Avoiding Circular Repetitions Hamoon Mousavi and Jeffrey Shallit - PowerPoint PPT Presentation

Avoiding Circular Repetitions Hamoon Mousavi and Jeffrey Shallit School of Computer Science University of Waterloo sh2mousa@uwaterloo.ca Canadian Discrete and Algorithmic Mathematics Conference CanaDAM June 10, 2013 H. Mousavi (University of Waterloo) Circular Repetitions June 10 1 / 31

Avoidability The seminal paper of Axel Thue (1906). Thue asked the existence of an infinite word over the alphabet { a , b , c } that avoids squares , i.e., factors of the form xx . Constructed a squarefree word by iterating the following morphism a → abcab b → acabcb c → acbcacb Iterating it gives us a → abcab → abcabacabcbacbcacbabcabacabcb → · · · Ternary alphabet is the smallest with an infinite squarefree word. H. Mousavi (University of Waterloo) Circular Repetitions June 10 2 / 31

Various themes Is the pattern ABCBABC avoidable? Is it 3 -avoidable ? Smallest n for which P is n -avoidable? Decidability Over circular words (necklaces) Smallest avoidable exponent (repetition threshold) H. Mousavi (University of Waterloo) Circular Repetitions June 10 3 / 31

Repetitions k k-power : x k = � �� � xx · · · x x 2 is 3-avoidable but not 2-avoidable (Thue). α -power : y = x ⌊ α ⌋ x ′ such that | y | | x | = α . We then write y = x α . Examples hotshots = ( hots ) 2 7 alfalfa = ( alf ) 2 a = ( alf ) 3 Definition w is α -power-free if none of its factors is a β -power for any β ≥ α . w is α + -power-free if none of its factors is a β -power for any β > α . H. Mousavi (University of Waterloo) Circular Repetitions June 10 4 / 31

Conjugation Two words x , y are conjugate if one is a cyclic shift of the other. That is if there exist words u , v such that x = uv and y = vu . ‘ bookcase ′ and ‘ casebook ′ . One simple observation is that all conjugates of a k -power are k -powers. abcabc = ( abc ) 2 cabcab = ( cab ) 2 H. Mousavi (University of Waterloo) Circular Repetitions June 10 5 / 31

Circular repetition Observation w is α -power-free if for every factor x w = x x is α -power-free Definition w is circularly α -power-free if for every factor x w = x x and all its conjugates are α -power-free . H. Mousavi (University of Waterloo) Circular Repetitions June 10 6 / 31

Circular repetition: example Example w = dividing x = dividi a conjugate of x is vididi 5 which has a 5 2 -power: ididi = ( id ) 2 So w is not circularly 5 2 -power-free. In fact, w is circularly ( 5 2 ) + -power-free. H. Mousavi (University of Waterloo) Circular Repetitions June 10 7 / 31

Circular repetition w is circularly α -power-free if for every pair of factors x and y y w = x yx is α -power-free. ( x , y ) is a circular α -power if yx is α -power. Example w = g v n d i i d i y x 5 yx = ididi = ( id ) 2 Hence ( x , y ) is a circular 5 2 -power. H. Mousavi (University of Waterloo) Circular Repetitions June 10 8 / 31

Repetition threshold Definition The repetition threshold , RT( n ), is the smallest α for which there exists an infinite α + -power-free word over Σ n . Example Dejean proved RT(3) = 7 4 by proving 1 there are only finitely many 7 4 -power-free words. 2 there are infinite ( 7 4 ) + -power-free words. ν ( a ) = abcacbcabcbacbcacba , ν ( b ) = bcabacabcacbacabacb , ν ( c ) = cabcbabcabacbabcbac . � �� � ν ω ( a ) = a bcacbca bcbacbcacba · · · H. Mousavi (University of Waterloo) Circular Repetitions June 10 9 / 31

Dejean’s conjecture Dejean’s conjecture Thue, Dejean, Pansiot, Moulin Ollagnier, Carpi, Currie, Mohammad- Noori, Rampersad, and Rao:  7 4 , if n = 3;   7 RT( n ) = if n = 4; 5 ,   n n − 1 , if n � = 3 , 4 . H. Mousavi (University of Waterloo) Circular Repetitions June 10 10 / 31

Repetition threshold for circular factors Definition The repetition threshold for circular factors , RTC( n ), is the smallest α for which there exists an infinite circularly α + -power-free word over Σ n . RT( n ) RTC( n ) n 2 2 4 7 13 3 4 4 7 5 4 5 2 5 105 5 4 46 6 1 + 6 5 = 11 6 5 6 . . . . . . . . . k 1 + RT( k ) = 2 k − 1 k k − 1 k − 1 H. Mousavi (University of Waterloo) Circular Repetitions June 10 11 / 31

Bounds on RTC( n ) n = 2 n = 3 n = 4 n = 5 n = 6 n = 7 n = 8 . . . 1 + RT( n ) 2 RT( n ) Theorem 1 + RT( n ) ≤ RTC( n ) ≤ 2 RT( n ) H. Mousavi (University of Waterloo) Circular Repetitions June 10 12 / 31

Thue-Morse word and RTC(2) Thue morphism h (0) = 01 h (1) = 10 . The Thue-Morse word t = h ω (0) = 01101001 · · · is 2 + -power-free. Theorem t is circularly 4 + -power-free. H. Mousavi (University of Waterloo) Circular Repetitions June 10 13 / 31

Thue-Morse word and RTC(2) Theorem t is circularly 4 + -power-free. Proof. Suppose ( x , y ) is a circular 4 + -power of t , i.e., y x t = · · · and yx is a 4 + -power. Then either y or x is a 2 + -power, a contradiction. H. Mousavi (University of Waterloo) Circular Repetitions June 10 14 / 31

RTC(2) = 4 Theorem RTC(2) = 4 . Proof. Since t is circularly 4 + -power-free, we have RTC(2) ≤ 4 . No binary word of length 12 is circularly 4-power-free, so RTC(2) ≥ 4 . H. Mousavi (University of Waterloo) Circular Repetitions June 10 15 / 31

RTC(3) = 13 4 Overview of the proof: A finite search shows that RTC(3) ≥ 13 4 . So to prove RTC(3) = 13 4 , we just need to construct an infinite word that is circularly ( 13 4 ) + -power-free. We give a pair of morphisms: ψ : Σ ∗ 6 → Σ ∗ 6 µ : Σ ∗ 6 → Σ ∗ 3 We prove µ ( ψ ω (0)) is circularly ( 13 4 ) + -power-free. H. Mousavi (University of Waterloo) Circular Repetitions June 10 16 / 31

Pair of morphisms ψ (0) = 0435 ψ (1) = 2341 ψ (2) = 3542 ψ (3) = 3540 ψ (4) = 4134 ψ (5) = 4105 . µ (0) = 012102120102012 µ (1) = 201020121012021 µ (2) = 012102010212010 µ (3) = 201210212021012 µ (4) = 102120121012021 µ (5) = 102010212021012 . H. Mousavi (University of Waterloo) Circular Repetitions June 10 17 / 31

µ ( ψ ω (0)) Theorem µ ( ψ ω (0)) is circularly ( 13 4 ) + -power-free. Proof idea The proof has two parts 1 r = ψ ω (0) is circularly cubefree. 2 s = µ ( r ) is circularly ( 13 4 ) + -power-free. s has no short circular ( 13 4 ) + -power. (This is checked by computer) 1 s has no long circular ( 13 4 ) + -power. 2 H. Mousavi (University of Waterloo) Circular Repetitions June 10 18 / 31

µ is well-behaved! µ : Σ ∗ 6 → Σ ∗ 3 is 15 -uniform | µ ( a ) | = 15 for all a ∈ Σ 6 . µ is synchronizing , i.e., for no a , b , c ∈ Σ 6 µ ( a ) µ ( b ) µ ( c ) µ is strongly synchronizing , i.e., for all a , b , c ∈ Σ 6 and x , y ∈ Σ ∗ 3 if µ ( a ) = x y µ ( b ) = y µ ( c ) = x either c = a or c = b . H. Mousavi (University of Waterloo) Circular Repetitions June 10 19 / 31

Main lemma Lemma Let φ be a strongly synchronizing q-uniform morphism. Let w be a circularly cubefree word. If ( x 1 , x 2 ) is a circular ( 13 4 ) + -power in φ ( w ) , i.e., φ ( w ) = x 1 x 2 x 2 x 1 is ( 13 4 ) + -power, then | x 2 x 1 | < 22 q . H. Mousavi (University of Waterloo) Circular Repetitions June 10 20 / 31

Main lemma: proof Proof is by contradiction. Suppose ( x 1 , x 2 ) is a circular ( 13 4 ) + -power of φ ( w ), and | x 2 x 1 | ≥ 22 q . x 1 x 2 φ ( w ) =

Main lemma: proof Proof is by contradiction. Suppose ( x 1 , x 2 ) is a circular ( 13 4 ) + -power of φ ( w ), and | x 2 x 1 | ≥ 22 q . x 1 x 2 φ ( w ) = φ ( a ) φ ( w 1 ) φ ( b ) φ ( c ) φ ( w 2 ) φ ( d ) w = a w 1 c w 2 b d H. Mousavi (University of Waterloo) Circular Repetitions June 10 21 / 31

Main lemma: proof y 1 y 2 y 3 y 4 x 1 x 2 φ ( w ) = φ ( a ) φ ( w 1 ) φ ( b ) φ ( c ) φ ( w 2 ) φ ( d ) x 1 = y 1 y 2 φ ( w 1 ) x 2 = y 3 y 4 φ ( w 2 ) z α = x 2 x 1 = y 3 φ ( w 2 ) y 4 y 1 φ ( w 1 ) y 2 Here z is a word and α > 13 4 . There are two cases to consider: 1 y 4 y 1 = ǫ 2 y 4 y 1 � = ǫ H. Mousavi (University of Waterloo) Circular Repetitions June 10 22 / 31

Main lemma: case 1 Case 1 is y 4 y 1 = ǫ . Therefore we have x 1 = φ ( w 1 ) y 2 , x 2 = y 3 φ ( w 2 ) . y 2 y 3 x 1 x 2 φ ( w ) = φ ( w 1 ) φ ( b ) φ ( c ) φ ( w 2 ) z α = x 2 x 1 = y 3 y 2 = y 3 y 2 φ ( w 2 ) φ ( w 1 ) φ ( w 2 w 1 ) Note that α > 13 4 = 3 . 25. We get that φ ( w 2 w 1 ) contains a cube. w 2 w 1 contains a cube. (synchronizing) w contains a circular cube, a contradiction. H. Mousavi (University of Waterloo) Circular Repetitions June 10 23 / 31

Main lemma: case 2 x 1 x 2 φ ( w ) = φ ( a ) φ ( w 1 ) φ ( b ) φ ( c ) φ ( w 2 ) φ ( d ) We would like to show that by shrinking x 1 and enlarging x 2 we can get another circular ( 13 4 ) + -power of the same length: x 1 x 2 φ ( w ) = φ ( a ) φ ( w 1 ) φ ( b ) φ ( c ) φ ( w 2 ) φ ( d ) x ′ x ′ 1 2 Then clearly ( x ′ 1 , x ′ 2 ) falls under case 1. H. Mousavi (University of Waterloo) Circular Repetitions June 10 24 / 31

Main lemma: case 2 Goal: x ′ 2 x ′ 1 = x 2 x 1 y 1 y 2 y 3 y 4 x 1 x 2 φ ( w ) = φ ( a ) φ ( w 1 ) φ ( b ) φ ( c ) φ ( w 2 ) φ ( d ) x ′ x ′ 1 2 H. Mousavi (University of Waterloo) Circular Repetitions June 10 25 / 31