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How do I verify that $\mathbb{Z}[\sqrt{2}] = \{ a +b\sqrt{2} \, | \, a,b \in \mathbb{Z} \}$ is a subring of $\mathbb{R}$ ? I'm thinking that i have to show that it's a subgroup which is closed under multiplication, is that correct?

Also, I have to show that $\mathbb{Z}[\sqrt{2}]^*$ is infinite. It furthermore says I'm supposed to consider powers of $1+\sqrt{2}$ . I am, however, completely lost on this one. I thought $\mathbb{Z}^* = \{ -1,1 \}$, and therefore $\mathbb{Z}[\sqrt{2}]^*$ would be $={-\sqrt{2},-1,1,\sqrt{2}}$, but I'm obviously wrong.

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Cathrine, the reason that your formulas were not showing correctly was that you needed to enclose them between $ symbols. If you click on the edit history you'll see the changes I made. Also, if I'm not mistaken, the "algebra" tag is no longer used. Instead you should use the "abstract algebra" tag for this kind of topics. –  Adrián Barquero Nov 12 '12 at 2:39
    
Oh, I'll remember that. Thank you so much for your help! –  Cathrine Nov 12 '12 at 2:42
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2 Answers

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I'm going to try to answer your second question, since showing that it's a subring only needs you to verify the conditions in the definition of a subring.

It seems that you're having some trouble understanding what $\mathbb{Z}[\sqrt{2}]^*$ is. By definition $\mathbb{Z}[\sqrt{2}]^*$ is the group of units of the ring $\mathbb{Z}[\sqrt{2}]$. It consists of elements $\alpha = a + b \sqrt{2}$ with $a, b \in \mathbb{Z}$ such that $\alpha$ has a multiplicative inverse in $\mathbb{Z}[\sqrt{2}]$.

This just means that if $\alpha = a + b \sqrt{2}$ is in $\mathbb{Z}[\sqrt{2}]^*$, then there must exist another element $\beta = c + d\sqrt{2}$ in $\mathbb{Z}[\sqrt{2}]$ such that $\alpha \beta = 1$.

Another fact that might be useful is that since $\mathbb{Z}[\sqrt{2}]^*$ is a group, then given any two elements $\alpha_1, \alpha_2 \in \mathbb{Z}[\sqrt{2}]^*$, then their product $\alpha_1 \alpha_2 \in \mathbb{Z}[\sqrt{2}]^*$ also.

Certainly $\pm 1 \in \mathbb{Z}[\sqrt{2}]^*$, but $\pm \sqrt{2} \notin \mathbb{Z}[\sqrt{2}]^*$ because for $\pm \sqrt{2}$ to be in $\mathbb{Z}[\sqrt{2}]^*$, there must be an element $c + d\sqrt{2}$ with $c, d \in \mathbb{Z}$ such that $\pm \sqrt{2}(c + d\sqrt{2}) = 1$. But observe that

$$ \begin{align} \pm \sqrt{2}(c + d\sqrt{2}) = 1 &\iff \pm c\sqrt{2} + \pm 2d = 1 \\ &\iff c = 0 \quad d = \pm\frac{1}{2} \end{align} $$

But then since $d = \pm \frac{1}{2} \notin \mathbb{Z}$, this shows that $\pm \sqrt{2} \notin \mathbb{Z}[\sqrt{2}]^*$.

On the other hand, since $(1 + \sqrt{2})(-1 + \sqrt{2}) = 1$, then $1 + \sqrt{2} \in \mathbb{Z}[\sqrt{2}]^*$.

Now, you have to show that $\mathbb{Z}[\sqrt{2}]^*$ has infinitely many elements, and for this you're provided with the hint to consider powers of $1 + \sqrt{2}$. Well, since we already know that $1 + \sqrt{2} \in \mathbb{Z}[\sqrt{2}]^*$ and that products of elements in $\mathbb{Z}[\sqrt{2}]^*$ also lie in $\mathbb{Z}[\sqrt{2}]^*$ then we can conclude that any power $(1 + \sqrt{2})^n \in \mathbb{Z}[\sqrt{2}]^*$. Finally, since $1 + \sqrt{2} > 1$ then each succesive positive power gets bigger, so that we have actually showed infinitely many different elements in $\mathbb{Z}[\sqrt{2}]^*$

$$ 1 < 1 + \sqrt{2} < (1 + \sqrt{2})^2 < (1 + \sqrt{2})^3 < \cdots $$

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Given a ring $R$, a subset $S\subseteq R$ is a subring if it contains the multiplicative identity of R and is closed under the "subtraction" and multiplication of $R$.

For the second question, recall that $\mathbb{Z}[\sqrt{2}]=\{a+b\sqrt{2}\,|\,a,b\in\mathbb{Z}\}$. So $\mathbb{Z}[\sqrt{2}]^*$ is the set of elements of the form $a+b\sqrt{2}$ ($a,b\in\mathbb{Z}$) that have multiplicative inverses. So think about the hint: does $x=1+\sqrt{2}$ have a multiplicative inverse? What about powers of $x$?

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I understand now. Thank you very much! –  Cathrine Nov 12 '12 at 3:05
    
Very glad to help –  Bey Nov 12 '12 at 3:14
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