The Cross-ratios or the Anharmonic-ratios are defined as, $$\frac{r_{ij}r_{kl}}{r_{ik}r_{jl}}, \text{ where } r_{ij}=|\mathbf{r}_i - \mathbf{r}_j|.$$ Now the claim is: conformal symmetry implies that for computing $N$ point correlation function there will be $N(N-3)/2$ number of independent cross-ratios.
I can't prove this claim. I have seen the Ginsparg's explanation on this claim but I can't understand that. I need the proof. Can anyone help me?