15-453 Formal Languages, Automata, and Computation
Lecture 12: Non-Context-Free Languages

We introduce a version of the pumping lemma for context-free languages which allows us to show that various languages are not context free. Because of the added expressive power of pushdown automata as compared to finite automata, the pumping lemma and its application are more complicated. As we will see later in this class, even more powerful models such as Turing machine require an entirely new technique for showing that they cannot recognize certain languages.

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Frank Pfenning