Table of Contents

8. Proving languages are not regular

8.1. The pumping lemma

8.1.1. Show that the pumping lemma holds for finite languages.

8.1.2.* Find a language which is not regular for which the pumping lemma holds.

8.2. Languages which are not regular

Show that each of the languages from the list below is not regular.

8.2.1. $ L = \{ \: A^n B^m \: | \: 0 \leq n \leq m \: \} $

8.2.2. $ L = \{ \: w \in \{A,B\}^* \: | \: \#A(w) = \#B(w) \: \} $

8.2.3. $ L = \{(01)^n(10)^n \mid n > 0 \} $

8.2.4. $ L = \{ \: w \in \{A,B\}^* \: | \: \text{w is a palindrome} \: \} $

8.2.5. $ L = \{ \: w \in \{0\}^* \: | \: \text{the length of w is a prime number} \: \} $

8.2.6. $ L = \{ \: w \in \{0\}^* \: | \: \text{the length of w is a power of two} \: \} $

8.2.7. $ L = \{ \: ww^R \: | \: w\in \{0,1\}^* \} $

8.3. Combining the pumping lemma with closure properties

8.3.1. Using the pumping lemma, prove that $ L = \{ \: A^nB^m \: | \: n \neq m \}$ is not a regular language.