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References:

Braekers, K., Caris, A. and Janssens, G. K. (2014). Exact and meta-heuristic approach for a general heterogeneous dial-a-ride problem with multiple depots, Transportation Research Part B: Methodological 67: 166-186.

Cordeau, J.-F. (2006). A branch-and-cut algorithm for the dial-a-ride problem, Operations Research 54(3): 573-586.

Cordeau, J.-F. and Laporte, G. (2003). A tabu search heuristic for the static multi-vehicle dial-a-ride problem, Transportation Research Part B: Methodological 37(6): 579-594.

Parragh, S. N. (2011). Introducing heterogeneous users and vehicles into models and algorithms for the dial-a-ride problem, Transportation Research Part C: Emerging Technologies 19(5): 912-930.

Ropke, S., Cordeau, J.-F. and Laporte, G. (2007). Models and branch-and-cut algorithms for pickup and delivery problems with time windows, Networks 49: 258-272.