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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

%I

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

%N 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.

%C "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].

%H T. Harju and M. Mueller, <a href="http://arxiv.org/abs/1309.2137">Square-free shuffles of words</a>, arxiv preprint, 2013, Table 3, page 7.

%e 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.

%K nonn,more

%O 1,3

%A _Jeffrey Shallit_, May 27 2014

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Last modified December 4 02:18 EST 2020. Contains 338921 sequences. (Running on oeis4.)