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Transparency No. 7-1 Formal Language and Automata Theory Chapter 7 Limitations of Finite Automata (lecture 11 and 12)

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Limitations of FAs Transparency No. 7-2 Limitations of FAs Problem: Is there any set not regular ? ans: yes! example: B = {a n b n | n 0 } = { ,ab,aabb,aaabbb,…} Intuition: Any machine accepting B must be able to remember the number of a’s it has scanned before encountering the first b, but this requires infinite amount of memory (states) and is beyond the capability of any FA, which has only a finite amount of memory (states). Pf: Assume B is regular = L(M) for some DFA M. Let k = |Q|. consider the string a n b n with n >> k. Let s a s 1 a s 2 a …. s n-1 a s n b s n+1 b …. b s 2n be any path witnessing the acceptance of a n b n. now consider the sequence of states s 0,s 1,…,s n, by pigeonhole principle there are 0 < i < j < n s.t. s i = s j. => (s i, a n-i b n ) = (s j, a n-j b n ) = s 2n F, and (s, a i ) = (s,a j ) = s i = s j. => (s, a n-(j-i) b n ) = (s, a i a n-j b n ) = ( (s,a i ), a n-j b n ) = (s j, a n- j b n ) = s 2n F => a n-(j-I) b n B, a contradiction.

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Limitations of FAs Transparency No. 7-3 Another nonregular set C= {a 2 n | n > 0} = {a, aa, aaaa, aaaaaaaa, … } is nonregular pf: assume C is regular and is accepted by a DFA with k states. Let n > k and x = a 2 n C. Now consider the sequence of states: (s,a), (s,aa),…., (s,a n ), by pigeonhole principle, there are 0<i<i+j n s.t. (s,a i ) = (s,a i+j ) [ = p] let 2 n = i + j + m. => (s, a 2 n +j ) = (s, a i a j a j a m ) = (s,a i a j a m ) = (s, a 2 n ) F. But since 2 n + j 2 n + n 2 n, a 2 n +j C => the DFA accept a string ∉ C, a contradiction! Hence C is not regular.

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Limitations of FAs Transparency No. 7-4 The pumping lemma Theorem 11.1: If A is a regular set, then (P): k 0 s.t. for any string x,y,z with xyz A and |y| k, there exist strings u,v,w s.t. y = uvw, v and for all i 0, the string xuv i wz A. pf: Similar to the previous two examples. Let k = |Q| where Q is the set of states in any FA accepting A. Theorem 11.2 (pumping lemma, the contropositive form) If A is any set of strings satisfying the property (~P): k 0 xyz A s.t. |y| k and u,v,w with uvw = y and v , there exists an i 0 s.t. xuv i vw A, then A is not regular. [I.e., ~P means for any k ≥ 0, there is a substring of length ≥ k [of a member] of A, a cut or a certain duplicates of the middle of any 3- segment decomposition of which will produce a string ∉ A. ]

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Limitations of FAs Transparency No. 7-5 Game-theoretical proof of nonregularity of a set 1. Two players: You (want to show that A is not regular) Demon (or Opponent want to show A is regular) 2 The game proceeds as follows: 1. D picks k (if A is regular, D’s best strategy is to pick k = #states of a FA accepting A) 2. Y picks x,y,x with xyz A and |y| k. 3. D picks u,v,w s.t. y = uvw and v . 4. Y picks i 0 3. Finally Y wins if xuv i wz A and D wins if xuv i wz A. 4. By Theorem 11.2, A is not regular if there is a winning strategy according to which Y always win. Note: P is a necessary but not a sufficient condition for the regularity of A (i.e., there is nonregular set A satisfying P).

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Limitations of FAs Transparency No. 7-6 Using the pumping lemma Ex1: Show the set A = {a n b m | n m } is not regular. the proof: 1. D gives k [for any k 0] 2. Y pick x = a k, y = b k, z = [ xyz in A with |y| k] ==> xyz = a k b k A 3. D decompose y = uvw with [for all uvw with uvw=y and |u|=j, |v|=m > 0 and |w|= n v ] 4. Y take i = 2. [ i 0 s.t. xuv i wz A] => xuv 2 wz = a k b j b 2m b n = a k b k+m A => Y wins. Hence A is not regular. Ex2: C = {a n! | n 0 } is not regular. pf: similar to Ex1. Left as an exercise. hint: for any k > 0 D chooses, let xyz =a kxk! a k! and let i = 0.

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Limitations of FAs Transparency No. 7-7 Other techniques: Using closure property of regular sets. Ex3: D = { x {a,b}* | #a(x) = #b(x) } = { , ab, ba, aabb, abab. baba, bbaa, abba, baab,… } is not regular. (Why ?) if regular => D a*b* = {a n b n | n 0 } = B is regular. But B is not regular, D thus is not regular. [H2E2:] A: any language; if A is regular, then rev(A) = def {x n x n-1 …x 1 | x 1 x 2 …x n A} is regular. Ex4: A = {a n b m | m n } is not regular. pf: If A is regular => rev(A) and h((rev(A)) = {a n b m | n m} is regular, where h(a) = b and h(b) = a. => A h(rev(A)) = {a n b n | n 0} is regular, a contradiction!

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