particle physics - Jarlskog Invariant and its mathematical origin

CP violation is present in the weak interactions if

1. There are no degeneracies in the up-quark/down-quark matrices


2. The Jarlskog invariant $J=Im(V_{us} V_{cb} V_{ub}^* V_{cs}^*)$ is nonvanishing

Furthermore, all CP violating effects are proportional to $J$.

I am getting stuck on showing how all CP violating effects are proportional to $J$. Also, is the Jarlskog invariant a well-known mathematical property of a unitary matrix? What is it quantifying? I'd like to know this to the extent I can generalize this to larger CKM matrices.

Edit: I did a bad job writing my question. I rewrite it here:

Question:

1. How do I constructively derive $J=Im(V_{us} V_{cb} V_{ub}^* V_{cs}^*)$, and how do I generalize this to arbitrary $n\times n$ unitary matrices ?


2. The Jarlskog invariant is invariant under a change of basis. What is the elegant way of showing this?