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# ch1 - b then two repetitions of a and ﬁnally aero...

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Undergraduate Course ELEMENTS OF COMPUTATION THEORY College of Computer Science Chapter 1 Zhejiang University Fall-Winter, 2007 P 46 1.7.4 Show each of the following. (c) If a and b are distinct symbols, then { a, b } * = { a } * ( { b }{ a } * ) * . (d) If Σ is an alphabet, e L 1 Σ * and e L 2 Σ * , then ( L 1 Σ * L 2 ) = Σ * . Solution: (c) It is obvious that { a } * ( { b }{ a } * ) * ⊆ { a, b } * . On the other hand, suppose that w ∈ { a, b } * , then w = a * or w = a * ba * ba * · · · ba * ∈ { a } * ( { b }{ a } * ) * . (d) Suppose w ( L 1 Σ * L 2 ). Then w = xyz , where x L 1 Σ * , y Σ * , z L 2 Σ * . Thus xyz * ) * = Σ * . On the other hand, suppose w Σ * . Then because e L 1 and e L 2 , w = ewe ( L 1 Σ * L 2 ). 1.7.6 Under what circumstances is L + = L * - { e } ? Solution: L + = L * - { e } exactly when e 6∈ L . P 51 1.8.3 Let Σ = { a, b } . Write regular expressions for the following sets: (c) All strings in Σ * with exactly one occurrence of the substring aaa . Solution: (( a aa b * ) b ) * aaa ( bb * ( a aa b * )) * .

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1.8.5 Which of the following are true? Explain. (a) baa a * b * a * b * (b) b * a * a * b * = a * b * (c) a * b * b * a * = (d) abcd ( a ( cd ) * b ) * Solution: (a) true. baaa consists of zero repetitions of a , followed by one repetition of
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Unformatted text preview: b , then two repetitions of a , and ﬁnally aero repetitions of b . (b) true. Any string described by a * b * consists of s string of a s followed by a string b s. If b * a * also describes this string, then there cannot be any as followed by a b in the string, so either there are zero a s or zero b s, making it into a string of any num-ber of a s or a string of any number of b s, by taking zero repetitions of b or a , res-pectively. (c) false. Any string consisting only of b s is described both by a * b * and by b * c * , so that their intersection is not the empty set, but rather b * . (d) false. If d appears in a string described by ( a ( cd ) * b ) * , it must be immediately followed by a c or a b . But this is not the case in abcd ....
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