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# hw06 - CS 154 Intro to Automata and Complexity Theory...

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CS 154 Intro. to Automata and Complexity Theory Handout 24 Autumn 2009 David Dill November 10, 2009 Problem Set 6 Due: November 17, 2009 Homework: (Total 100 points) Do the following exercises. Problem 1 [20 points] This was the example CFG in lecture for regular expressions over the alphabet Σ = { a, b } . Rewrite the grammar into an equivalent grammar (i.e., one with the same language) that is unambiguous. The resulting grammar should group operations according to their precedence ( * should be “stickier” than · , which should be “stickier” than +). Draw the parse tree for ( a + b · a ) * using your new grammar. R → ∅ R e R a R b R R + R R R · R R R * R ( R ) Problem 1’ [20 points] If you felt that the previous problem was too easy, you can do this one instead: Give a context-free grammar for the set of all strings over the al- phabet { a, b } that are not of the form ww (i.e, the same string concatenated with itself). (This is exercise 5.1.1 (c) in the textbook.)

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hw06 - CS 154 Intro to Automata and Complexity Theory...

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