1. Use Pumping Lemma to show that the following languages over {a, b} are not regular.

a) L1 = {x | n_a (x) = n_b (x)}

b) L2 = {xx | x {a,b} ^* }

c) L3 = {xx^R | x {a,b} ^* }

d) L4 = {a^n b^m | n < m}