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Let $\mathfrak p $ be a prime ideal in a Dedekind domain $O$ with field of fractions $K$. Define $$\mathfrak p^{-1}= \{x \in K: x\mathfrak p \subset O\}.$$ Let $\mathfrak a \subset \mathfrak p$ and $\mathfrak a \neq \mathfrak p$ be an ideal of $O$. How do I prove

i) $\mathfrak a\mathfrak p^{-1} \neq \mathfrak p\mathfrak p^{-1}$;

ii)$(\mathfrak a\mathfrak p^{-1})\mathfrak p = \mathfrak a$.

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@Downvoters why the downvotes? –  Mohan Mar 4 '13 at 14:36
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1 Answer 1

up vote 3 down vote accepted

Massive hint: (1) and (2) follow immediately from a result in algebraic number theory that $pp^{-1} = \mathcal{O}$. I can add a proof of this result if you would like. For example in (1) if we had $ap^{-1} = pp^{-1} = \mathcal{O}$ then we get $a = p\mathcal{O} = p$ contradicting $a \neq p$.

Edit: If I recall correctly the result that $pp^{-1} = \mathcal{O}$ can be found in Neukirch.

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