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Is $\mathbb{Z}[\sqrt{2},\sqrt{3}]$ flat over $\mathbb{Z}[\sqrt{2}]$? The definitions doesn't seem to help. An idea of how to look at such problems would be helpful.

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    Dear rola, It is pretty obviously free, and free modules are flat. Regards,2012-07-24
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    @MattE That's a lot better — would you mind posting that as an answer? I can incorporate it into what I wrote if you don't have the time, but it wouldn't be as good. I somehow convinced myself that the module structure would be weird, but of course it isn't. Cheers,2012-07-24
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    Dear Dylan, Done. Cheers,2012-07-24
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    @rola Note that there is still something to do here: take what you believe is a basis for $\mathbb Z[\sqrt2, \sqrt3]$ over $\mathbb Z[\sqrt2]$ and prove that it is one.2012-07-24

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$\mathbb Z[\sqrt{2},\sqrt{3}]$ is freely generated as a $\mathbb Z[\sqrt{2}]$-module (exercise). Free modules are flat. QED

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    Thanks a lot Matt for taking time to answer.2012-07-24
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I have a way of deciding this, although I don't like it very much.

The ring $\mathbb Z[\sqrt2]$ is a Dedekind domain — it's the ring of integers of $\mathbb Q(\sqrt2)$. A module over a Dedekind domain is flat if and only if it is torsion-free. Why? Well, flatness can be checked at each prime, each localization of a Dedekind domain at a prime is a PID, and the result is true for PIDs.

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    Unlike you, I like this answer: +1 !2012-07-24