How to prove that $\Delta f = 0$ for $f(x)=\frac{1}{||x||}$?

I am given a function $f:\mathbb{R}^3\rightarrow\mathbb{R}:f(x)=\frac{1}{||x||}$, and I am not really sure that the norm must be Euclidian (anyway it wasn't mentioned in the task), and I have to prove that $\Delta f = 0$ (Laplace operator: $\Delta f(x_1,..,x_n) = \sum_{i=1}^n \frac{\partial^2f}{\partial x_i ^2}$). But even if I assume that the norm is Euclidian, I still don't get the right result:

$$\frac{\partial ^2f}{\partial x_i^2} = -\frac{x_i^2}{(x_1^2+x_2^2+x_3^2)^{\frac{3}{2}}} + \frac{1}{(x_1^2+x_2^2+x_3^2)^{\frac{1}{2}}}$$

So:

$$\Delta f = \frac{2}{(x_1^2+x_2^2+x_3^2)^{\frac{1}{2}}}$$

So something is wrong in my calculations. Could you help me?

$$\frac{\partial ^2}{\partial x_i^2}\frac{1}{(x_1^2+x_2^2+x_3^2)^{1/2}} = \frac{2x_i^2-x_j^2-x_k^2}{(x_1^2+x_2^2+x_3^2)^{5/2}}$$
• Is it worth mentioning the case of what happens when $|| x || = 0$? Jun 5 '13 at 23:41
• @AndrewD: There is a singularity at $0$, so the function is not harmonic on the unit ball.