Is it possible to a real-valued function of two variables defined on an open set to have partial derivatives of all order and to be discontinuous at some point or maybe at each point?
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$f(x,y)= 2xy/ x^2+y^2$ when $x^2+y^2>0$ and $0$ when $(x,y)=0$ f is differentiable at each point of other than the origin and $D_1f(0,0)=0=D_2f(0,0)$ since $f(x,0)=0=f(0,y) \forall x,y$ but But $f(t,t)=1 \forall t\neq 0$ so f is not continuous at $0$