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Sep 12, 2011 at 20:11 comment added Kevin Ventullo Heh, yes I suppose that is an easier way of saying it.
Sep 10, 2011 at 15:12 comment added Kevin Buzzard Oh -- you mean there's just one prime above $p$. Thanks for the clarification!
Sep 10, 2011 at 8:38 comment added Kevin Ventullo Sorry, I'm not sure what to call it. I mean that the field can be written as $L>K>\mathbb{Q}$, where $p$ is totally ramified in $K/\mathbb{Q}$ and inert in $L/K$.
Sep 10, 2011 at 8:02 comment added Kevin Buzzard Thanks -- this sounds like another reference for the "well-known" part. What does "some prime has no splitting" mean?
Sep 9, 2011 at 21:02 comment added Kevin Ventullo In Example 3 on pg. 122 of Algebraic Groups and their Birational Invariants by Voskresenskii, there is an explicit description of a torus which does not satisfy "weak approximation." The example is $T=R^{(1)}_{L/\mathbb{Q}}(\mathbb{G}_m)$, where $R^{(1)}$ denotes the norm 1 elements of $\mathrm{Res}_{L/\mathbb{Q}}$, and $L$ is any biquadratic extension in which some prime has no splitting.
Sep 8, 2011 at 16:34 history answered Kevin Buzzard CC BY-SA 3.0