Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Gouveia and Martins [7] reviewed exact and lower bounding schemes, including earlier works of Gavish [4, 5], the branch-and-bound algorithm of Malik and Yu [10], the Lagrangean relaxation approach of Gouveia [6], and the cutting plane method of Hall [9].
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