Ranking pages and the topology of the web
  This paper presents our studies on the rearrangement of links from the structure of websites for the purpose of improving the valuation of a page or group of pages as established by a ranking function as Google's PageRank. We build our topological taxonomy starting from unidirectional and bidirectional rooted trees, and up to more complex hierarchical structures as cyclical rooted trees (obtained by closing cycles on bidirectional trees) and PR--digraph rooted trees (digraphs whose condensation digraph is a rooted tree that behave like cyclical rooted trees). We give different modifications on the structure of these trees and its effect on the valuation given by the PageRank function. We derive closed formulas for the PageRank of the root of various types of trees, and establish a hierarchy of these topologies in terms of PageRank. We show that the PageRank of the root of cyclical and PR--digraph trees basically depends on the number of vertices per level and the number of cycles of distinct lengths among levels, and we give a closed vector formula to compute PageRank. 
Argimiro Arratia and Carlos Mariju\'an
cs.DM math.CO