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Unformatted text preview: 3.3. DERIVATIVES 237 A third example is defined by a straightforward formula (no cases): f ( x,y ) = x 1 / 3 y 1 / 3 . Again, the function is constant along the coordinate axes, so both partials are zero. However, if we try to evaluate the limit in Equation ( 3.6 ) using any vector not pointing along the axes, we get d (0 , 0) f ( + ) = d dt vextendsingle vextendsingle vextendsingle vextendsingle t =0 bracketleftBig 1 / 3 1 / 3 t 2 / 3 bracketrightBig ; since t 2 / 3 is definitely not differentiable at t = 0, the required linear map d (0 , 0) f ( + ) cannot exist. From all of this, we see that having the partials at a point x is not enough to guarantee differentiability of f ( x ) at x = x . It is not even enough to also have partials at every point near x all our examples above have this property. However, a slight tweaking of this last condition does guarantee differentiability. We callguarantee differentiability....
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 Fall '08
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 Calculus, Derivative

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