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Please can you help me to find all functions of class $C^2$, $f:\mathbb R^2\to\mathbb R$ such that $\frac{\partial^2f}{\partial x\partial y} = 0$.

Thank you so much!

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    yes.. i'm sorry because i have not written correctly2012-12-07
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    I have made the title more meaningful.2012-12-07
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    Here is an outline solution: - Engage your brain. - Solve the problem.2012-12-07
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    Note first that $\dfrac{\partial^2{f}}{\partial{x} \partial{y}}=\dfrac{\partial}{\partial{x}}\left(\dfrac{\partial{f}}{\partial{y}} \right).$ Let $g(x, \,y):=\dfrac{\partial{f}}{\partial{y}}.$ What $g(x, \,y)$ satisfies $\dfrac{\partial{g}}{\partial{x}}=0$?2012-12-07

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