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arXiv 1408.0451math.COcs.DM
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广义梯形词

Generalized trapezoidal words

Amy Glen, Florence Levé

摘要:

有限或无限字$w$的因子复杂度函数$C_w(n)$对每个$n \ge 0$的长度为$n$的$w$的不同因子的数目进行计数。长度为$|w|$的有限字$w$被称为梯形,如果其因子复杂度$C_w(n)$作为$n$的函数的图(对于$0 \leq n \leq |w|$)是规则梯形(也可能是等腰三角形);即,在长度为$r$某个区间上,$C_w(n)$随着每个$n$增加1,则$C_w(n)$在长度为$s$的某个区间上是常数,最后,在相同长度$r$的间隔上,随着每个$n$,$C_w(n)$减1。必须是$C_w(1)=2$(因为有一个长度因子$0$,即空字),所以任何梯形字都是二进制字母表。梯形词首先由de Luca(1999)在研究有限Sturmian词的因子复杂度的行为时引入,即,因子的无限"切割序列",通过编码序列的削减在一个整数格的正象限的$\mathbb{R}^2$由一条线的无理斜率。每个有限的Sturmian词都是梯形的,但不是相反。然而,这两个词族(梯形和Sturmian)都是所谓的“丰富词”(也称为“完整词”)的特殊类别-一个更广泛的有限和无限词族,其特征是包含最大数量的回文-由第一作者和其他人在2009年深入研究。在本文中,我们介绍了一个自然推广的梯形字在任意有限的字母表$\mathcal{A}$,称为广义梯形字(或GT字简称)。特别是,我们研究了这一类新的词的组合和结构特性,我们表明,与二进制的情况下,并不是所有的GT-词是丰富的回文时,$|\mathcal{A}| \geq 3$,但我们可以描述所有那些丰富的。

英文摘要:

The factor complexity function $C_w(n)$ of a finite or infinite word $w$ counts the number of distinct factors of $w$ of length $n$ for each $n \ge 0$. A finite word $w$ of length $|w|$ is said to be trapezoidal if the graph of its factor complexity $C_w(n)$ as a function of $n$ (for $0 \leq n \leq |w|$) is that of a regular trapezoid (or possibly an isosceles triangle); that is, $C_w(n)$ increases by 1 with each $n$ on some interval of length $r$, then $C_w(n)$ is constant on some interval of length $s$, and finally $C_w(n)$ decreases by 1 with each $n$ on an interval of the same length $r$. Necessarily $C_w(1)=2$ (since there is one factor of length $0$, namely the empty word), so any trapezoidal word is on a binary alphabet. Trapezoidal words were first introduced by de Luca (1999) when studying the behaviour of the factor complexity of finite Sturmian words, i.e., factors of infinite "cutting sequences", obtained by coding the sequence of cuts in an integer lattice over the positive quadrant of $\mathbb{R}^2$ made by a line of irrational slope. Every finite Sturmian word is trapezoidal, but not conversely. However, both families of words (trapezoidal and Sturmian) are special classes of so-called "rich words" (also known as "full words") - a wider family of finite and infinite words characterized by containing the maximal number of palindromes - studied in depth by the first author and others in 2009. In this paper, we introduce a natural generalization of trapezoidal words over an arbitrary finite alphabet $\mathcal{A}$, called generalized trapezoidal words (or GT-words for short). In particular, we study combinatorial and structural properties of this new class of words, and we show that, unlike the binary case, not all GT-words are rich in palindromes when $|\mathcal{A}| \geq 3$, but we can describe all those that are rich.

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