Exercise 14:

{gCFG({a,b})L(g)={a,b}}{GL(G)={wa:w{a,b}}}\{g\in\mathtt{CFG}(\{a,b\})\mid\mathcal{L}(g)=\{a,b\}^*\}\quad\leq\quad\{G\mid\mathcal{L}(G)=\{wa : w\in\{a,b\}^*\}\}
Reduce the universality problem on CFGs over {a,b}\{a,b\} to the problem of whether a CFG generates all words over {a,b}\{a,b\} that end with aa, in order to prove that such problem is not semi-decidable (not recursively enumerable).
Authors: Carles Creus, Guillem Godoy / Documentation:
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