I want to calculate $\nabla\cdot(\frac{\hat{r}}{r^2})$ Naively applying the expression for divergence in spherical coordinates gives $\nabla\cdot(\frac{\hat{r}}{r^2})=\frac{1}{r^2}\frac{\partial}{\partial r}(r^2 *\frac{1}{r^2})=\frac{1}{r^2}\frac{\partial}{\partial r}(1)=0$ However, based on the context in which this problem arose, I believe there should be a delta function. You can get this by noticing: $\nabla^2\frac{1}{r}=\nabla\cdot(\nabla\frac{1}{r})=-\nabla\cdot (\frac{\hat{r}}{r^2})$ Thus $\nabla\cdot(\frac{\hat{r}}{r^2})=-\nabla^2\frac{1}{r}=4\pi\delta(\vec{r})$ Can anyone point out where the error is? I suspect there is an issue cancelling out the $r^2$ in the first method due to that covering up the zero-in-denominator issue, but I would like to understand precisely what goes wrong.
Why am I getting zero divergence of function $\vec{f} = \frac{1}{r^2} \hat{r}$, where $r$ is the distance from the origin and $\hat{r}$ is the unit vector in the radial direction. The divergence of this function over a sphere of radius $R$, which includes the origin.  $$\nabla \cdot f = \frac{1}{r^2} \frac{\partial}{\partial r}(r^2 f_r) = \frac{1}{r^2} \frac{\partial}{\partial r}(r^2 \frac{1}{r^2}) = 0$$
How can you get the names of method parameters? If I have a method such as:  public void MyMethod(int arg1, string arg2)   How would I go about getting the actual names of the arguments? I can't seem to find anything in the MethodInfo which will actually give me the name of the parameter.  I would like to write a method which looks like this:  public static string GetParamName(MethodInfo method, int index)   So if I called this method with:  string name = GetParamName(MyMethod, 0)   it would return "arg1". Is this possible? What did Ken Thompson mean when he said, "I'd spell creat with an 'e'."? What is Ken referring to? Is there a "creat" command? Motion tracked Video doesn't show up in the render or in the compositing tab I tried to add a 3D object to a clip by following a youtube video. But clip doesn't show up in the render or in the compositing tab. It show up even in the camera view. I didn't change any settings. please help. Thank you.    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