4.3, due on 2/4

1.  I'm not sure if I understood the definition of a unit before.  It seemed trivial to me but Theorem 4.8 seemed very clear and non-trivial to me.

2. I thought it was interesting that in fields irreducible is prime and reducible is composite.  I also thought it was interesting that every polynomial of degree 1 is irreducible, which means that in a field there are an infinite number of primes (irreducibles).