“…optimization (IPO) on the ILP model under the framework of variable neighborhood searches (VNS), known as IPO-ILP-VNS, to obtain a near-optimal solution whose quality can be controlled by the given CPU time. VNS is a new top-level meta-heuristics approach proposed byMladenovic and Hansen (1997) and it has been successfully applied to combinatorial optimization problems in various fields, such as the travelling salesman problem, the Pmedian problem, the vehicle routing problem, and the multi-level lot-sizing problem(Hansen et al, 2010;Xiao et al, 2011aXiao et al, , 2014a. The mechanism of VNS is to perform local search with designed changes in multilevel neighborhoods, which gives many desirable properties of meta-heuristics such as simplicity, robustness, userfriendliness and generality(Hansen et al, 2008).…”