## 2 Literature Review
The research of [27] used mathematical models for the picker routing problem; also, they determined that the performance of these strategies (optimal or heuristic) depends on the characteristics of the problem, such as the number of pick location and picking aisles, the position of the deposit and the number of items requested in the picking list. The detailed description of these picker routing and their variations can be found in [14; 30; 31]; research such as [16; 18; 20; 27] provided optimal solutions; and [19; 22; 32; 33], among others, showed heuristic solutions. It is important to mention that for all studies reviewed by [10], the problems were solved independently. Figure 1: Proportional times for each activity. Drawing on an extensive research carried out by [10], the authors decided that there are three combinations of planning problems to improve the efficiency of order picking, those being the order batching process--the picker routing problem (the focus of the largest number of case studies); the storage location assignment and routing problem (this being the option analyzed in this work); and finally, the storage location and order batching process. Although most of the literature concentrates on giving solutions to the first combination, in [6], the picker routing problem is very interdependent on the storage location problem; therefore, it is important to optimize both processes simultaneously. Also, according to [24], there is a statistically significant interaction between these problems, since the elements that move fast must be assigned to storage locations that can be accessed more easily, which depends largely on the routing policy. The literature on the routing problem and storage assignment has highlighted several investigations, such as [34] that focuses on proposing different storage assignment strategies and heuristics to picker routing in a case study. That work proposed a mathematical model with an optimal solution. Similarly, in a study conducted by [22], they proposed different storage assignment strategies and an exact solution algorithm (modification of the algorithm of [16]) based on the concept of dynamic programming, to solve such problems in a home products company. In that study, the processes of storing assignment and routing were not performed, since once the products were assigned to the storage spaces, they were subsequently collected. On the contrary, in our work, these decisions were made simultaneously. Nevertheless, both investigations considered precedence constraints based on the weight of the products in both processes. These constraints were also very important for the works conducted by [35; 36]. The studies presented provide evidence that the research presented by [26] is the only report wherein a model is formulated for simultaneously determining the assignment and sequencing decisions, but it does not consider the weight of the products in the picking tour and the products are stored in two or more locations in the warehouse; instead, in this work, each product must be set in only location. A summary of the main articles that have been reviewed for this research are shown in Table 1.