The following graph represents an instance of the Fixed Charge Network Flow problem (see Slides
“Network Design: Part II Integer programming methods”):
Red numbers are supply or demand of flow, while the figures (uij, cij) on each arc represent respectively arc capacity and arc activation cost.
1. Write a ZIMPL model that formulates the Fixed Charge Network Flow problem by choosing an appropriate representation of the graph by text files.
2. Compare the optimal solution of the above model with the case in which all arcs have the same activation cost (for instance, cij = 100)
3. Delete arcs from the graph until you get an infeasible solution (discuss your choices)