Minimizing this loss function with respect to θ and φ is straightforward.
• ext-LexRank (Erkan and Radev, 2004) is a graph-based approach that computes sentence importance based on the concept of eigenvector centrality in a graph representation of source sentences; • ext-SumBasic (Vanderwende et al., 2007) is an extractive approach that assumes words occurring frequently in a document cluster have a higher chance of being included in the summary; • ext-KL-Sum (Haghighi and Vanderwende, 2009) describes a method that greedily adds sentences to the summary so long as it decreases the KL divergence; • abs-Opinosis (Ganesan et al., 2010) generates abstractive summaries by searching for salient paths on a word co-occurrence graph created from source documents; Nodes (Oracle) Nodes Edges (Oracle) Edges Approach  P  R  F  P  R  F  P  R  F  P  R  F   LCS 16.7 26.7 19.9 31.5 49.5 37.6 6.7 8.0 6.9 16.1 18.7 16.6 Smatch 20.9 33.2 24.9 33.2 52.0 39.6 9.3 10.7 9.4 17.2 20.1 17.8 Concept Cov.