Let there be a function f[x,y]: RxR->R(adsbygoogle = window.adsbygoogle || []).push({});

Is there any connection between the differentiability

(I am not sure that this is the right English term - I meant f[x,y]= a*dx+b*dy +something of smaller order)

and the equality fxy=fyx, where fxy means the derivative of f[x,y] first by y, and than by x ?

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# A question about mixed partial derivative

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