February 27

• Know the pumping lemma is used to prove a language is not regular
• Be able to figure out when a language is regular or not
• Ideally: be able to use the pumping lemma to prove a language is not regular. We looked at several proofs

  1. a^nb^n 

  2. same number of a's as b's
  3. doubleword ww : where w is a string of a's and b's
  4. 1^*0^n1^n
  5. strings that have a different number of a's than b's. Use the fact that the complement of a regular language is regular. We know that same number of a's as b's is not regular.

  6. a^nb^m, n >= m