a multi-product inventory model for selecting the first and second layers of suppliers in a supply chain

in recent years, supplier evaluation and selection, an important element in supply chain management, has been gaining attention in both academic literature and industrial practice. the mixed integer multi-objective non-linear programming model (mimonlp) presented in this paper aimed to evaluate and select the appropriate set of suppliers considering quantitative and qualitative criteria and in addition to selecting the first layer's suppliers which relate directly to the organization, analyses the characteristics of second-layers suppliers, and design a network to determine the flow rate of products and materials between buyers and best suppliers in both layers. another important feature of this model is considering holding costs of different products over the planning horizon and quantity discounts for the first layer's suppliers at the same time. finally, the model is solved by using goal programming approach and numerical examples are presented to test the performance of proposed model.