By Jean Berstel, Aaron Lauve, Christophe Reutenauer, and Franco V. Saliola

ISBN-10: 0821844806

ISBN-13: 9780821844809

The 2 components of this article are in accordance with sequence of lectures brought by way of Jean Berstel and Christophe Reutenauer in March 2007 on the Centre de Recherches Mathematiques, Montreal, Canada. half I represents the 1st smooth and complete exposition of the idea of Christoffel phrases. half II provides quite a few combinatorial and algorithmic elements of repetition-free phrases stemming from the paintings of Axel Thue--a pioneer within the concept of combinatorics on phrases. A newbie to the speculation of combinatorics on phrases might be prompted via the various examples, and the massive number of workouts, which make the ebook specified at this point of exposition. The fresh and streamlined exposition and the large bibliography can be favored. After studying this e-book, newcomers might be able to learn sleek study papers during this speedily transforming into box and give a contribution their very own study to its improvement. skilled readers might be drawn to the finitary method of Sturmian phrases that Christoffel phrases supply, in addition to the radical geometric and algebraic technique selected for his or her exposition. they'll additionally take pleasure in the old presentation of the Thue-Morse be aware and its functions, and the unconventional effects on Abelian repetition-free phrases.

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**Extra resources for Combinatorics on words: Christoffel words and repetitions in words**

**Example text**

Suppose |ad − bc| = 1. Then (1, 0) and (0, 1) are in the span of (a, b) and (c, d). Indeed, d(a, b) − b(c, d) = (ad − bc, 0) = ±(1, 0) and a(c, d) − c(a, b) = ±(0, 1). Thus (a, b) and (c, d) generate Z2 . Conversely, suppose (a, b) and (c, d) generate Z2 . Then the matrix equation a c x=b b d has a unique solution x ∈ Z2 for all vectors b ∈ Z2 . For b = (0, 1)T , we have x= a c b d −1 0 1 = 1 ad − bc d −c −b a 0 1 = 1 ad − bc −c a ∈ Z2 . It follows that |ad − bc| divides |a| and |c|. Since (a, b) and (c, d) generate Z2 , there exists i, j ∈ Z such that i(a, b) + j(c, d) = (1, 0).

Example. 6, marching upwards from the vertex (xxxy, xxxyxxy) to the root (x, y). 26 CHAPTER 3. STANDARD FACTORIZATION some Christoffel words w ... 6: Paths in the Christoffel tree from (u, v) to the root (x, y) preserve the cutting points for standard factorizations. Note that we have found a characterization of those Christoffel morphisms that preserve Christoffel words. Namely, f : (x, y) → (w1 , w2 ) is such a morphism if and only if (w1 , w2 ) is a standard factorization of a Christoffel word.

1. u = Pal(v) for some v ∈ {x, y}∗ if and only if xuy is a Christoffel word. 2. u = Pal(v) for some v ∈ {x, y}∗ if and only if u has relatively prime periods p and q and |u| = p + q − 2. Proof of 1. 6, if u = Pal(v) then xuy is a Christoffel word. Conversely, let w = xuy be a Christoffel word and let (w1 , w2 ) be its standard factorization. 4, |w1 |x |w2 |x |w1 |y |w2 |y ∈ SL2 (Z) ∩ N2×2 38 CHAPTER 4. PALINDROMIZATION (writing N2×2 for the set of 2 × 2 matrices with nonnegative integer entries).

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