| 21. | Decision problems become harder to answer or completely undecidable.
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| 22. | Philip K . Hooper proved in 1966 that the mortality problem is undecidable.
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| 23. | Earlier, Alfred Tarski proved elementary group theory undecidable.
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| 24. | Higher-order unification is known to be undecidable.
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| 25. | These are natural mathematical equivalents of the G�del " true but undecidable " sentence.
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| 26. | The list of undecidable problems gives additional examples of problems with no computable solution.
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| 27. | The equivalence of two lambda expressions is undecidable.
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| 28. | For more expressive formalisms, this problem can be harder, or even undecidable.
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| 29. | :A source of confusion may be that " undecidable " has several meanings.
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| 30. | Partially decidable problems and any other problems that are not decidable are called undecidable.
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