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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").

0, 0, 6, 18, 36, 78, 150, 306, 420, 792, 1338, 2082, 3228, 4830, 7050, 9954, 13920, 18738, 24666, 32610

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OFFSET

0,3

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, A180439.

Sequence in context: A116367 A225384 A276480 * A202366 A185223 A299272

Adjacent sequences: A180435 A180436 A180437 * A180439 A180440 A180441

KEYWORD

nonn,more

AUTHOR

Fred Lunnon, Sep 05 2010

STATUS

approved