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It is not the case that Among the unsolvable decision problems of recursively enumerable sets, there is a highest degree of unsolvability.
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Reasons For
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Reason for 1 of 2
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1.
Post's problem (1944) demonstrated that intermediate degrees exist between decidable and complete r.e. sets, undermining claims of a single highest degree.
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2.
Friedberg and Muchnik (1956) independently proved incomparable r.e. degrees exist, showing the r.e. degrees form a non-linearly-ordered structure.
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3.
A non-linear partial order with incomparable elements cannot possess a unique maximal element in any straightforward sense without further qualification.
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Reason for 2 of 2
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1.
The halting problem is Turing-complete for r.e. sets, but 'highest degree' conflates m-reducibility and Turing reducibility, which diverge extensionally.
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2.
Lachlan and Yates showed the r.e. degrees lack a definable well-ordering, making 'highest' a structurally ambiguous predicate absent a fixed reducibility notion.
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Reasons Against
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Reason against
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1.
The theory of recursively enumerable sets admits a primary problem of determining degrees of unsolvability of their unsolvable decision problems.
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2.
The structure of reducibility among these problems yields at least one problem to which all others are reducible.
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