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May 31, 2017 at 3:57 history edited Stanley Yao Xiao CC BY-SA 3.0
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Aug 11, 2011 at 16:06 comment added Daniel Erman @Joe :) I agree it's useful to leave this answer.
Aug 11, 2011 at 15:02 history edited JSE CC BY-SA 3.0
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Aug 11, 2011 at 14:44 comment added Joe Silverman @Daniel: Sorry. I knew I must be missing something (you wouldn't have asked such an easy question!). But maybe it's useful to leave my "answer" to indicate why the result is trivial for a valuation ring (as opposed to a ring that happens to have a valuation).
Aug 11, 2011 at 14:25 comment added Daniel Erman I think this works when $Q=R$. But $Q$ is a polynomial ring over a valuation ring and is not itself a valuation ring (in particular it is not local). So if $a$ has valuation 0, then this does not imply that a is a unit. For instance, if $R=\mathbb Q[[u]]$, then a could be something like $s^2+ut^2$.
Aug 11, 2011 at 14:18 history answered Joe Silverman CC BY-SA 3.0