Math and science::Theory of Computation

# Context-free languages: closed operations

### Context-free language closure

If $$L$$ and $$M$$ are context-free languages, then the following are also context-free languages:

• $$L \cup M$$
• [operation 2]
• [operation 3]

In other words, context-free languages are closed under union, [operation 2] and [operation 3].

There are important operations under which context free languages are not closed. Can you remember them?