ABSTRACT

This chapter explains how to interpret the results from LP solver software, and points out features of problems that resist LP formulation. The LP formulation defines variables the amount of flow of commodity k along edge. The user frequently has secondary considerations, not included in the objective function, would be delighted to sacrifice a small fraction of the primary objective in order to get a significant gain with respect to a secondary one. The chapter discusses the LP formulation, how to build an LP model that accurately describes the critical elements of a problem. The main steps in model building are: understand and visualize the problem; define index sets and data; define the decision variables; write the objective function as a linear combination of the decision variables; and write the constraints as linear inequalities in the decision variables. Network models with integrality constraints constitute the exception to the general rule that non-convexity of the feasible region interferes with LP modeling.