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Figure 1 Cocitation analysis based on cited references that appear in the 123 surveyed articles. Circle size represents the number of times the paper has been cited and line weight represents the number of cocitations (cited in the same article) with a threshold of 8 cocitations to enhance clarity. Centrality (betweeness) is determined by the fraction of all directed paths between any two vertices that pass through a node; closeness (distance of one vertex to others) depends on inverse distance to other vertices. Full citations are provided in Appendix B 
