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Homework Statement
I'm reading through Taylor's advanced calculus and came across this question in section 7.2 :
http://gyazo.com/6b0c5a2e4e605ff77bf6584eb3295948
Homework Equations
The definition of the partial of f with respect to some variable at some point (a,b), let's say the partial with respect to x of some f(x,y) would be :
[itex]\frac{∂f}{∂x} (a, b) = \lim_{h→0} \frac{f(a+h, b) - f(a, b)}{h}[/itex]
The Attempt at a Solution
Okay, so the question wants me to compute the second partials given the conditions on f.
So first I note that f is continuous everywhere for x,y ≠ 0. I also note that f(x,0) = f(0,y) = f(0,0) = 0 has been defined nicely for me as to make my function continuous at (0,0).
I suppose first that I'll show that [itex]\frac{∂^2f}{∂y∂x} = -1[/itex]. So I first have to do a subsidiary calculation to find the first partial with respect to x.
[itex]\frac{∂f}{∂x} (0, 0) = \lim_{h→0} \frac{f(h, 0) - f(0, 0)}{h} = \lim_{h→0} \frac{0 - 0}{h} = 0[/itex]
Okay, that was not so bad since f was nicely defined for us. Now here comes my problem when I try to find the second partial. Note that when I use subscripts I refer to the partial of the function with respect to that variable. So [itex]f_x = \frac{∂f}{∂x}[/itex].
[itex]\frac{∂^2f}{∂y∂x} (0, 0) = \lim_{k→0} \frac{f_x(0, k) - f_x(0, k)}{k} (***)[/itex]
I'm labeling this equation (***) so I can return to it after I calculate fx(0,k). Now to calculate it :
[itex]\frac{∂f}{∂x} (0, k) = \lim_{h→0} \frac{f(h, k) - f(0, k)}{h} = \lim_{h→0} \frac{h^2arctan(k/h) - h^2arctan(h/k)}{h} = 0[/itex]
Now this produces a big problem when I plug it back into (***) as I don't get the answer of -1 which I'm apparently supposed to get :
[itex]\frac{∂^2f}{∂y∂x} (0, 0) = \lim_{k→0} \frac{f_x(0, k) - f_x(0, k)}{k} = \lim_{k→0} \frac{0 - 0}{k} = 0[/itex]
What am I doing wrong here? Have I missed something or have I done a calculation wrong?