Formally, the amount of new information contained in a summary S is given by the crossentropy Inf (S, K) = CE(S, K): For Relevance the cross-entropy between S and D should be low.
if c i is the i-th bigram selected in the summary and w i is its weight computed from D, then: LexRank: (Erkan and Radev, 2004) This is a well-known graph-based approach.