Degrees of Infinite Words, Polynomials and Atoms

Jörg Endrullis*, Juhani Karhumäki, Jan Willem Klop, Aleksi Saarela

*Corresponding author for this work

Research output: Contribution to JournalArticleAcademicpeer-review


We study finite-state transducers and their power for transforming infinite words. Infinite sequences of symbols are of paramount importance in a wide range of fields, from formal languages to pure mathematics and physics. While finite automata for recognising and transforming languages are well-understood, very little is known about the power of automata to transform infinite words. The word transformation realised by finite-state transducers gives rise to a complexity comparison of words and thereby induces equivalence classes, called (transducer) degrees, and a partial order on these degrees. The ensuing hierarchy of degrees is analogous to the recursion-theoretic degrees of unsolvability, also known as Turing degrees, where the transformational devices are Turing machines. However, as a complexity measure, Turing machines are too strong: they trivialise the classification problem by identifying all computable words. Finite-state transducers give rise to a much more fine-grained, discriminating hierarchy. In contrast to Turing degrees, hardly anything is known about transducer degrees, in spite of their naturality. We use methods from linear algebra and analysis to show that there are infinitely many atoms in the transducer degrees, that is, minimal non-trivial degrees.

Original languageEnglish
Pages (from-to)825-843
Number of pages19
JournalInternational Journal of Foundations of Computer Science
Issue number5
Publication statusPublished - Aug 2018

Bibliographical note

This article is part of the Special Issue: Developments in Language Theory (DLT 2016); Guest Editors: Alexandre Blondin Massé, Srečko Brlek and Christophe Reutenauer


This research has been supported by the Academy of Finland under the grant 257857.

FundersFunder number
Academy of Finland257857


    • degrees of infinite words
    • finite-state transducers
    • Infinite words


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