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diff --git a/posts/2015-05-30-infinite-binary-words-containing-repetitions-odd-periods.md b/posts/2015-05-30-infinite-binary-words-containing-repetitions-odd-periods.md deleted file mode 100644 index 8f2ff8e..0000000 --- a/posts/2015-05-30-infinite-binary-words-containing-repetitions-odd-periods.md +++ /dev/null @@ -1,94 +0,0 @@ ---- -template: oldpost -title: AMS review of 'Infinite binary words containing repetitions of odd period' by Badkobeh and Crochemore -date: 2015-05-30 -comments: true -archive: false ---- -This paper is about the existence of pattern-avoiding infinite binary -words, where the patterns are squares, cubes and \\(3^+\\)-powers. - There are mainly two kinds of results, positive (existence of an -infinite binary word avoiding a certain pattern) and negative -(non-existence of such a word). Each positive result is proved by the -construction of a word with finitely many squares and cubes which are -listed explicitly. First a synchronising (also known as comma-free) -uniform morphism \\(g\\: \\Sigma_3^\* \\to -\\Sigma_2^\*\\) - -is constructed. Then an argument is given to show that the length of -squares in the code \\(g(w)\\) -for a squarefree \\(w\\) is bounded, hence all the squares can be obtained by examining all \\(g(s)\\) for \\(s\\) -of bounded lengths. The argument resembles that of the proof of, e.g., -Theorem 1, Lemma 2, Theorem 3 and Lemma 4 in \[N. Rampersad, J. O. -Shallit and M. Wang, Theoret. Comput. Sci. **339** -(2005), no. 1, 19–34; -[MR2142071](http://www.ams.org/mathscinet/search/publdoc.html?r=1&pg1=MR&s1=2142071&loc=fromrevtext)\]. -The negative results are proved by traversing all possible finite words -satisfying the conditions. - - Let \\(L(n_2, n_3, S)\\) be the maximum length of a word with \\(n_2\\) distinct squares, \\(n_3\\) -distinct cubes and that the periods of the squares can take values only -in \\(S\\) -, where \\(n_2, n_3 \\in \\Bbb N \\cup -\\{\\infty, \\omega\\}\\) -and \\(S \\subset \\Bbb N_+\\) -. - \\(n_k = 0\\) -corresponds to \\(k\\)-free, \\(n_k = \\infty\\) -means no restriction on the number of distinct \\(k\\)-powers, and \\(n_k = \\omega\\) -means \\(k^+\\)-free. - - Below is the summary of the positive and negative results: - -(1) (Negative) \\(L(\\infty, \\omega, 2 \\Bbb N) - < \\infty\\) - : \\(\\nexists\\) - an infinite \\(3^+\\) - -free binary word avoiding all squares of odd periods. - (Proposition 1) - -(2) (Negative) \\(L(\\infty, 0, 2 \\Bbb N + 1) \\le - 23\\) - : \\(\\nexists\\) - an infinite 3-free binary word, avoiding squares of even periods. - The longest one has length \\(\\le 23\\) - (Proposition 2). - -(3) (Positive) \\(L(\\infty, \\omega, 2 \\Bbb N + - 1) = \\infty\\) - : \\(\\exists\\) - an infinite \\(3^+\\) - -free binary word avoiding squares of even periods (Theorem 1). - -(4) (Positive) \\(L(\\infty, \\omega, \\{1, 3\\}) = - \\infty\\) - : \\(\\exists\\) - an infinite \\(3^+\\) - -free binary word containing only squares of period 1 or 3 - (Theorem 2). - -(5) (Negative) \\(L(6, 1, 2 \\Bbb N + 1) = - 57\\) - : \\(\\nexists\\) - an infinite binary word avoiding squares of even period containing - \\(< 7\\) - squares and \\(< 2\\) - cubes. The longest one containing 6 squares and 1 cube has length 57 - (Proposition 6). - -(6) (Positive) \\(L(7, 1, 2 \\Bbb N + 1) = - \\infty\\) - : \\(\\exists\\) - an infinite \\(3^+\\) - -free binary word avoiding squares of even period with 1 cube and 7 - squares (Theorem 3). - -(7) (Positive) \\(L(4, 2, 2 \\Bbb N + 1) = - \\infty\\) - : \\(\\exists\\) - an infinite \\(3^+\\) - -free binary words avoiding squares of even period and containing 2 - cubes and 4 squares (Theorem 4). - - -Copyright notice: This review is published at http://www.ams.org/mathscinet-getitem?mr=3313467, its copyright owned by the AMS. |