I have got following link:

https://www.cs.colostate.edu/~massey/Teaching/cs301/RestrictedAccess/Slides/301lecture06.pdf

In slide 13/15 they have shown that a^nb^n is not regular when n>=0.

However I can’t understand the last slide where they show that there is a contradiction.

contradiction.

I have attached the first and last slides.

Zulfi.