

A242949


Number of distinct ternary squarefree words of length 2n that can be obtained as the selfshuffle of a ternary squarefree word of length n.


1



0, 0, 6, 12, 30, 24, 42, 78, 138, 228, 396, 168, 1008
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OFFSET

1,3


COMMENTS

"squarefree" means it contains no block of the form xx, with x nonempty. A length2n word w is in the selfshuffle of a lengthn word x if there is a disjoint partition of the indices {1,2,..., 2n} into two increasing sequences of length n, say s and t, such that x = w[s] = w[t].


LINKS

Table of n, a(n) for n=1..13.
T. Harju and M. Mueller, Squarefree shuffles of words, arxiv preprint, 2013, Table 3, page 7.


EXAMPLE

a(3) = 6, as the sequences are 010212 (arising from selfshuffle of 012), and the 5 others arising from permutation of the letters 0, 1, 2.


CROSSREFS

Sequence in context: A294730 A079390 A124679 * A143272 A153877 A229491
Adjacent sequences: A242946 A242947 A242948 * A242950 A242951 A242952


KEYWORD

nonn,more


AUTHOR

Jeffrey Shallit, May 27 2014


STATUS

approved



