COMP-330 Theory of Computation. Fall Prof. Claude Crépeau. Lec. 5 : DFA minimization

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1 COMP-33 Theory of Computation Fall Prof. Claude Crépeau Lec. 5 : DFA minimization

2 COMP 33 Fall 27: Lectures Schedule 4. Context-free languages 5. Pushdown automata 6. Parsing 7. The pumping lemma for CFLs 8. Introduction to computability -2. Introduction.5. Some basic mathematics 2-3. Deterministic finite automata +Closure properties, 3-4. Nondeterministic finite automata 5. Minimization+ Myhill-Nerode theorem 9. Models of computation 6. Determinization+Kleene s theorem 7. Regular Expressions+GNFA 8. Regular Expressions and Languages 9-. The pumping lemma. Duality 2. Labelled transition systems 3. MIDTERM Basic computability theory 2. Reducibility, undecidability and Rice s theorem 2. Undecidable problems about CFGs 22. Post Correspondence Problem 23. Validity of FOL is RE / Gödel s and Tarski s thms 24. Universality / The recursion theorem 25. Degrees of undecidability 26. Introduction to complexity

3 N, {,2} {,3} {} {2} {3} {,4}, {4} {,2,3} {,2,4} {,3,4} {2,3,4} {2,3} {2,4} {3,4} {,2,3,4}

4 Unreachable States {,2} {,3} {,4} {} {2} {3} {4} {,2,3} {,2,4} {,3,4} {2,3,4} {2,3} {2,4} {3,4} {,2,3,4}

5 Reachable States {} {,2,3} {,3} {,3,4} {,4} {,2,3,4}

6 Redondant States 2 3, 4, 5

7 Redondant States 2,, 5 4

8 Myhill-Nerode Theorem John R. Myhill Anil Nerode

9 Myhill-Nerode Theorem Let x and y be strings and L be a language. We say that x and y are distinguishable by L if there exists a z such that xz L and yz L or yz L and xz L. If x and y are indistinguishable by L we write x L y, ( L is an euivalence relation ). If x, y are distinguishable by L we write x L y.

10 Distinguishable Strings L 2 3, 4, 5 there exists a z such that xz L and yz L. z= is such that L while L.

11 Indistinguishable Strings L 2 3, 4, 5 There does not exist a z such that xz L and yz L nor yz L and xz L. For all z, xz and yz are both in L or neither in L.

12 Myhill-Nerode Theorem Let L be a language and X a set of strings. We say that X is pairwise distinguishable by L if eve r y two elem ents in X are distinguishable by L (For all x,x in X, x L x ). Define the index of L to be the size of a m a x i m u m s e t X t h at i s p a i r w i s e distinguishable by L. The index may be finite or infinite.

13 Distinguishable Strings 2 3, 4, 5 While this automaton has 5 states, the index of L is only 4: E,, and L while L, L while L, L while E L, L while L, L while L, L while L.

14 Myhill-Nerode Theorem a. If L is recognized by a DFA with k states, L has index at most k. b. If the index of L is a finite number k, it is recognized by a DFA with k states. c. L is regular iff it has finite index. This index is the size of the smallest DFA recognizing L.

15 Myhill-Nerode Theorem a. If L is recognized by a DFA with k states, L has index at most k. (a) Let M be a k state DFA recognizing L. Suppose L has index larger than k. Some X with k+ elements is distinguishable by L. But since the number of states < k+ there must exist x,y in X such that δ(,x) = δ(,y).but then, x and y are not distinguishable. A contradiction.

16 Myhill-Nerode Theorem a. If L is recognized by a DFA with k states, L has index at most k. (a) Let M be a k state DFA recognizing L. Suppose L has index larger than k. Some X with k+ elements is distinguishable by L. But since the number of states < k+ there must exist x,y in X such that δ(,x) = δ(,y).but then, x and y are not distinguishable. A contradiction.

17 Myhill-Nerode Theorem b. If the index of L is a finite number k, it is recognized by a DFA with k states. (b) Let X={s,...,s k } be pairwise distinguishable by L. Let Q={,..., k } be the states of a DFA recognizing L and define δ( i,a)= j s.t. s j L s i a. Let be the i s.t. s i L E. Let F={ i s i L }. M is s.t. { s δ(,a)= i } = { s s L s i }.

18 Myhill-Nerode Theorem c. L is regular iff it has finite index. This index is the size of the smallest DFA recognizing L. (c) L is regular implies the existence of a DFA recognizing L. By (a), L has index at most k. If L has index k then by (b) there exists a DFA with k states (i.e. L is regular). As for the minimality, if the index of L is not the size of the minimal DFA then there exists a DFA with index- states recognizing L. But this is impossible by part (a).

19 Minimizing via Myhill-Nerode Theorem Let L be a regular language. Compute the index of L by finding the set X of all the strings that are pairwise distinguishable by L. All strings considered as x, y, xz and yz may be shorter than the number of states of a DFA accepting L. Every string which is longer is euivalent to a shorter one obtained by pumping down.

20 Computing Index 2 3, 4, 5 If we consider all 63 strings of length up to 5, we get: E L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L

21 Minimizing via Myhill-Nerode Theorem Let L be a regular language. Compute the index of L by finding the set X of all the strings that are pairwise distinguishable by L. Using part (b) of the Myhill-Nerode Theorem we construct a minimal DFA to accept L.

22 Minimal DFA (b) Let X={s,...,s k } be pairwise distinguishable by L. Let Q={,..., k } be the states of a DFA recognizing L and define δ( i,a)= j s.t. s j L s i a. Let be the i s.t. s i L E. Let F={ i s i L }. M is s.t. { s δ(,a)= i } = { s s L s i }. E,,

23 Application of the Myhill-Nerode Theorem Given two regular expressions R and R we can find out whether they generate the same regular language or not : Given R and R, compute NFA N and N accepting them as in Lemma.55. Compute DFA M and M as in Theorem.39. Using part (b) of the Myhill-Nerode Theorem we construct a minimal DFA W and W for each of them. L(R)=L(R ) iff W W.

24 Application of the Myhill-Nerode Theorem B = { n n n } is non-regular because it has infinite index. Consider the set X={ n n }. It s an infinite set that is pairwise distinguishable by B. Proof: For all n, n is distinguishable from all previous i, i<n, because there exists a z= n s.t. n z B while i z B, i<n. QED

25 Application of the Myhill-Nerode Theorem F = { ww w * } is non-regular because it has infinite index. Consider the set X={ n n }. It s an infinite set that is pairwise distinguishable by F. Proof: For all n, n is distinguishable from all previous i, i<n, because there exists a z= n s.t. n z B while i z B, i<n. QED

26 COMP-33 Theory of Computation Fall Prof. Claude Crépeau Lec. 5 : DFA minimization