Showing that Z[Sqrt[-3]] is a Euclidean Domain

**roninpro** · November 22nd 2010, 09:56 PM

Hello everyone,

I'm having a bit of trouble showing that $\mathbb{Z}[\sqrt{-3}]$ is a Euclidean domain. My attempt is:

For this ring, define the norm $N(a+b\sqrt{-3})=a^2+3b^2$ . We need to show that there is a division algorithm, so let $\alpha=a+b\sqrt{-3}$ and $\beta=c+d\sqrt{-3}$ . We will compute $\alpha/\beta$ as complex numbers and fish for an appropriate quotient and remainder. We have
$\displaystyle \frac{\alpha}{\beta}=\frac{ac+3bd}{c^2+3d^2}+\frac {bc-ad}{c^2+3d^2}\sqrt{-3}$
Now choose a quotient $q\in \mathbb{Z}[\sqrt{-3}]$ such that $|Re(q)-\frac{ac+3bd}{c^2+3d^2}|\leq 1/2$ and $|Im(q)-\frac{bc-ad}{c^2+3d^2}|\leq \sqrt{3}/2$ . Then we have $\alpha/\beta=q+r/\beta$ , where $r/\beta$ is the leftover "fractional" term. From the choice of $q$ , we have $N(r/\beta)\leq 1$ , and $N(r)\leq N(\beta)$ .

At this point, I can't shake the possible equality. I would appreciate any suggestions on this issue.

**tonio** · November 23rd 2010, 01:50 AM

Originally Posted by roninpro

Hello everyone,

I'm having a bit of trouble showing that $\mathbb{Z}[\sqrt{-3}]$ is a Euclidean domain.

No wonder: it is not. For example, check $4=2\cdot 2=(1+\sqrt{-3})(1-\sqrt{-3})$ ...

Tonio

My attempt is:

For this ring, define the norm $N(a+b\sqrt{-3})=a^2+3b^2$ . We need to show that there is a division algorithm, so let $\alpha=a+b\sqrt{-3}$ and $\beta=c+d\sqrt{-3}$ . We will compute $\alpha/\beta$ as complex numbers and fish for an appropriate quotient and remainder. We have
$\displaystyle \frac{\alpha}{\beta}=\frac{ac+3bd}{c^2+3d^2}+\frac {bc-ad}{c^2+3d^2}\sqrt{-3}$
Now choose a quotient $q\in \mathbb{Z}[\sqrt{-3}]$ such that $|Re(q)-\frac{ac+3bd}{c^2+3d^2}|\leq 1/2$ and $|Im(q)-\frac{bc-ad}{c^2+3d^2}|\leq \sqrt{3}/2$ . Then we have $\alpha/\beta=q+r/\beta$ , where $r/\beta$ is the leftover "fractional" term. From the choice of $q$ , we have $N(r/\beta)\leq 1$ , and $N(r)\leq N(\beta)$ .

At this point, I can't shake the possible equality. I would appreciate any suggestions on this issue.

.

**roninpro** · November 23rd 2010, 06:09 AM

Ah, yes - the statement of the problem in the book was a little bit confusing. I think that they meant to put $\alpha=\frac{1+\sqrt{-3}}{2}$ and to show $\mathbb{Z}[\alpha]$ is a Euclidean domain. I'll go back and look at it again.

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