cm0081 undecidable problems about turing machines
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Automata and Formal Languages - CM0081Undecidable Problems About Turing Machines
Andrรฉs Sicard-Ramรญrez
Universidad EAFIT
Semester 2018-1
Reductions
DefinitionLet ๐1 and ๐2 be two problems. A reduction from ๐1 to ๐2 is a Turingmachine that takes an instance of ๐1 written on its tape and halts with aninstance of ๐2 that have the same answer (i.e. a reduction is an algorithm).
yes
no
yes
no
๐1 ๐2
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Reductions
Theorem (9.7.a)If there is a reduction from ๐1 to ๐2 then if ๐1 is undecidable then so ๐2.(Hint: Suppose the ๐2 is decidable and find a contradiction).
Theorem (9.7.b)If there is a reduction from ๐1 to ๐2 then if ๐1 is not recursively enumerablethen so ๐2. (Hint: Suppose the ๐2 is recursively enumerable and find acontradiction).
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Reductions
Theorem (9.7.a)If there is a reduction from ๐1 to ๐2 then if ๐1 is undecidable then so ๐2.(Hint: Suppose the ๐2 is decidable and find a contradiction).
Theorem (9.7.b)If there is a reduction from ๐1 to ๐2 then if ๐1 is not recursively enumerablethen so ๐2. (Hint: Suppose the ๐2 is recursively enumerable and find acontradiction).
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Turing Machines that Accept the Empty Language
NotationHenceforth, weโll regard strings as the Turing machines they represent.
Two languagesLe = {๐ โฃ ๐ฟ(๐) = โ },
Lne = {๐ โฃ ๐ฟ(๐) โ โ }.
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Turing Machines that Accept the Empty Language
Theorem (9.8)Lne is recursively enumerable,
ProofConstruction of a non-determinist TM to accept Lne:โ
โ Figure from Hopcroft, Motwani and Ullman [2007, Fig. 9.8].Undecidable Problems About Turing Machines 6/28
Turing Machines that Accept the Empty Language
Theorem (9.8)Lne is recursively enumerable,
ProofConstruction of a non-determinist TM to accept Lne:โ
โ Figure from Hopcroft, Motwani and Ullman [2007, Fig. 9.8].Undecidable Problems About Turing Machines 7/28
Turing Machines that Accept the Empty Language
Theorem (9.9)Lne is not recursive.
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Turing Machines that Accept the Empty Language
Proof.1. Reduction from Lu to Lne where the pair (๐, ๐ค) is converted in ๐ โฒ,
such that ๐ค โ ๐ฟ(๐), if only if, ๐ฟ(๐ โฒ) โ โ .
2. The key is to have ๐ โฒ ignore its input.โ
3. Lne is not recursive by Theorem 9.7.a.
โ Figure from Hopcroft, Motwani and Ullman [2007, Fig. 9.9].Undecidable Problems About Turing Machines 9/28
Turing Machines that Accept the Empty Language
Proof.1. Reduction from Lu to Lne where the pair (๐, ๐ค) is converted in ๐ โฒ,
such that ๐ค โ ๐ฟ(๐), if only if, ๐ฟ(๐ โฒ) โ โ .2. The key is to have ๐ โฒ ignore its input.โ
3. Lne is not recursive by Theorem 9.7.a.
โ Figure from Hopcroft, Motwani and Ullman [2007, Fig. 9.9].Undecidable Problems About Turing Machines 10/28
Turing Machines that Accept the Empty Language
Proof.1. Reduction from Lu to Lne where the pair (๐, ๐ค) is converted in ๐ โฒ,
such that ๐ค โ ๐ฟ(๐), if only if, ๐ฟ(๐ โฒ) โ โ .2. The key is to have ๐ โฒ ignore its input.โ
3. Lne is not recursive by Theorem 9.7.a.
โ Figure from Hopcroft, Motwani and Ullman [2007, Fig. 9.9].Undecidable Problems About Turing Machines 11/28
Turing Machines that Accept the Empty Language
Theorem (9.10)Le is not recursively enumerable. (Hint: Le is the complement of Lne).
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Riceโs Theorem
Set of the recursively enumerable languagesREL = {๐ฟ โฃ ๐ฟ is recursively enumerable }.
Properties (subsets) of the recursively enumerable languagesProperty ๐ of REL: ๐ โ REL.
Example (Trivial properties)๐ (๐ฟ) = โ or ๐(๐ฟ) = REL.
Example๐ (๐ฟ): ๐ฟ is a language regular.
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Riceโs Theorem
Set of the recursively enumerable languagesREL = {๐ฟ โฃ ๐ฟ is recursively enumerable }.
Properties (subsets) of the recursively enumerable languagesProperty ๐ of REL: ๐ โ REL.
Example (Trivial properties)๐ (๐ฟ) = โ or ๐(๐ฟ) = REL.
Example๐ (๐ฟ): ๐ฟ is a language regular.
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Riceโs Theorem
Set of the recursively enumerable languagesREL = {๐ฟ โฃ ๐ฟ is recursively enumerable }.
Properties (subsets) of the recursively enumerable languagesProperty ๐ of REL: ๐ โ REL.
Example (Trivial properties)๐ (๐ฟ) = โ or ๐(๐ฟ) = REL.
Example๐ (๐ฟ): ๐ฟ is a language regular.
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Riceโs Theorem
Set of the recursively enumerable languagesREL = {๐ฟ โฃ ๐ฟ is recursively enumerable }.
Properties (subsets) of the recursively enumerable languagesProperty ๐ of REL: ๐ โ REL.
Example (Trivial properties)๐ (๐ฟ) = โ or ๐(๐ฟ) = REL.
Example๐ (๐ฟ): ๐ฟ is a language regular.
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Riceโs Theorem
Theorem (Riceโs theorem, 9.11)Every non-trivial property of REL is undecidable.
How to prove Riceโs theorem?We identify a property ๐ by the Turing machines ๐ such that ๐ฟ(๐) โ ๐ .
Theorem (Riceโs theorem (second version))If ๐ โ REL is a non-trivial property then
๐ฟ๐ = {๐ โฃ ๐ฟ(๐) โ ๐}
is undecidable.
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Riceโs Theorem
Theorem (Riceโs theorem, 9.11)Every non-trivial property of REL is undecidable.
How to prove Riceโs theorem?We identify a property ๐ by the Turing machines ๐ such that ๐ฟ(๐) โ ๐ .
Theorem (Riceโs theorem (second version))If ๐ โ REL is a non-trivial property then
๐ฟ๐ = {๐ โฃ ๐ฟ(๐) โ ๐}
is undecidable.
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Riceโs Theorem
Theorem (Riceโs theorem, 9.11)Every non-trivial property of REL is undecidable.
How to prove Riceโs theorem?We identify a property ๐ by the Turing machines ๐ such that ๐ฟ(๐) โ ๐ .
Theorem (Riceโs theorem (second version))If ๐ โ REL is a non-trivial property then
๐ฟ๐ = {๐ โฃ ๐ฟ(๐) โ ๐}
is undecidable.
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Riceโs Theorem
Proof: Case โ โ ๐ .1. Let ๐ฟ be a language and ๐๐ฟ be a Turing machine such ๐ฟ โ โ ,
๐ฟ โ ๐ and ๐ฟ = ๐ฟ(๐๐ฟ). Reduction from Lu to ๐ฟ๐ where the pair(๐, ๐ค) is converted in ๐ โฒ such that:โ
i) ๐ฟ(๐ โฒ) = โ (i.e. ๐ โฒ โ ๐ฟ๐ ) if ๐ค โ ๐ฟ(๐) andii) ๐ฟ(๐ โฒ) = ๐ฟ (i.e. ๐ โฒ โ ๐ฟ๐ ) if ๐ค โ ๐ฟ(๐).
2. ๐ฟ๐ is not recursive by Theorem 9.7.a.
โ Figure from Hopcroft, Motwani and Ullman [2007, Fig. 9.10].Undecidable Problems About Turing Machines 20/28
Riceโs Theorem
Proof: Case โ โ ๐ .1. Let ๐ฟ be a language and ๐๐ฟ be a Turing machine such ๐ฟ โ โ ,
๐ฟ โ ๐ and ๐ฟ = ๐ฟ(๐๐ฟ). Reduction from Lu to ๐ฟ๐ where the pair(๐, ๐ค) is converted in ๐ โฒ such that:โ
i) ๐ฟ(๐ โฒ) = โ (i.e. ๐ โฒ โ ๐ฟ๐ ) if ๐ค โ ๐ฟ(๐) andii) ๐ฟ(๐ โฒ) = ๐ฟ (i.e. ๐ โฒ โ ๐ฟ๐ ) if ๐ค โ ๐ฟ(๐).
2. ๐ฟ๐ is not recursive by Theorem 9.7.a.
โ Figure from Hopcroft, Motwani and Ullman [2007, Fig. 9.10].Undecidable Problems About Turing Machines 21/28
Riceโs Theorem
Proof: Case โ โ ๐ .1. Let ๐ฟ be a language and ๐๐ฟ be a Turing machine such ๐ฟ โ โ ,
๐ฟ โ ๐ and ๐ฟ = ๐ฟ(๐๐ฟ). Reduction from Lu to ๐ฟ๐ where the pair(๐, ๐ค) is converted in ๐ โฒ such that:โ
i) ๐ฟ(๐ โฒ) = โ (i.e. ๐ โฒ โ ๐ฟ๐ ) if ๐ค โ ๐ฟ(๐) andii) ๐ฟ(๐ โฒ) = ๐ฟ (i.e. ๐ โฒ โ ๐ฟ๐ ) if ๐ค โ ๐ฟ(๐).
2. ๐ฟ๐ is not recursive by Theorem 9.7.a.โ Figure from Hopcroft, Motwani and Ullman [2007, Fig. 9.10].
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Riceโs Theorem
Proof: Case โ โ ๐ .1. By the previous case, ๐ is undecidable, i.e. ๐ฟ๐ is undecidable.2. ๐ฟ๐ = ๐ฟ๐ .3. Suppose ๐ฟ๐ is decidable then ๐ฟ๐ would be also decidable
(contradiction).4. Therefore, ๐ฟ๐ is undecidable.
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Riceโs Theorem
All the problems about Turing machines that involve only the languagethat the TM accepts are undecidable.
ExamplesIs the language accepted by the TM empty?Is the language accepted by the TM finite?Is the language accepted by the TM regular?Is the language accepted by the TM context-free language?Does the language accepted by the TM contain the string โhelloworldโ?Does the language accepted by the TM contain all the even numbers?
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Riceโs Theorem
All the problems about Turing machines that involve only the languagethat the TM accepts are undecidable.
ExamplesIs the language accepted by the TM empty?Is the language accepted by the TM finite?Is the language accepted by the TM regular?Is the language accepted by the TM context-free language?Does the language accepted by the TM contain the string โhelloworldโ?Does the language accepted by the TM contain all the even numbers?
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Riceโs Theorem
Riceโs theorem does not imply that everything about Turing machines isundecidable.
ExampleIt is decidable if a Turing machine has five states.
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Riceโs Theorem
Riceโs theorem does not imply that everything about Turing machines isundecidable.ExampleIt is decidable if a Turing machine has five states.
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References
Hopcroft, J. E., Motwani, R. and Ullman, J. D. (2007). Introduction toAutomata theory, Languages, and Computation. 3rd ed. Pearson Education.
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