The pumping lemma for regular languages:
If a language L is regular, there exists a constant p  1 such that every sentence s  L of
length  p can be written as xyz =s satisfying these three conditions: |y|>0, |xy|p, and for all
i0, xyi z  L.
Problem 2.
a) Prove that L2 is decidable.
b) Use the minimum-state lemma to prove that L4 is not regular.