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Jan 8, 2010 at 13:54 comment added Pete L. Clark In fact this was already explained by Poonen in a "motivational comment" to his question linked to above.
Jan 8, 2010 at 12:18 comment added Chandan Singh Dalawat Put another way, the only smooth proper curve over $\mathbb{Q}$ which has good reduction everywhere is $\mathbb{P}_1$.
Jan 8, 2010 at 11:44 comment added David E Speyer That doesn't work. If f:C -> Z is a curve of genus zero, then omega_{C/Z}^{-1} has degree 2 and f^*(omega_{C/Z}^{-1}) is a free Z module of rank 3. (Locally free over a general base, but Z is a PID.) So C can be written as a conic in P^2_{Z}. The Hasse principle applies to conics.
Jan 8, 2010 at 11:35 history answered Hagen CC BY-SA 2.5