# Is this integral a convolution ?

1. Oct 7, 2011

### Mentz114

I'm struggling to find a function $E(t)$ which is the energy inside a sphere with energy density $\rho(t,r)$ where the radius $r \equiv r(t)$ is itself a function of time. This
$$E(t) = 8\pi \int_0^{r(t)} \rho(r,t) dr$$
doesn't make sense, does it ? Is the thing I'm looking for some kind of convolution of r and $\rho$ ?

2. Oct 7, 2011

### HallsofIvy

Why do you say it doesn't make sense? It looks perfectly reasonable to me.

3. Oct 7, 2011

### mathman

You should try to be more careful in your notation. You have ρ(t,r) in one place and ρ(r,t) someplace else. More important if r is the upper limit of the integral, it should not be used as the dummy for integration.

4. Oct 7, 2011

### Mentz114

Thanks for the replies. Sorry about the sloppiness.

Yes, I thought there was something wrong but I'm still baffled.