I am trying to study the Elementary new proofs of classical limit theorems for Galton Watson processes written by Jochen Geiger. I don't understand what Z_(n,i) stand for. And in the proof of Theorem 3.2, they discuss n-N_n, I really can't get this. I have been trying to find some documents to understand this problems but failed. I know their definitions, I just can't visualize what they are in the shape of the backward tree they discuss earlier in the paper.
Please anyone could help me?