Chapter 0: Q20E (page 1)
For each of the following languages, give two strings that are members and two strings that are not members鈥攁 total of four strings for each part. Assume the alpha-alphabet in all parts.
Short Answer
The solution is,

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Chapter 0: Q20E (page 1)
For each of the following languages, give two strings that are members and two strings that are not members鈥攁 total of four strings for each part. Assume the alpha-alphabet in all parts.
The solution is,

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Let . For each , let be the language consisting of all strings that contain an a exactly K places from the right-hand end.
Thus . Describe an NFA with states that recognizes in terms of both a state diagram and a formal description.
In the fixed-point version of the recursion theorem (Theorem 6.8), let the transformation t be a function that interchanges the states and in Turing machine descriptions. Give an example of a fixed point for t.
a). Let C be a context-free language and R be a regular language. Prove that the languageis context free.
b). Let A= { contains equal numbers of }. Use part to show that A is not a CFL
Modify the proof of Theorem 3.16 to obtain Corollary 3.19, showing that a language is decidable if some nondeterministic Turing machine decides it. (You may assume the following theorem about trees. If every node in a tree has finitely many children and every branch of the tree has finitely many nodes, the tree itself has finitely many nodes.)
Question: Describe the error in the following 鈥減roof鈥 that is not a regular language. (An error must exist because is regular.) The proof is by contradiction. Assume that is regular. Let p be the pumping length for localid="1662103472623" given by the pumping lemma. Choose s to be the string 0p1p . You know that s is a member of 0*1*, but Example 1.73 shows that s cannot be pumped. Thus you have a contradiction. So is not regular.
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