Santo Fortunato Marián Boguñá Alessandro Flammini Filippo Menczer Approximating PageRank from In-Degree. 59-71 2006 WAW http://dx.doi.org/10.1007/978-3-540-78808-9_6 conf/waw/2006 db/conf/waw/waw2006.html#FortunatoBFM06