Not possible to find a function g such that g

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Why is it not possible to find a function g such that g [tex]\in[/tex] C1 at (0,0) and g is not differentiable at (0,0)
 
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Also given that f(x,y) = x|y| how do I show that f [tex]\notin[/tex] C1 at (0,0)
 


squenshl said:
Why is it not possible to find a function g such that g [tex]\in[/tex] C1 at (0,0) and g is not differentiable at (0,0)

As far as I know, a function is defined to be C1 if it is continuously differentiable, so the question doesn't make sense.
 


squenshl said:
Also given that f(x,y) = x|y| how do I show that f [tex]\notin[/tex] C1 at (0,0)

Remember the definition of differentiability: all approaches to the point (0,0) must yield the same derivative. Consider approaching the derivative along the x-axis and then the derivative along the y-axis. Are they equal?