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In 1949 Tolstov proved that there exists $f:\mathbb{R^2} \to \mathbb{R}$ so that $f_x$ and $f_y$ exist and are continuous everywhere and $f_{xy}$ and $f_{yx}$ exist everywhere and differ on a set of positive Lebesgue measure. I have also heard that he provided an example where the cross partials do not exist everywhere but differ on a set of full measure. The only copies of the papers I have heard of are in Russian so I could not read them to find his examples. In any case I would still like to play around with those questions a bit more on my own before looking up the answers.

I am wondering if any further progress on this question is reasonably well known? It seems like this question is natural enough that it should either be a known open research problem or solved, though I may be wrong on that. My intuition says that if the cross partials can differ on a set of positive measure or even a.e. (but not necessarily exist everywhere) then it should probably be the case that they can exist and be different everywhere, though I would not be surprised to be proven completely wrong.

More broadly, if anyone has a good way of thinking about functions with cross partial derivatives which exist everywhere and are different on some "large" set I would be interested in getting a better feel for what kind of pathological behavior I should be looking for.

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A related question. –  J. M. Feb 2 '12 at 0:52
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1 Answer 1

My posts in the following 2006 and 2007 sci.math threads may be of interest:

second partial derivatives commute (Clairaut's Thm.)

http://groups.google.com/group/sci.math/msg/78a769e88081ec8b

http://groups.google.com/group/sci.math/msg/3b7eeedf51fc5b92

http://groups.google.com/group/sci.math/msg/cd695899296573b6

History of mixed partial derivatives equality

http://groups.google.com/group/sci.math/msg/14aa228dadc9ed5b

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