As for document summarization using combinatorial optimization techniques, many studies employ explicit solution techniques such as branch and bound method, dynamic programming, integer linear programming, and so on (Mcdonald, 2007;Yih et al., 2007;Gillick et al., 2008;Takamura et al., 2009;Lin et al., 2010).
the explicit solution techniques often face NP-hard, they require much calculation time for solving a problem of combinatorial optimization, though they provide optimal solution.