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Must a polynomial function $f \in \mathbb{R}[x_1, \ldots, x_n]$ that's lower bounded by some $\lambda \in \mathbb{R}$ have a global minimum over $\mathbb{R}^n$?

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Something for your perusal: – J. M. Sep 1 '10 at 23:38
...and also – J. M. Sep 1 '10 at 23:50
up vote 27 down vote accepted

No. The polynomial $f(x, y) = (1 - xy)^2 + x^2$ is bounded below by $0$, cannot actually take the value $0$, but can take the value $\epsilon$ for any $\epsilon > 0$. I learned this example from Richard Dore on MO.

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@Agusti Roig. $f$ has no critical points in the interior of the disc, so you have to look for the minimum at the unit circle. One way would be to parametrize $x=\cos\theta, y=\sin\theta$, and minimize.

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I just realised your question was meant for Chen only. Sorry. – Weltschmerz Sep 2 '10 at 2:32
Don't worry: it was just a silly question. Maybe I should delete it. – a.r. Sep 2 '10 at 11:15

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