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The graph vertices are labeled by a string representing the user id for example: "@John". The topology of the Twitter graph varies in time. In consequence the Twitter Graph is formalized by G(n) = ( V(n), E(n) ) where n denotes the graph at the discrete time sample n. A( n ) is the adjancency matrix and k( n ) is the degree vector.  Twitter NER:  Users @ at sign  RT  OpenNLP:  people,  places,  dates  $ dollar sign  stock market#  hash http  youtube  Freebase