiards words and some Applications, Theoretical Computer Science 380 (2007) 286--303 Laurent Vuillon, Balanced Words, Bull. Belg. Math. Soc. 10 (2003), 787-805. FORMULA Expensive linear programming inequality analysis may be reduced by projecting each candidate word onto the axis hyperplanes, yielding m new (m-1)-symbol words which are necessarily also billiard, and can be validated from a precomputed list for dimension m-1. If any of these fails, the candidate fails; and if only one candidate remains after n-th symbols are attached to a valid (n-1)-length word, there is still no need for inequality analysis --- the ball cannot avoid bouncing next against some wall pair! EXAMPLE For n = 5 there are a(5) = 856 words, permutations on {1,2,3,4} of the 42 words 11111, 11112, 11121, 11123, 11211, 11212, 11213, 11231, 11234, 12111, 12112, 12113, 12121, 12122, 12123, 12131, 12132, 12134, 12212, 12213, 12221, 12222, 12223, 12231, 12232, 12234, 12311, 12312, 12313, 12314, 12321, 12322, 12323, 12324, 12331, 12332, 12333, 12334, 12341, 12342, 12343, 12344. CROSSREFS See A005598 for 2 symbols, A180238 for 3 symbols. Sequence in context: A228735 A289694 A232425 * A006811 A269685 A269614 Adjacent sequences:  A180236 A180237 A180238 * A180240 A180241 A180242 KEYWORD nonn AUTHOR Fred Lunnon, Aug 18 2010 STATUS approved

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Last modified September 28 17:43 EDT 2020. Contains 337393 sequences. (Running on oeis4.)