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Resynchronizing Classes of Word Relations

Authors María Emilia Descotte, Diego Figueira, Gabriele Puppis

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María Emilia Descotte
  • LaBRI, Université de Bordeaux
Diego Figueira
  • CNRS, LaBRI, Université de Bordeaux
Gabriele Puppis
  • CNRS, LaBRI, Université de Bordeaux

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María Emilia Descotte, Diego Figueira, and Gabriele Puppis. Resynchronizing Classes of Word Relations. In 45th International Colloquium on Automata, Languages, and Programming (ICALP 2018). Leibniz International Proceedings in Informatics (LIPIcs), Volume 107, pp. 123:1-123:13, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2018)


A natural approach to define binary word relations over a finite alphabet A is through two-tape finite state automata that recognize regular languages over {1, 2} x A, where (i,a) is interpreted as reading letter a from tape i. Accordingly, a word w in L denotes the pair (u_1,u_2) in A^* x A^* in which u_i is the projection of w onto i-labelled letters. While this formalism defines the well-studied class of Rational relations (a.k.a. non-deterministic finite state transducers), enforcing restrictions on the reading regime from the tapes, which we call synchronization, yields various sub-classes of relations. Such synchronization restrictions are imposed through regular properties on the projection of the language onto {1,2}. In this way, for each regular language C subseteq {1,2}^*, one obtains a class Rel({C}) of relations. Regular, Recognizable, and length-preserving rational relations are all examples of classes that can be defined in this way. We study the problem of containment for synchronized classes of relations: given C,D subseteq {1,2}^*, is Rel({C}) subseteq Rel({D})? We show a characterization in terms of C and D which gives a decidability procedure to test for class inclusion. This also yields a procedure to re-synchronize languages from {1, 2} x A preserving the denoted relation whenever the inclusion holds.

Subject Classification

ACM Subject Classification
  • Theory of computation → Formal languages and automata theory
  • synchronized word relations
  • containment
  • resynchronization


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