http://img17.imageshack.us/img17/3937/vjj6.png [Broken] I don't understand the very last bit, where it says a and b are central and so c is, hence a is not irreducible. Basically what replaces the "hence"? I can't seem to figure out the link between c being central meaning a isn't irreducible. a being irreducible means it isn't a unit, hence D[x]a is proper but if it is not maximal it sits inside an ideal D[x]b where b is a central non-unit. Hence a = cb for some c. Since a and b are non-units, to be irreducible c must be a unit. But then c being central contradicts it being a unit? Why? There are central units contained in H[x] for instance (the ring of quaternions), a real number is a central unit.