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Jan 13, 2019 at 11:47 history closed Nik Weaver
Piotr Hajlasz
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Alexandre Eremenko
Jan-Christoph Schlage-Puchta
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Jan 12, 2019 at 14:47 vote accept emTaMa
Jan 11, 2019 at 16:01 answer added Nik Weaver timeline score: 3
Jan 11, 2019 at 15:59 comment added Nik Weaver You know what, come to think of it, there's a proof of this in a book I just wrote ...
Jan 11, 2019 at 15:54 comment added Nik Weaver @StanleySnelson: you have to check the reverse inequality too, which is where convexity comes in: any two points are joined by a line segment and you can apply the one-dimensional mean value theorem.
Jan 11, 2019 at 15:05 review Close votes
Jan 13, 2019 at 11:47
Jan 11, 2019 at 15:00 comment added user126920 It may be hard to find the proof in a textbook, but you can prove it yourself by taking a point $x$ with $\|\nabla f(x) \|\geq K-\varepsilon$ and finding a $y$ with $|f(y)-f(x)| \geq (K-2\varepsilon)|y-x|$ using the definition of differentiability at $x$.
Jan 11, 2019 at 14:49 comment added Nik Weaver Undoubtedly this is in Geometric Measure Theory by Federer. But I think this question belongs on math.stackexchange.
Jan 11, 2019 at 14:21 history edited Martin Sleziak CC BY-SA 4.0
MathJax: \sup
Jan 11, 2019 at 14:15 review First posts
Jan 11, 2019 at 14:32
Jan 11, 2019 at 14:11 history asked emTaMa CC BY-SA 4.0