I came across the following derivation from a paper "Pfeiffer III, Joseph J., et al. "Fast generation of large scale social networks with clustering." arXiv preprint arXiv:1202.4805 (2012)."
In the attached equation,
I am not able to understand how they got rid of $D_k^2$ in the numerator and the denominator from line 3 to 4. Although the final results hold even while using $D_k^2$, I am unable to understand if this is just a typo or some reduction step that I did not understand.