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$\{g\in\mathtt{CFG}(\{a,b\})\mid\mathcal{L}(g)=\{a,b\}^*\}\quad\leq\quad\{G\mid\mathcal{L}(G)=\{w\in\{a,b\}^* : |w|_a=|w|_b\}\}$
Reduce the universality problem on CFGs over $\{a,b\}$ to the problem of
whether a CFG generates the words over $\{a,b\}$ with as many occurrences of
$a$ as occurrences of $b$, in order to prove that such problem is not
semi-decidable (not recursively enumerable).
Authors: Carles Creus, Guillem Godoy
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