lecture 34 - rice's theorem
TRANSCRIPT
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8/6/2019 Lecture 34 - Rice's Theorem
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8/6/2019 Lecture 34 - Rice's Theorem
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Rices
Theorem
Robb T.Koether
CollectedProblems
Homework
Review
FunctionalProperties
RicesTheorem
Assignment
Outline
1 Collected Problems
2 Homework Review
3 Functional Properties
4 Rices Theorem
5 Assignment
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8/6/2019 Lecture 34 - Rice's Theorem
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Rices
Theorem
Robb T.Koether
CollectedProblems
Homework
Review
FunctionalProperties
RicesTheorem
Assignment
Collected Problems
Due Mon, Nov 17, 2008
Exercises 4.3, 4.5.
Problems 4.12, 4.16.
Exercise 5.1.
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Rices
Theorem
Robb T.Koether
CollectedProblems
Homework
Review
FunctionalProperties
RicesTheorem
Assignment
Homework Review
Problem 5.9, page 211.
Let
T = {M | M is a TM that accepts wR whenever it
accepts w}.
Show that T is undecidable.
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8/6/2019 Lecture 34 - Rice's Theorem
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Rices
Theorem
Robb T.Koether
CollectedProblems
Homework
Review
FunctionalProperties
RicesTheorem
Assignment
Homework Review
Solution
We will show that T is reducible to ATM.
Suppose that T is decidable.
Let DT be a decider for T.
Let M be a Turing machine and w a string.
Given M,w, construct the Turing machine Mwdescribed in the following diagram.
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Rices
Theorem
Robb T.Koether
CollectedProblems
Homework
Review
FunctionalProperties
RicesTheorem
Assignment
Homework Review
Solution
COMPx M, w
Mw
Ux= w
x= wR
xwor wR
yes
no
acc
rej
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Rices
Theorem
Robb T.Koether
CollectedProblems
Homework
Review
FunctionalProperties
RicesTheorem
Assignment
Homework Review
Solution
The language of Mw is
L(Mw) =
{w,wR}, w L(M);{wR}, w / L(M).
Then, as shown in the following diagram, we can use
Mw to build a decider for ATM, which is a contradiction.
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8/6/2019 Lecture 34 - Rice's Theorem
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Rices
Theorem
Robb T.Koether
CollectedProblems
Homework
Review
FunctionalProperties
RicesTheorem
Assignment
Homework Review
Solution
M, w
DA yes
no
MwDT
yes
no
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8/6/2019 Lecture 34 - Rice's Theorem
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Rices
Theorem
Robb T.Koether
CollectedProblems
Homework
ReviewFunctionalProperties
RicesTheorem
Assignment
Functional Properties
Definition (Functional properties)
A functional property P is a set of Turing-machine
descriptions M with the property that if L(M1) = L(M2),then either M1, M2 P or M1, M2 / P
Definition (Trivial functional property)
A functional property P is trivial if P = orP = {M | M is a Turing machine}.
The following are nontrivial functional properties.
M accepts input 0.M computes n + 1 on input n.M recognizes {w | w contains 111}.
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Rices
Theorem
Robb T.Koether
CollectedProblems
Homework
ReviewFunctionalProperties
RicesTheorem
Assignment
Rices Theorem
Theorem (Rices Theorem)LetP be a nontrivial functional property. ThenP is
undecidable.
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Rices
TheoremRobb T.Koether
CollectedProblems
Homework
ReviewFunctionalProperties
RicesTheorem
Assignment
Proof.
Let P be a nontrivial functional property.
LetM
1 be a Turing machine that rejects all inputs.That is, L(M1) = .
Without loss of generality, assume that M1 / P.
(Otherwise, we reverse the roles of P and P.)
Let M2 P for some Turing machine M2.
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Rices
TheoremRobb T.Koether
CollectedProblems
Homework
ReviewFunctionalProperties
RicesTheorem
Assignment
Proof.
Suppose that P is decidable.
Let DP be a decider for P.
MP yes
no
DP
M
MP
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Rices
TheoremRobb T.Koether
CollectedProblems
HomeworkReview
FunctionalProperties
RicesTheorem
Assignment
Proof.
To build DA, we first describe how to build another
Turing machine Mw to use as a module.
Given M,w, build a Turing machine Mw that does thefollowing:
Given input x, simulate M on w.If Mhalts and rejects w, then Mw rejects x.If Mhalts and accepts w, then Mw simulates M2 on x.
If M loops, then (obviously) Mw loops.
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Rices
TheoremRobb T.Koether
CollectedProblems
HomeworkReview
FunctionalProperties
RicesTheorem
Assignment
x
M rejects w
Maccepts w
Mw
M, wU
no
M2
M2 rejects x
yesM2 accepts x
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Rices
TheoremRobb T.Koether
CollectedProblems
HomeworkReview
FunctionalProperties
RicesTheorem
Assignment
Proof.
Therefore,
L(Mw) = , if M rejects w (or loops);L(M2), if M accepts w.
That is, Mw P if and only if M accepts w.
Now let DA use DP to decide whether Mw P:
If Mw P, then DA accepts M,w.If Mw / P, then DA rejects M,w.
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Rices
TheoremRobb T.Koether
CollectedProblems
HomeworkReview
FunctionalProperties
RicesTheorem
Assignment
DA
DP
yes
no
MwP
MwM, w
MwP
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Rices
TheoremRobb T.Koether
CollectedProblems
HomeworkReview
FunctionalProperties
RicesTheorem
Assignment
Proof.
That is, DA decides ATM, which is a contradiction.
Therefore, P is undecidable.
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Rices
TheoremRobb T.Koether
CollectedProblems
HomeworkReview
FunctionalProperties
RicesTheorem
Assignment
Assignment
Homework
Read Problem 5.28, page 213.
Apply Rices Theorem to Problem 9.
Determine whether the properties described in
Problems 10 - 15 are functional properties. If they are,
then apply Rices Theorem.
Problems 30, 31, page 213.
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