A180439 - OEIS

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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").

1, 3, 3, 3, 9, 15, 33, 63, 153, 219, 261, 351, 585, 879, 933, 1233, 1401, 1899, 2301, 3111

(list; graph; refs; listen; history; text; internal format)

OFFSET

0,2

COMMENTS

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.

LINKS

Table of n, a(n) for n=0..19.

CROSSREFS

Cf. A005598, A180238, A180239, A180437, A180438.

Sequence in context: A217457 A231753 A231663 * A298315 A299189 A231651

Adjacent sequences: A180436 A180437 A180438 * A180440 A180441 A180442

KEYWORD

nonn,more

AUTHOR

Fred Lunnon, Sep 05 2010

STATUS

approved