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A242949 Number of distinct ternary squarefree words of length 2n that can be obtained as the self-shuffle of a ternary squarefree word of length n. 1
0, 0, 6, 12, 30, 24, 42, 78, 138, 228, 396, 168, 1008 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

"squarefree" means it contains no block of the form xx, with x nonempty.  A length-2n word w is in the self-shuffle of a length-n 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, Square-free shuffles of words, arxiv preprint, 2013, Table 3, page 7.

EXAMPLE

a(3) = 6, as the sequences are 010212 (arising from self-shuffle 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

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Last modified January 22 15:57 EST 2019. Contains 319364 sequences. (Running on oeis4.)