This was not homework but a practice question. To show (what they called a volume element) \frac{d^{3}k}{E_{k}} is Lorentz invariant. The post above answers my question, thanks to both of you.
Yes, that's exactly what I was trying to show.
I looked up Jacobian's and for that transformation I calculated the determinant to be: \gamma (Lorentz factor). So I tried another approach and got a 3x4 matrix which I can't find how to solve.
I know it should be quite trivial and only a few...
Simple question.. How do you prove the volume element of momentum space (d3k/Ek) is Lorentz Invariant?
I tried making it proportional to "velocity volume element" derived from the Lorentz transformations but didn't seem to get very far.