| 1. | Since the halting problem is undecidable, ? cannot be computed.
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| 2. | There are many known examples of undecidable problems from ordinary mathematics.
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| 3. | The latter problem is called the Halting problem and is undecidable.
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| 4. | Many word problems are undecidable based on the Post correspondence problem.
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| 5. | Problems that are not decidable are called " undecidable ".
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| 6. | This is a very general setting for formulating the undecidable in general.
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| 7. | Problems which are undecidable using classical computers remain undecidable using quantum computers.
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| 8. | Problems which are undecidable using classical computers remain undecidable using quantum computers.
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| 9. | In fact, every consistent extension will be essentially undecidable.
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| 10. | The theory of fields is undecidable but not essentially undecidable.
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