Ayse Karaman
In this paper, we analyze the periodic factors of Sturmian words for the findings to lead to a linear-time algorithm for the computation of runs in this class of words which, to our best knowledge, is an open problem in literature.
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For an infinite word $x$, Bugeaud and Kim introduced a quantity $\mathrm{rep}(x)$ called the exponent of repetition of $x$. We prove that $\mathrm{rep}(x) = \mathrm{rep}(y)$ holds for a Sturmian word $x$ and every suffix $y$ of $x$. Let $c$ be the characteristic Sturmian word of slope $θ$. When $θ$ is a quadratic irrational, a formula of $\mathrm{rep}(c)$ is given. This formula shows that $\mathrm{rep}(c) = \mathrm{rep}(c')$ if $c'$ is the characteristic Sturmian word whose slope is a quadratic irrational equivalent to $θ$.
By measuring second occurring times of factors of an infinite word $x$, Bugeaud and Kim introduced a new quantity ${\rm rep}(x)$ called the exponent of repetition of $x$. It was proved by Bugeaud and Kim that $1 \leq {\rm rep}(x) \leq r_{\max} = \sqrt{10} - 3/2$ if $x$ is a Sturmian words. In this paper, we determine the value $r_1$ such that there is no Sturmian word $x$ satisfying $r_1 < {\rm rep}(x) < r_{\max}$ and $r_1$ is an accumulation point of the set of ${\rm rep}(x)$ when $x$ runs over the Sturmian words.
We give a combinatorial definition of the second parameter describing Sturmian words, that we call formal intercept, as an infinite Ostrowski expansion. We recall the well-known properties of Sturmian words, study factors graphs and repetition function, and end with the definition of formal intercepts.
We investigate abelian repetitions in Sturmian words. We exploit a bijection between factors of Sturmian words and subintervals of the unitary segment that allows us to study the periods of abelian repetitions by using classical results of elementary Number Theory. We prove that in any Sturmian word the superior limit of the ratio between the maximal exponent of an abelian repetition of period $m$ and $m$ is a number $\geq\sqrt{5}$, and the equality holds for the Fibonacci infinite word. We further prove that the longest prefix of the Fibonacci infinite word that is an abelian repetition of pe…
The repetition threshold of episturmian sequences
本文研究 d 元 episturmian 序列的重复阈值，证明其由 d-bonacci 序列达到，值为 2+1/(t-1)，其中 t 为特定多项式的唯一正根。
Let p be a maximal palindrome in a Sturmian word s=ul_1pl_2v so that p is a palindrome and l_1pl_2 is not for letters l_1 and l_2. Let α(p,p') be a morphism mapping letters a and b respectively to a^pb and a^p'b, |p-p'|=1. In this paper, we characterize the palindromes in a Sturmian word and show that the number of maximal palindromes in a Sturmian word X= α(p,p')(Y) for finite Y and thus X is 2|X|-2|Y|. We show that the set of maximal palindromes in a finite Sturmian word X has the cardinality Σ i=1..n max(pi,p'i) where X is characterized by subsequent mappings of i=1..n.
Richomme, Saari and Zamboni (J. Lond. Math. Soc. 83: 79-95, 2011) proved that at every position of a Sturmian word starts an abelian power of exponent $k$ for every $k > 0$. We improve on this result by studying the maximum exponents of abelian powers and abelian repetitions (an abelian repetition is an analogue of a fractional power) in Sturmian words. We give a formula for computing the maximum exponent of an abelian power of abelian period $m$ starting at a given position in any Sturmian word of rotation angle $α$. vAs an analogue of the critical exponent, we introduce the abelian critical …
For an infinite word $\mathbf{x}$, Bugeaud and Kim introduced a new complexity function $\text{rep}(\mathbf{x})$ which is called the exponent of repetition of $\mathbf{x}$. They showed $1\le \text{rep}(\mathbf{x}) \le \sqrt{10}-\frac{3}{2}$ for any Sturmian word $\mathbf{x}$. Ohnaka and Watanabe found a gap in the set of the exponents of repetition of Sturmian words. For an irrational number $θ\in(0,1)$, let \[ \mathscr{L}(θ):=\{\text{rep}(\mathbf{x}):\textrm{$\mathbf{x}$ is an Sturmian word of slope $θ$}\}.\] In this article, we look into $\mathscr{L}(θ)$. The minimum of $\mathscr{L}(θ)$ is d…
$α$-numbers, diophantine exponent and factorisations of sturmian words
该研究引入Sturmian字的$α$-数和形式截距概念，通过Rauzy图构建形式体系，推导其一般分解公式，还计算了重复函数与丢番图指数，为Sturmian字分析提供新工具。
Infinite string bricks and Sturmian words over some gentle algebras
本文分类了双Kronecker温和代数上的无限串砖模，证明其与Sturmian词双射，并推广至更广的温和代数。
The cornerstone of any algorithm computing all repetitions in a string of length n in O(n) time is the fact that the number of runs (or maximal repetitions) is O(n). We give a simple proof of this result. As a consequence of our approach, the stronger result concerning the linearity of the sum of exponents of all runs follows easily.
We give a presentation by generators and relations of a certain monoid generating a subgroup of index two in the group Aut(F_2) of automorphisms of the rank two free group F_2 and show that it can be realized as a monoid in the group B_4 of braids on four strings. In the second part we use Christoffel words to construct an explicit basis of F_2 lifting any given basis of the free abelian group Z^2. We further give an algorithm allowing to decide whether two elements of F_2 form a basis or not. We also show that, under suitable conditions, a basis has a unique conjugate consisting of two palind…
Let $A$ be an $a$-letter alphabet. We consider fractional powers of $A$-strings: if $x$ is a $n$-letter string, $x^r$ is a prefix of $xxxx...$ having length $nr$. Let $l$ be a positive integer. Ilie, Ochem and Shallit defined $R(a,l)$ as the infimum of reals $r>1$ such that there exist a sequence of $A$-letters without factors (substrings) that are fractional powers $x^{r'}$ where $x$ has length at least $l$ and $r'\ge r$. We prove that $1+\frac{1}{la}\le R(a,l)\le 1+\frac{c}{la}$ for some constant $c$.
String attractors and bi-infinite words
本文刻画了具有有限字符串吸引子的双向无限词为特征Sturmian词及其态射像，并研究了无限字符串吸引子的结构性质。
Optimal Bounds for Distinct Quartics
本研究解决了二维字符串中不同四次方的渐近紧界问题，证明其数量为$\u00a0(n^2 \u00a0 n)$，并给出了最坏情况下最优的$\u00a0(n^2 \u00a0 n)$时间计算算法，完善了高维字符串重复结构的研究。
If $x$ is a non-empty string then the repetition $xx$ is called a tandem repeat. Similarly, a tandem in a two dimensional array $X$ is a configuration consisting of a same primitive block $W$ that touch each other with one side or corner. In \cite{Apostolico:2000}, Apostolico and Brimkov have proved various bounds for the number of tandems in a two dimensional word of size $m \times n$. Of the two types of tandems considered therein, they also proved that, for one type, the number of occurrences in an $m \times n$ Fibonacci array attained the general upper bound, $\mathcal{O}(m^{2}n \hspace{0.…
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