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From squares in words to squares in permutations Sergey Kitaev Reykjavík University.

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Presentation on theme: "From squares in words to squares in permutations Sergey Kitaev Reykjavík University."— Presentation transcript:

1 From squares in words to squares in permutations Sergey Kitaev Reykjavík University

2 This talk is related to combinatorics on words, a very powerful discipline. 2111=-12+ twoeleven = twelve For example, imaging you have to prove the following identity. Proving it is almost trivial using combinatorics on words!

3 A word is a finite or infinite sequence of symbols (letters) taken from a (usually finite) set called alphabet. A factor of a word w is a block of consecutive letters in w. Example: w = abca has 9 distinct factors {a,b,c,ab,bc,ca,abc,bca,abca} A square is a word of the form XX. Example: abcabc, aa, aaaa, baba are squares, whereas abcbca, ab, bab are not squares

4 To avoid squares means not to contain them as factors. Example: abaac, 2009, do not avoid squares, whereas ABA, eFGFeG, 234 avoid squares. Squares (or any set of prohibited words) are avoidable if there exists an infinite sequence avoiding them. avoidablenon-avoidable 1 letter+ 2 letters+ 3 letters+

5 Axel Thue (1906) a abc b ac c b Iterate the following morphism to obtain a square-free sequence abcacbabcbac a abcab b acabcb c acbcacb Other morphisms doing the same job: (Thue, 1912) a abcab b abcbac c abcacbc (Thue, 1912) a abcbacbcabcba b bcacbacabcacb c cabacbabcabac (Leech, 1957)

6 Axel Thue (1906) a abc b ac c b Iterate the following morphism to obtain a square-free sequence abcacbabcbac (n-1)n Other morphisms doing the same job: (Arshon, 1937) n1 n n12...(n-1)(n-2)... 1 n(n-1) n n (n-2)(n-1)...21n... odd positions even positions

7 How many square-free words of length n do we have over k letter alphabet? A tough question! For k=3 the asymptotic answer to the question is 3 cn(1-e ) n where < c < and e → when n →. ∞ 0 n Kolpakov, 2006Ochem, Reix, 2006

8 Clearly, any permutation is a square-free word. We use one line notation while talking on permutations. Examples of permutations: 25341, 321, , etc. However, we define squares in permutations differently. We say that, e.g., 4257 forms the pattern A permutation is square-free if no two consecutive factors of length at least 2 are equal as patterns. Example: contains the square 4615; is square-free. in pattern form 12-12

9 .....

10 Question 1: Is the number of square-free permutations finite or infinite? Question 2: If it is infinite, then how does the number of n-permutations grow with growing n? We answer both questions above using a “hot dog”-like construction. An important structural observation:... A B C D E F G H... a segment in a square-free permutation Schematically any square-free permutation looks like this:

11 Any permutation! Any square-free permutation!

12

13

14 Resulting square-free permutation is

15 Asymptotic enumeartion n/2 n/4

16 Asymptotic enumeartion n/2 n/4 coincides with the asymptotics for n!

17 Asymptotic enumeartion n/2 n/4

18 Directions of further reseach Improving upper bound Connection to permutation patterns avoidance theory: to have a shape above, a permutation must avoid 12 consecutive patterns: 1234, 4321, 2143, 3412, 3142, 2413, 4231, 1324, 4132, 2314, 1423, and 3241.

19 Directions of further reseach Improving lower bound

20 Directions of further reseach Improving lower bound Resulting square-free permutation is

21 Thank you for your attention!


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