Q1.9

The Halting Problem of Turing machines is undecidable

Determining whether a context-free grammar is ambiguous is undecidable

Given two arbitrary context-free grammars $G_1$ and $G_2$ it is undecidable whether $L(G_1) = L(G_2)$

Given two regular grammars $G_1$ and $G_2$ it is undecidable whether $L(G_1) = L(G_2)$