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Unformatted text preview: CS 154 Intro. to Automata and Complexity Theory Handout 12 Autumn 2008 David Dill October 13, 2009 Solution Set 2 1 Problem 1 L ( M ) = ( ab ) c Problem 2 a. L 1 = { w  w contains twice as many 1s as 0s } is not regular. Pumping Lemma: Proof Assume L 1 is regular and apply the Pumping Lemma. By P.L. there exists some pumping constant n > 0. We choose w = 0 n 1 2 n , which is in L 2 and satisfies  w  n . By P.L. w = xyz such that  xy  n and y negationslash = . Note that the first condition implies that both x and y contain only 0s. Let  x  = a and  y  = b , the second condition implies b > 0. We choose k = 0: xz L 1 by P.L., but xz = 0 n b 1 2 n which is not in L 1 since b negationslash = 0. Thus we get a contradiction, which implies our initial assumption was wrong. We conclude that L 1 is not regular. Closure Properties: Proof Suppose L 1 were regular. Define h : { , 1 } { , 1 } so that h (0) = 0 and h (1) = 11. Then { i 1 i  i } = h 1 [ L 1 L (0 1 )]. To see this, we show that w { i 1 i  i } iff h ( w ) h 1 [ L 1 L (0 1 )]. If w { i 1 i  i } , it is obvious that h ( w ) L 1 L (0 1 ). If w negationslash { i 1 i  i } , then it must either have different numbers of 0s and 1s, in which case h ( w ) will not have twice as many 1s and 0s as required to be in L 1 , or w has a 1 before a 0, in which case h ( w ) will have the same property and will not be a member of L (0 1 ).) Since the regular languages are closed under inverse homomorphisms, { i 1 i  i } would be regular, but it is known not to be. Therefore, L 1 must not be regular.must not be regular....
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This note was uploaded on 01/12/2010 for the course CS 154 at Stanford.
 '08
 Motwani,R

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