
A social network is constituted by a number of units (nodes) that are connected to each other by a defined relationship -- for example, "x cites y", "x sends 5 email messages a week to y", "x and y belong to an organization in common." There are a few wrinkles -- the units may be persons, organizations, cities, journal articles, or other types of entities; the relationships may be uni-directional or bi-directional; and the linking relationships may represent categorical relationships or intensity relationships. "x and y are friends" is a bi-directional relationship; "x and y are close friends" is a bi-directional relationship recording intensity.Some of the basic questions about a social network are easy to formulate but difficult to assess....