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Michael Sipser Edition 3 Exercise 4 Question 31 (Page No. 212)

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Say that a variable $A$ in $CFL\: G$ is usable if it appears in some derivation of some string $w \in G$. Given a $CFG\: G$ and a variable $A$, consider the problem of testing whether $A$ is usable. Formulate this problem as a language and show that it is decidable.
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Michael Sipser Edition 3 Exercise 4 Question 29 (Page No. 212)
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Michael Sipser Edition 3 Exercise 4 Question 15 (Page No. 212)
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