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 Mat. Zametki: Year: Volume: Issue: Page: Find

 Mat. Zametki, 2012, Volume 92, Issue 1, Pages 3–18 (Mi mz9483)

On a Method for Proving Exact Bounds on Derivational Complexity in Thue Systems

Steklov Mathematical Institute of the Russian Academy of Sciences

Abstract: In this paper, the following system of substitutions in a $3$-letter alphabet
$$\mathbf\Sigma=\langle a,b,c\mid a^2 \to bc, b^2\to ac, c^2\to ab\rangle$$
is considered. A detailed proof of results that were described briefly in the author's paper [1] is presented. They give an answer to the specific question on the possibility of giving a polynomial upper bound for the lengths of derivations from a given word in the system $\mathbf\Sigma$ stated in the literature. The maximal possible number of steps in derivation sequences starting from a given word $W$ is denoted by $\mathbf D(W)$. The maximum of $\mathbf D(W)$ for all words of length $|W|=l$ is denoted by $\mathbf D(l)$. It is proved that the function $\mathbf D(W)$ on words $W$ of given length $|W|=m+2$ reaches its maximum only on words of the form $W=c^2b^m$ and $W=b^ma^2$. For these words, the following precise estimate is established:
$$\mathbf D(m+2)=\mathbf D(c^2b^m)=\mathbf D(b^ma^2) =\rceil\frac{3m^2}{2}\lceil+m+1<\frac{3(m+1)^2}{2},$$
where $\lceil{3m^2}/{2}\rceil$ for odd $|m|$ is the round-up of ${3m^2}/{2}$ to the nearest integer.

Keywords: word rewriting system, derivational complexity, Thue system, polynomial upper bound, left (right) divisibility of a word

 Funding Agency Grant Number Russian Foundation for Basic Research Ministry of Education and Science of the Russian Federation

DOI: https://doi.org/10.4213/mzm9483

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English version:
Mathematical Notes, 2012, 92:1, 3–15

Bibliographic databases:

Document Type: Article
UDC: 510.52+512.54.05

Citation: S. I. Adian, “On a Method for Proving Exact Bounds on Derivational Complexity in Thue Systems”, Mat. Zametki, 92:1 (2012), 3–18; Math. Notes, 92:1 (2012), 3–15

Citation in format AMSBIB
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