Crucial words and the complexity of some extremal problems for sets of prohibited words
Journal of Combinatorial Theory Series A
Crucial Words for Abelian Powers
DLT '09 Proceedings of the 13th International Conference on Developments in Language Theory
A note on efficient computation of all Abelian periods in a string
Information Processing Letters
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Let k=2 be an integer. An abeliankth power is a word of the form X"1X"2...X"k where X"i is a permutation of X"1 for 2@?i@?k. A word W is said to be crucial with respect to abelian kth powers if W avoids abelian kth powers, but Wx ends with an abelian kth power for any letter x occurring in W. Evdokimov and Kitaev (2004) [2] have shown that the shortest length of a crucial word on n letters avoiding abelian squares is 4n-7 for n=3. Furthermore, Glen et al. (2009) [3] proved that this length for abelian cubes is 9n-13 for n=5. They have also conjectured that for any k=4 and sufficiently large n, the shortest length of a crucial word on n letters avoiding abelian kth powers, denoted by @?"k(n), is k^2n-(k^2+k+1). This is currently the best known upper bound for @?"k(n), and the best known lower bound, provided in Glen et al., is 3kn-(4k+1) for n=5 and k=4. In this note, we improve this lower bound by proving that for n=2k-1, @?"k(n)=k^2n-(2k^3-3k^2+k+1); thus showing that the aforementioned conjecture is true asymptotically (up to a constant term) for growing n.