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$\{g\in\mathtt{CFG}(\{a,b\})\mid\mathcal{L}(g)=\{a,b\}^*\}\quad\leq\quad\{\langle G_1,G_2\rangle\mid\overline{\mathcal{L}(G_1)}\subseteq\overline{\mathcal{L}(G_2)}\}$
Reduce the universality problem on CFGs over $\{a,b\}$ to the inclusion problem
between the complement of the language of a CFG and the complement of the
language of another CFG, in order to prove that such problem is not
semi-decidable (not recursively enumerable).
Authors: Carles Creus, Guillem Godoy
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