Let and defined by
How should we choose such that:
a) is continuous.
b) is differentiable.
c) has continuous partial derivatives on every point on
Spoiler:
Kind of, it will give you that the function can't be differentiable if , but to prove differentiablity in case n>1 you'll need the definition I gave you. Before looking at the hint, notice that in case n>1 the function approaches zero very fast near the origin (try to picture it in one dimension) so the derivative must be...
Hint:Spoiler: