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1 Introduction to Computability Theory Lecture15: Reductions Prof. Amos Israeli

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The rest of the course deals with an important tool in Computability and Complexity theories, namely: Reductions. The reduction technique enables us to use the undecidability of to prove many other languages undecidable. Introduction 2

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A reduction always involves two computational problems. Generally speaking, the idea is to show that a solution for some problem A induces a solution for problem B. If we know that B does not have a solution, we may deduce that A is also insolvable. In this case we say that B is reducible to A. Introduction 3

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In the context of undecidability: If we want to prove that a certain language L is undecidable. We assume by way of contradiction that L is decidable, and show that a decider for L, can be used to devise a decider for. Since is undecidable, so is the language L. Introduction 4

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Using a decider for L to construct a decider for, is called reducing L to. Note: Once we prove that a certain language L is undecidable, we can prove that some other language, say L’, is undecidable, by reducing L’ to L. Introduction 5

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1.We know that A is undecidable. 2.We want to prove B is undecidable. 3.We assume that B is decidable and use this assumption to prove that A is decidable. 4.We conclude that B is undecidable. Note: The reduction is from A to B. Schematic of a Reduction 6

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1.We know that A is undecidable. The only undecidable language we know, so far, is whose undecidability was proven directly. So we pick to play the role of A. 2.We want to prove B is undecidable. Demonstration 7

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2.We want to prove B is undecidable. We pick to play the role of B that is: We want to prove that is undecidable. 3.We assume that B is decidable and use this assumption to prove that A is decidable. Demonstration 8

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3.We assume that B is decidable and use this assumption to prove that A is decidable. In the following slides we assume (towards a contradiction) that is decidable and use this assumption to prove that is decidable. 4.We conclude that B is undecidable. Demonstration 9

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Consider Theorem is undecidable. Proof By reducing to. The “Real” Halting Problem 10

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Assume by way of contradiction that is decidable. Recall that a decidable set has a decider R : A TM that halts on every input and either accepts or rejects, but never loops!. We will use the assumed decider of to devise a decider for. Discussion 11

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Recall the definition of : Why is it impossible to decide ? Because as long as M runs, we cannot determine whether it will eventually halt. Well, now we can, using the decider R for. Discussion 12

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Assume by way of contradiction that is decidable and let R be a TM deciding it. In the next slide we present TM S that uses R as a subroutine and decides. Since is undecidable this constitutes a contradiction, so R does not exist. Proof 13

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S= “On input where M is a TM: 1. Run R on input until it halts. 2. If R rejects, (i.e. M loops on w ) - reject. (At this stage we know that R accepts, and we conclude that M halts on input w.) 3. Simulate M on w until it halts. 4. If M accepts - accept, otherwise - reject. “ Proof (cont.) 14

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In the discussion, you saw how Diagonalization can be used to prove that is not decidable. We can use this result to prove by reduction that is not decidable. Another Example 15

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Note: Since we already know that both and are undecidable, this new proof does not add any new information. We bring it here only for the the sake of demonstration. Another Example 16

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1.We know that A is undecidable. Now we pick to play the role of A. 2.We want to prove B is undecidable. We pick to play the role of B, that is: We want to prove that is undecidable. 3.We assume that B is decidable and use this assumption to prove that A is decidable. Demonstration 17

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3.We assume that B is decidable and use this assumption to prove that A is decidable. In the following slides we assume that is decidable and use this assumption to prove that is decidable. 4.We conclude that B is undecidable. Demonstration 18

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Let R be a decider for. Given an input for, R can be run with this input : If R accepts, it means that. This means that M accepts on input w. In particular, M stops on input w. Therefore, a decider for must accept too. Discussion 19

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If however R rejects on input, a decider for cannot safely reject: M may be halting on w to reject it. So if M rejects w, a decider for must accept. Discussion 20

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How can we use our decider for ? The answer here is more difficult. The new decider should first modify the input TM, M, so the modified TM,, accepts, whenever TM M halts. Since M is a part of the input, the modification must be a part of the computation. Discussion 21

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Faithful to our principal “ If it ain’t broken don’t fix it”, the modified TM keeps M as a subroutine, and the idea is quite simple: Let and be the accepting and rejecting states of TM M, respectively. In the modified TM,, and are kept as ordinary states. Discussion 22

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We continue the modification of M by adding a new accepting sate. Then we add two new transitions: A transition from to, and another transition from to. This completes the description of. It is not hard to verify that accepts iff M halts. Discussion 23

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Discussion 24

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The final description of a decider S for is: S= “On input where M is a TM: 1. Modify M as described to get. 2. Run R, the decider of with input. 3. If R accepts - accept, otherwise - reject. ” Discussion 25

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It should be noted that modifying TM M to get, is part of TM S, the new decider for, and can be carried out by it. It is not hard to see that S decides. Since is undecidable, we conclude that is undecidable too. Discussion 26

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We continue to demonstrate reductions by showing that the language, defined by is undecidable. Theorem is undecidable. The TM Emptiness Problem 27

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The proof is by reduction from : 1.We know that is undecidable. 2.We want to prove is undecidable. 3.We assume toward a contradiction that is decidable and devise a decider for. 4.We conclude that is undecidable. Proof Outline 28

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Assume by way of contradiction that is decidable and let R be a TM deciding it. In the next slides we devise TM S that uses R as a subroutine and decides. Proof 29

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Given an instance for,, we may try to run R on this instance. If R accepts, we know that. In particular, M does not accept w so a decider for must reject. Proof 30

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What happens if R rejects? The only conclusion we can draw is that. What we need to know though is whether. In order to use our decider R for, we once again modify the input machine M to obtain TM : Proof 31

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We start with a TM satisfying. Description of___ 32

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Now we add a filter to divert all inputs but w. Description of___ 33 filter no yes

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TM has a filter that rejects all inputs excepts w, so the only input reaching M, is w. Therefore, satisfies: Proof 34

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Here is a formal description of : “On input x : 1. If - reject. 2. If - run M on w and accept if M accepts. ” Note: M accepts w if and only if. Proof 35

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This way, if R accepts, S “can be sure” that and accept. Note that S gets the pair as input, thus before S runs R, it should compute an encoding of. This encoding is not too hard to compute using S ’s input. Proof 36

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S= “On input where M is a TM: 1. Compute an encoding of TM. 2. Run R on input. 3. If R rejects - accept, otherwise - reject. Proof 37

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Recall that R is a decider for. If R rejects the modified machine,, hence by the specification of,, and a decider for must accept. If however R accepts, it means that, hence, and S must reject. QED Proof 38

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