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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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- Spring '09
- Computer Science, Following, Regular expression, a∗ b∗