On Sets of Words of Rank Two

Research output: Chapter in Book/Report/Conference proceedingConference contribution

2 Citations (Scopus)


Given a (finite or infinite) subset X of the free monoid A∗ over a finite alphabet A, the rank of X is the minimal cardinality of a set F such that X⊆ F∗. A submonoid M generated by k elements of A∗ is k-maximal if there does not exist another submonoid generated by at most k words containing M. We call a set X⊆ A∗ primitive if it is the basis of a |X|-maximal submonoid. This extends the notion of primitive word: indeed, w is a primitive set if and only if w is a primitive word. By definition, for any set X, there exists a primitive set Y such that X⊆ Y∗. The set Y is therefore called a primitive root of X. As a main result, we prove that if a set has rank 2, then it has a unique primitive root. This result cannot be extended to sets of rank larger than 2. For a single word w, we say that the set x, y is a binary root of w if w can be written as a concatenation of copies of x and y and x, y is a primitive set. We prove that every primitive word w has at most one binary root x, y such that |x|+|y|<|w|. That is, the binary root of a word is unique provided the length of the word is sufficiently large with respect to the size of the root. Our results are also compared to previous approaches that investigate pseudo-repetitions, where a morphic involutive function θ is defined on A∗. In this setting, the notions of θ -power, θ -primitive and θ -root are defined, and it is shown that any word has a unique θ -primitive root. This result can be obtained with our approach by showing that a word w is θ -primitive if and only if w, θ(w) is a primitive set.
Original languageEnglish
Title of host publicationLecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics)
Number of pages14
Publication statusPublished - 2019

Publication series


All Science Journal Classification (ASJC) codes

  • Theoretical Computer Science
  • General Computer Science


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