Kleinberg is best known for his work on networks and particularly for his HITS algorithm, developed while he was at IBM. HITS is an algorithm for web search that builds on the eigenvector-based methods used in algorithms and served as the full-scale model for PageRank by recognizing that web pages or sites should be considered important not only if they are linked to by many others, but also if they link to many others. Search engines themselves are examples of sites that are important because they link to many others. Kleinberg realized that this generalization implies two different classes of important web pages, which he called "hubs" and "authorities". The HITS algorithm is an algorithm for automatically identifying the leading hubs and authorities in a network of hyperlinked pages. Kleinberg is also known for his work on algorithmic aspects of the small world experiment. He was one of the first to realize that Stanley Milgram's famous "six degrees" letter-passing experiment implied not only that there are short paths between individuals in social networks but also that people seem to be good at finding those paths, an apparently simple observation that turns out to have profound implications for the structure of the networks in question. The formal model in which Kleinberg studied this question is a two dimensional grid, where each node has both short-range connections to neighbours in the grid and long-range connections to nodes further apart. For each node v, a long-range edge between v and another node w is added with a probability that decays as the second power of the distance between v and w. This is generalized to a d-dimensional grid, where the probability decays as the d-th power of the distance. Kleinberg has written numerous papers and articles as well as a textbook on computer algorithms, Algorithm Design, co-authored the first edition with Éva Tardos and sole authored the second edition. Among other honors, he received a MacArthur Foundation Fellowship also known as the "genius grant" in 2005 and the Nevanlinna Prize in 2006, an award that is given out once every four years along with the Fields Medal as the premier distinction in Computational Mathematics. His new book is entitled "Networks, Crowds, and Markets: Reasoning About a Highly Connected World", published by Cambridge University Press in 2010. Cornell's Association of Computer Science Undergraduates awarded him the "Faculty of the Year" award in 2002.
