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Feb 17, 2011 at 5:53 comment added Bill Thurston @Andrea Altomani: Good point. I think it depends exactly how you're given the information. If you're told that the function is analytic, it's not enough, as Brouwer-style examples show: e.g. in the plane take the graph of the function $ \Sum(a_i x^{2i})$ where $a_i$ = 1 if the i'th element of an enumeration of all possible formal deductions in some formal system yields the Riemann hypothesis, $a_i = -1$ if the $i$th zero gives a counterexample, and 0 otherwise. It's analytic, but there's no way to know when to stop checking. But more ypically you'd have some other way to understand a curve
Feb 16, 2011 at 19:18 vote accept AndreA
Feb 16, 2011 at 19:18 comment added AndreA Thanks for the answer. I still have a question though. Even assuming that we are in a "nice" case (e.g. $M$ is real-analytic) how do i know how many jet spaces i should consider? If for example $M$ contains a straight line, for any order of approximation and any linear functional strict positivity will fail.
Feb 16, 2011 at 1:31 history edited Bill Thurston CC BY-SA 2.5
Fixed blotch where text was missing.
Feb 15, 2011 at 18:41 history answered Bill Thurston CC BY-SA 2.5