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Question: Is it true that $$\mathbb{Z}[\sqrt{-3}] + \mathbb{Z}[\sqrt{-5}]$$ not a ring?

I think it is not because it is not closed under multiplication. More precisely, if $a_1+b_1\sqrt{-3}, a_2+b_2\sqrt{-3}\in \mathbb{Z}[\sqrt{-3}]$ and $c_1+d_1\sqrt{-5},c_2+d_2\sqrt{-5}\in \mathbb{Z}[\sqrt{-5}],$ then $$(a_1+b_1\sqrt{-3} + c_1+d_1\sqrt{-5}) \cdot (a_2+b_2\sqrt{-3} + c_2+d_2\sqrt{-5})$$ is not an element of $\mathbb{Z}[\sqrt{-3}] + \mathbb{Z}[\sqrt{-5}].$

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  • $\begingroup$ That would be correct. $\endgroup$ – Parcly Taxel Nov 17 '18 at 4:50
  • $\begingroup$ That can be an element of $\Bbb Z[\sqrt{-3}]+\Bbb Z[\sqrt{-5}]$ for some values of the variables. To show it's not closed under multiplication, all one has to do is give an example. The easy example is $\sqrt{-3}\times\sqrt{-5}$. $\endgroup$ – Lord Shark the Unknown Nov 17 '18 at 5:28
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You don't even have to be so general about it. A single counterexample is enough (and often easier to actually prove).

For instance, $\sqrt{-3}$ and $\sqrt{-5}$ are both in your set, but their product (whether that's $-\sqrt{15}$ or $\sqrt{15}$) isn't.

However, $\Bbb Z[\sqrt{-3}]+\Bbb Z[\sqrt{-5}]+\Bbb Z[\sqrt{15}]$ is a ring.

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