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The Halting problem is a natural undecidable language which is complete for the set of recursivly enumrable sets. I am interested in undecidable but not Turing-complete language such that we can not reduce the Halting problem to it.

What is the most natural undecidable RE problem which is not Turing-complete?

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There indeed are sets that are undecidable but not Turing complete. However, I'm not sure there is such a thing as a natural undecidable not Turing complete language.

Emil Post set out on a search for exactly this (See Post's problem) and his research resulted in the Simple sets. To quote from the Wikipedia article:

[Its existence] was affirmed by Friedberg and Muchnik in the 1950s using a novel technique called the priority method. They give a construction for a set that is simple (and thus non-recursive), but fails to compute the halting problem.

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I will wait for a natural problem. –  Mohammad Al-Turkistany Mar 23 '13 at 13:33
    
Some have argued that there are none. –  Andrej Bauer Mar 23 '13 at 14:04
    
Could you provide a pointer? –  Mohammad Al-Turkistany Mar 23 '13 at 14:48
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