# Homework Help: Factorization Problem

1. May 21, 2006

### Oxymoron

I need to determine whether or not $\mathcal{O}_{-10} = \mathbb{Z}[\sqrt{-10}]$ is a unique factorization domain.

Now, I think the short answer is simply: NO.

The question is meant to be simple (I think).

I just finished proving that $\mathcal{O}_{-5} = \mathbb{Z}[\sqrt{-5}]$ is NOT a unique factorization domain and it took me two pages. It involved me finding an integer which had two DIFFERENT factorizations into irreducibles. Using maple and the "factorEQ" command with (numtheory) I found that

$$21 = 3\cdot 7 = (1+2\sqrt{-5})(1-2\sqrt{-5})[/itex] But now for the question at hand, Maple cannot find an integer which equals two factorizations of this sort because, unlike 5, 10 is not a prime. Ill let you guys muse over this for a while. Last edited: May 21, 2006 2. May 21, 2006 ### Oxymoron I suppose I could just use [tex]26 = 2\cdot 13 = (4+\sqrt{-10})(4-\sqrt{-10})$$

and show that no element of $\mathbb{Z}[\sqrt{-10}]$ of norm 2 or 13 hence 2 and 13 are irreducible. And then show there is no element of norm $4+\sqrt{-10}$ or $4-\sqrt{-10}$ and thus, they are irreducible. If I can show that then I have shown that 26can be written as two DIFFERENT factorizaruib into irreducibles which implies the ring is a UFD. This shouldnt be too hard if its the right thing to do. Is it the right thing to do?