Parse Trees and CFGs in Chomsky Normal Form

Here we look at the structure of parse trees for context-free grammars in Chomsky Normal Form. The idea is that since the right hand side of any rule has at most 2 variables or terminals, the corresponding parse tree is always binary for all internal nodes (i.e., has two children), except for the nodes at the bottom, which all have exactly 1 or 0 children. We will use this to prove that there are some languages that are not context-free.

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