An explicit solution to Post's Problem over the reals

  • Authors:
  • Klaus Meer;Martin Ziegler

  • Affiliations:
  • Department of Mathematics and Computer Science, Syddansk Universitet, Campusvej 55, 5230 Odense M, Denmark;Department of Mathematics and Computer Science, Syddansk Universitet, Campusvej 55, 5230 Odense M, Denmark and PaSCo and HNI, University of Paderborn, 33095 Paderborn, Germany

  • Venue:
  • Journal of Complexity
  • Year:
  • 2008

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Abstract

In the BSS model of real number computations we prove a concrete and explicit semi-decidable language to be undecidable yet not reducible from (and thus strictly easier than) the real Halting Language. This solution to Post's Problem over the reals significantly differs from its classical, discrete variant where advanced diagonalization techniques are only known to yield the existence of such intermediate Turing degrees. Then we strengthen the above result and show as well the existence of an uncountable number of incomparable semi-decidable Turing degrees below the real Halting Problem in the BSS model. Again, our proof will give concrete such problems representing these different degrees. Finally we show the corresponding result for the linear BSS model, that is over (R,+,-,