On the presence of periodic configurations in Turing machines and in counter machines

  • Authors:
  • Vincent D. Blondel;Julien Cassaigne;Codrin Nichitiu

  • Affiliations:
  • Division of Applied Mathematics, Center Cesame, Université catholique de Louvain, 4 avenue Georges Lemaitre, B-1348 Louvain-la-Neuve, Belgium;Institut de Mathématiques de Luminy--CNRS, 163 avenue de Luminy, Case 907, 13288 Marseille Cedex 9, France;EURISE, Université Jean Monnet St Étienne, 23, rue du Dr Paul Michelon, 42023 St Étienne Cedex 2, France

  • Venue:
  • Theoretical Computer Science
  • Year:
  • 2002

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Abstract

A configuration of a Turing machine is given by a tape content together with a particular state of the machine. Petr Kůrka has conjectured that every Turing machine--when seen as a dynamical system on the space of its configurations--has at least one periodic orbit. In this paper, we provide an explicit counterexample to this conjecture. We also consider counter machines and prove that, in this case, the problem of determining if a given machine has a periodic orbit in configuration space is undecidable.