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

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