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Math Help - Fermat's theorem (stationary points) of higher dimensions

  1. #1
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    Fermat's theorem (stationary points) of higher dimensions

    Look at this page and the Proof part,

    Fermat's theorem (stationary points) - Wikipedia, the free encyclopedia

    How to change the proof 2 into a proof of higher dimensions or can you give a proof of Fermat's theorem of higher dimensions?

    To clarify, I need to prove this:
    Let E\subset \mathbb{R}^n and f:E\rightarrow\mathbb{R} be a continuous function. Prove that if a is a local maximum point for f, then either f is differentiable at x=a with Df(a)=0 or f is not differentiable at a.
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  2. #2
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    Re: Fermat's theorem (stationary points) of higher dimensions

    Df(x) is, for each x, a vector (or can be represented by such a vector, depending upon exactly how you are defining Df(x)) pointing in the direction of fastest increase. Show that if f is differentiable at a and Df(a) is not 0 then moving a short distance in the direction of Df(a) will give a higher value of f and in the opposite direction will give a lower value. You can use the mean value theorem for that.
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