The halting problem for Turing machine is un-decidable

Determining whether ambiguity a context free grammar is un-decidable

Given two arbitrary context free grammars $${G_1}$$ and $${G_2}$$ whether $$L\left( {{G_1}} \right) = L\left( {{G_2}} \right)$$

Given two regular grammars $${{G_1}}$$ and $${{G_2}},$$ it is un-decidable whether $$L\left( {{G_1}} \right) = L\left( {{G_2}} \right)$$