14. The document ranking system of claim 13 , wherein a sub-graph having a number of documents greater than or equal to a certain number is determined to be the community.
14. The document ranking system of claim 13 , wherein a sub-graph having a number of documents greater than or equal to a certain number is determined to be the community. 15. The document ranking system of claim 14 , wherein, in response to determining a link is deleted from or added to a link path comprising a plurality of steps of links from the second document to the first document, the contribution coefficient determining unit is configured to re-determine contribution coefficients of documents corresponding to links included in the link path.