Decidability (What, stuff is unsolvable?)

Transcription

1 University of Georgia Fall 2014

2 Outline Decidability Decidable Problems for Regular Languages Decidable Problems for Context Free Languages The Halting Problem Countable and Uncountable Sets Diagonalization Unrecognizable Problems

3 Turing Machines Definition A Turing machine is a 7-tuple (Q, Σ, Γ, δ, q 0, q accept, q reject ) such that 1. Q is a finite set of states; 2. Σ is the input alphabet, and / Σ; 3. Γ is the tape alphabet, where Γ and Σ Γ; 4. δ : (Q Γ) (Q Γ {L, R}) is the transition function; 5. q 0 Q is the start state; 6. q accept Q is the accept state; 7. q reject Q is the reject state, and q reject q accept.

4 Configurations Definition The state, tape contents, and tape head position constitute a configuration. An accepting configuration (rejecting configuration) is one in which the machine is in state q accept (q reject ). The start configuration has the tape head in the leftmost position, and the input string is written on it. A string 01010q may be used to indicate a configuration. The transition function maps one configuration to another. Definition A Turing Machine M accepts a string w if there exists a sequence of configurations C 1, C 2,... C n such that 1. C 1 is the starting configuration of M on input w, 2. C n is an accepting configuration, 3. for each 1 i < n, C i yields C i+1. L(M), language of M is the set of strings accepted by M.

5 Turing Recognizable and Decidable Languages Definition A language L is Turing recognizable (recursively enumerable) if there is a Turing machine M such that L(M) = L. It is possible for a TM to never reach a halting configuration. On any given input w, it might instead loop. These are the three options for any input: accept, reject, loop. A TM M that halts on every input is called a decider. If L(M) = L, then M is said to decide L. Definition A language L is Turing decidable (also called recursive) if there is a Turing machine M that decides L.

6 Decidability and Undecidability The definitions indicate that not all languages are decidable. The Church-Turing Thesis equates algorithmically solvable with solvable by Turing machines. A yes/no problem is algorithmically decidable if there is some Turing machine that will decide it. Given any instance of the problem (input), the machine will always halt in finite time with the correct answer (yes or no). A problem is not algorithmically solvable if every TM loops on at least one input. Chapter 4 discusses problems that are solvable/unsolvable by TMs. Knowing which problems are unsolvable is important (and pretty cool). Why try to write a program to solve a problem when someone s already told you it can t be done.

7 Decidability and Undecidability I can t find an... algorithm, I guess I m just too lame. source: MR Garey and DS Johnson. Computers and Intractability. W. H. Freeman, image:

8 Decidability and Undecidability I can t find an... algorithm, because no such algorithm is possible. source: MR Garey and DS Johnson. Computers and Intractability. W. H. Freeman, image:

9 Decidable Problems for Regular Languages The following languages/problems are all decidable. A DFA = { B, w B is a DFA and w L(B)} For any DFA B, L(B) is decidable. A NFA = { B, w B is an NFA and w L(B)} A REX = { R, w R is a regular expression that generates w} E DFA = { B B is a DFA and L(B) = } EQ DFA = { A, B A, B are DFAs and L(A) = L(B)} In each case, the input is a string representing, for instance, a DFA and an input string w. We design a TM to decide the problem (typically by simulating the machines described in the input). A reasonable representation of the DFA is needed. E.g., q 0,..., q n #a 1,..., a m #(q 0, a 0, q i ),... #q 0 #q i, q j,... #w 0... w k

10 Decidable Problems for Regular Languages A DFA is decidable, where A DFA = { B, w B is a DFA and w L(B)}

11 Decidable Problems for Regular Languages A DFA is decidable, where A DFA = { B, w B is a DFA and w L(B)} (Proof idea) We construct a Turing machine M to decide the problem. Scan the input string B, w, determining whether the input constitutes a valid DFA and string w. If not, then reject. Scan the input string repeatedly. Keep track of the current DFA state and position of w. Find the appropriate transition to update the state appropriately. After processing w k, check whether B is in an accept state. If so, then accept B, w. Otherwise reject B, w.

12 Decidable Problems for Regular Languages If B is a DFA, then L(B) is decidable. This is clearly true, as every DFA halts on every input. We can easily create a TM M B to recognize L(B) by modifying the transition function of B to fit the form for TMs. If δ B (q, a) = q, then δ MB (q, a) = (q, a, R). Alternatively, we can use M from the previous proof in M B. On input w, run M on B, w. If M accepts B, w, accept w. Otherwise reject w.

13 Decidable Problems for Regular Languages A NFA is decidable, where A NFA = { B, w B is an NFA and w L(B)}

14 Decidable Problems for Regular Languages A NFA is decidable, where A NFA = { B, w B is an NFA and w L(B)} (Proof idea) We construct another Turing machine N to decide A NFA : Scan the input string B, w, determining whether the input constitutes a valid NFA and string w. If not, then reject. Convert B to a DFA C (using the construction described in Ch 1). Run M on C, w. Accept B, w if M accepts C, w. Otherwise reject B, w.

15 Decidable Problems for Regular Languages A REX is decidable, where A REX = { R, w R is a regular expression that generates w}

16 Decidable Problems for Regular Languages A REX is decidable, where A REX = { R, w R is a regular expression that generates w} (Proof idea) We construct another Turing machine P to decide A REX : Scan the input string R, w, determining whether the input constitutes a valid reg-exp and string w. If not, then reject. Convert R to an NFA A (using the construction described in Ch 1). Run N on A, w. Accept R, w if N accepts A, w. Otherwise reject R, w.

17 Decidable Problems for Regular Languages E DFA is decidable, where E DFA = { A A is a DFA and L(A) = } Idea: Check whether an accept state is reachable from A s start state.

18 Decidable Problems for Regular Languages E DFA is decidable, where E DFA = { A A is a DFA and L(A) = } Idea: Check whether an accept state is reachable from A s start state. (Proof idea) We construct Turing machine T to decide E DFA : Scan the input string A, determining whether the input constitutes a valid DFA. If not, then reject. We assume states of A are listed left to right, with q 0 being leftmost. Mark q 0. Using the transition function of A, mark any unmarked state q such that δ(q, a) = q and q is already marked. Continue until no new nodes can be marked. If an accept state is marked, then reject A. Otherwise accept it.

19 Decidable Problems for Regular Languages EQ DFA is decidable, where EQ DFA = { A, B A, B are DFAs and L(A) = L(B)} Regular languages are closed under complementation, union, intersection. Given A and B, we can construct a DFA for the symmetric difference: Diff = (L(A) L(B)) (L(B) L(A)) If Diff is nonempty, then L(A) L(B).

20 Decidable Problems for Regular Languages EQ DFA is decidable, where EQ DFA = { A, B A, B are DFAs and L(A) = L(B)} Regular languages are closed under complementation, union, intersection. Given A and B, we can construct a DFA for the symmetric difference: Diff = (L(A) L(B)) (L(B) L(A)) If Diff is nonempty, then L(A) L(B). (Proof idea) We construct a Turing machine F to decide EQ DFA : Scan the input string A, B determining whether the input constitutes two valid DFAs A and B. If not, then reject. Construct a string representing the DFA C, where L(C) is the symmetric difference of L(A) and L(B). Run Turing Machine T on C. (Recall that T decides E DFA.) If T accepts C, accept A, B. Otherwise reject it.

21 Decidable Problems for Regular Languages Answer all questions for the following DFA M and give reasons for your answers. 1. Is M, 0110 A DFA? 2. Is M, 001 A DFA? 3. Is M A DFA? 4. Is M, 0110 A REX? 5. Is M E DFA? 6. Is M, M EQ DFA?

22 Decidable Problems for Context Free Languages The following languages/problems are decidable. A CFG = { G, w G is a CFG generating w} E CFG = { G G is a CFG and L(A) = } If G is a CFG, then L(G) is decidable. The following language/problem is NOT decidable. EQ CFG = { G, H G, H are CFGs and L(G) = L(H)} Why won t using (L(G) L(H)) (L(H) L(G)) work in this case?

23 An Undecidable Problem for Context Free Languages The following language/problem is NOT decidable. EQ CFG = { G, H G, H are CFGs and L(G) = L(H)} Context free grammars are not closed under complementation or intersection, and so we cannot use as was done for EQ DFA. (L(G) L(H)) (L(H) L(G))

24 Decidable Problems for Context Free Languages A CFG is decidable, where A CFG = { G, w G is a CFG generating w}.

25 Decidable Problems for Context Free Languages A CFG is decidable, where A CFG = { G, w G is a CFG generating w}. Idea: If G is in CNF, then it takes at most 2n 1 steps to generate w. (Proof idea) We construct a Turing machine S to decide the problem. Scan the input string G, w, determining whether the input constitutes a valid CFG and string w. If not, then reject. Convert G to CNF, using the procedure described in Ch 2. If w = ε, look for grammar rule S ε. If present, then accept. Otherwise reject. If w ε, systematically generate all derivations of at most 2n 1 steps, where w = n. If any of these derivations produce w, then accept. If not, reject.

26 Decidable Problems for Context Free Languages E CFG is decidable, where E CFG = { G G is a CFG and L(G) = }.

27 Decidable Problems for Context Free Languages E CFG is decidable, where E CFG = { G G is a CFG and L(G) = }. Idea: Work backwards from terminal symbols to start symbol S. (Proof idea) We construct a Turing machine R to decide the problem. Scan the input string G, determining whether the input constitutes a valid CFG. If not, then reject. Convert G to Chomsky Normal Form, as described in Ch 2. For each rule A a, mark every occurrence of A in the rules. For each rule of the form A BC, if both B and C are marked, then mark every occurrence of A in the rules. Continue until no new variables are marked. If S is NOT marked, then accept G. Otherwise Reject.

28 Decidable Problems for Context Free Languages E CFG is decidable, where E CFG = { G G is a CFG and L(G) = }. An easier alternative. (Proof idea) We construct a Turing machine R to decide the problem. Scan the input string G, determining whether the input constitutes a valid CFG. If not, then reject. Mark every terminal in every rule. For each rule of the form A w, if every symbol of w is marked, mark every occurrence of A in the rules. Continue until no new variables are marked. If S is NOT marked, then accept G. Otherwise Reject.

29 Decidable Problems for Context Free Languages If G is a CFG, then L(G) is decidable.

30 Decidable Problems for Context Free Languages If G is a CFG, then L(G) is decidable. We can use the machine S, which decides A CFG = { H, w H is a CFG generating w}, to construct a TM M G to decide L(G). On input w, run S on G, w. If S accepts G, w, accept w. Otherwise reject w.

31 Closure Property?Is it closed under Set of Languages Operations RL CFL T-Recog T-Deci Union Y Y Y Y Concatenation Y Y Y Y Star Y Y Y Y Intersection Y N Y Y Complementation Y N N Y RL: Regular Languages; CFL: Context-Free Languages; T-Recog: Turing-Recognizable Languages; T-Deci: Turing Decidable Languages. The intersection of a CFL and a RL is a CFL. It is recommended by Ian Lindsey Berrigan.

32 Undecidable Problems There are problems/languages that are undecidable. In general, program verification (deciding whether a program runs correctly ) is undecidable. The Halting Problem: The most famous undecidable problem in CS. A TM is undecidable, where A TM = { M, w M is a TM and M accepts w}. The proof of this is a proof by contradiction.

33 Undecidable Problems Before we get to the proof, let us first observe that A TM is Turing-recognizable. Thus this theorem shows that recognizers are more powerful than deciders. The following TM U recognizes A TM. U = On input M, w, where M is a TM and w is a string: 1. Simulate M on input w. 2. If M ever enters its accept state, accept; if M ever enters its reject state, reject. Note that this machine loops on M, w if M loops on w. Hence A TM sometimes called the halting problem.

34 The Halting Problem A TM is undecidable, where A TM = { M, w M is a TM and M accepts w}. Proof. Suppose A TM is decidable. Then there s a machine H deciding it. The input of H is a string M, w. M is a TM, and w is a string. H runs M on w. If M accepts w, then H stops and accepts. If M does not accept w, then H stops and rejects. Observe that there is no string on which H does not halt.

35 The Halting Problem continued. Given H, we construct a new Turing machine D: The input of D is a string M. D runs H on M, M. If H accepts M, M, then D rejects M. If H rejects M, M, then D accepts M. Given the definition of H and D: On input M, D accepts M if M does not accept M. D rejects M if M accepts M.

36 The Halting Problem Given the definition of H and D: On input M, D accepts M if M does not accept M. D rejects M if M accepts M. On input D, we then find D accepts D if D does not accept D. D rejects D if D accepts D. Under the assumption that A TM is decidable, there is a machine D which simultaneously accepts and rejects its own specification D. This is a contradiction! And so the assumption that A TM is decidable is false. We conclude that A TM is undecidable.

37 Countability and Uncountability/Diagonalization Implicitly, the proof that A TM is undecidable relies on Cantor s diagonalization method. This is used to prove that a set is uncountable. Recall that a set S is countable if either it is finite or there is a one-to-one correspondence between elements of S and those of N. That is, there is a bijective function f from N to S. injective: For all a, b N, if a b, then f (a) f (b). surjective: For all b S, there is an a N such that f (a) = b. Intuitively, S is countable if we can write the elements of S as a list, and each element of S appears exactly once. A set A is countable if either it is finite or it has the same size as N. The set of positive even integers is countable, as is the set of positive odd numbers. The set of positive rational numbers is countable as well.

38 The set of finite strings are countable The set of finite strings S over any finite alphabet Σ is countably infinite. Assume an ordering of symbols in A. The strings of S can be ordered as follows: For any n 0, all string of length n come before all those of length n + 1. For any given n, arrange the strings of length n in according to the ordering of the alphabet symbols (as in a dictionary). Crucially, for any n, only a finite number of strings have length n. As a corollary to this, the set of Turing machines is countably infinite. Each TM can be written as a finite string over a given alphabet.

39 The set of infinite strings are uncountable In contrast, the set of infinite strings S over a finite alphabet Σ is not countable. We ll prove this using Cantor s diagonalization method, a type of proof by contradiction. Assume Σ = {0, 1} as the alphabet. An infinite binary sequence is an unending sequence of 0s and 1s. Each element of S is an infinitely long string of 0s and 1s. E.g., We will refer to particular bits of a string: b 1b 2b The Set B of infinitely long strings over Σ = {0, 1} is uncountable.

40 The set of infinite strings are uncountable The Set B of infinite strings over Σ = {0, 1} is uncountable. Proof. Assume B is countable. As such, the elements of B can be listed as a sequence s 1, s 2,... (there s a first element, a second element, a third...). Every element of B appears exactly once in the sequence. s 1 = b 1,1b 1,2b 1,3b 1,4b 1,5... s 2 = b 2,1b 2,2b 2,3b 2,4b 2,5... s 3 = b 3,1b 3,2b 3,3b 3,4b 3, Above, b i,j refers to the jth bit of the ith element of B.

41 The set of infinite strings are uncountable The Set B of infinite strings over Σ = {0, 1} is uncountable. Proof, continued. We construct an infinite bitstring X = c 1 c 2 c 3... by examining the diagonal: s 1 = b 1,1 b 1,2b 1,3b 1,4b 1,5... s 2 = b 2,1b 2,2 b 2,3b 2,4b 2,5... s 3 = b 3,1b 3,2b 3,3 b 3,4b 3, For each i 1, if b i,i = 1, then c i = 0. Otherwise c i = 1. For instance: If b 1,1 b 2,2 b 3,3 b 4,4 b 5,5... = Then c 1 c 2 c 3 c 4 c 5... = Observe that for any k, b k,k c k.

42 The set of infinite strings are uncountable The Set B of infinite strings over Σ = {0, 1} is uncountable. Proof, continued. We construct an infinite bitstring X = c 1 c 2 c 3...: If b 1,1 b 2,2 b 3,3 b 4,4 b 5,5... = Then c 1 c 2 c 3 c 4 c 5... = Observe that X is an infinite bitstring and so must be some s n on the list (it must appear somewhere on the list). And so b n,n (the nth bit of s n ) is the same as c n. However, by construction of X, b n,n c n. A contradiction! And so the assumption that B is countable must be false.

43 Characteristic Sequences For any finite Σ, Σ is a set of finite strings and so is countable. That is, we can list out the strings of Σ : s 1, s 2, s 3,.... Any language L over Σ can thus be represented as an infinite bitstring b 1 b 2 b s i L if and only if b i = 1. s i / L if and only if b i = 0. The string b 1 b 2 b 3... is called the characteristic sequence for L. It is clear that, for any Σ, there is a one-to-one correspondence between the languages over Σ and the set of infinite bit-strings. Given the previous proof, the following may be inferred. For any Σ, the set of languages over Σ is uncountable. Corollary There exist languages that are not Turing Recognizable.

44 Back to the Halting Problem The set of Turing Machines is countable, and so we can list them. The set of finite strings over Σ is also countable. We then can construct the following table. M 1 M 2 M 3 M 4... M M M M A 1 in a cell i, j means that M i accepts M j. A 0 in a cell i, j means that M i does not accept M j. And so the table gives a partial description of H.

45 Back to the Halting Problem Recall D: On input M i, D accepts M i if M i does not accept M i. D rejects M i if M i accepts M i. M 1 M 2 M 3 M 4... M M M M So, the output of D can be found by flipping bits in the diagonal.

46 Back to the Halting Problem Recall D: On input M i, D accepts M i if M i does not accept M i. D rejects M i if M i accepts M i. M 1 M 2 M 3 M 4... D... M M M M D ??? So, the output of D can be found by flipping bits in the diagonal. But D must be equal to some M n and so appear on the list. At cell (n, n), the contents is both 1 and 0. A contradiction!

47 Unrecognizable Languages Definition Language L is co-turing recognizable (co-recursively enumerable) if it is the complement of a Turing recognizable language. L is Turing decidable if and only if L is recognizable and co-recognizable. Proof. If L is decidable, then it is clearly recognizable. Also, if M is the machine deciding L, we construct a new machine M to decide L by simply doing the opposite of M. So M recognizes L. As such, L is co-recognizable. Now assume that L is recognizable and co-recognizable. Then L is recognizable, and TMs M 1 and M 2 recognize L and L. We construct TM M deciding L. On input w, M runs M 1 and M 2 in parallel. If M 1 accepts w, then M accepts. If M 2 accepts w, then M rejects. It is clear that it accepts all strings of L and reject all those not in L.

48 Unrecognizable Languages L is Turing decidable if and only if L is recognizable and co-recognizable. A TM = { M, w M is a TM and M accepts w} is undecidable. A TM is clearly recognizable, however. H runs M on w and accepts if and only if M accepts w. From this, the below holds. A TM is not recognizable. Proof. If A TM is recognizable, then a machine M recognizes it. We can construct a machine N to decide A TM by running M and H simultaneously. On any input w, one of them must accept w. If H accepts w, then N accepts. If M accepts, then N rejects. But A TM is not decidable, and so no such machine M exists.

49 More Decidable Problems Show the following languages/problems are decidable. EQ DFA,REX = { A, R A is a DFA, R is a regular expression and L(A) = L(R)} A = { M M is a DFA which does not accept any string containing an odd number of 1s } A ε CFG = { G G is a CFG that generates ε}

50 More Decidable Problems Show the following languages/problems are decidable. S = { M M is a DFA that accepts w R whenever it accepts w} ALL DFA = { A A is a DFA and L(A) = Σ } A = { R, S R and S are regular expressions and L(R) L(S)} A = { R R is a regular expression describing a language containing at least one string w that has 111 as a substring(i.e., w = x111y for some x and y }

51 More Decidable Problems Show the following languages/problems are decidable. INFINITE DFA = { A A is a DFA and L(A) is an infinite language} INFINITE PDA = { B B is a PDA and L(B) is an infinite language}

52 More Decidable Problems INFINITE DFA is decidable, where INFINITE DFA = { A A is a DFA and L(A) is an infinite language}. Proof. The following TM X decides INFINITE DFA. X = On input A where A is a DFA: 1. Let k be the number of states of A. 2. Construct a DFA D that accepts all strings of length k or more. 3. construct a DFA M such that L(M) = L(A) L(D). 4. Test L(M) =, using the E DFA decider T. 5. If T accepts, reject; if T rejects, accept.

53 More Decidable Problems INFINITE PDA is decidable, where INFINITE PDA = { B B is a PDA and L(B) is an infinite language}. Proof. The following TM Y decides INFINITE PDA. Y = On input B where B is a PDA: 1. Convert B to a CFG G and compute G s pumping length p. 2. Construct a regular expression E that contains all strings of length p or more. 3. construct a CFG H such that L(H) = L(G) L(E). 4. Test L(H) =, using the E CFG decider R. 5. If R accepts, reject; if R rejects, accept.