## 2 Literature Review  
Most of the literature considers the design of assembly lines with the assignment of tasks exclusively to one station. The classical assembly line balancing problem did not concern the possibility of sharing a task among two or more stations. A complete survey on dynamic line balancing was proposed by [18]. They provide an exhaustive classification of the tactical and operational models proposed for the dynamic line balancing problem. As discussed in this work as a future research path, studies of how shared workloads vary and the identity of shareable activities can provide important information. A dynamic line balancing problem can be defined as the assignment of some tasks exclusively to a particular station, while other tasks can be shared between two consecutive stations [19]. The numerical analysis shows how this approach reduces the cycle time of the line, improving productivity. [PERSON] and [PERSON] [20] proposed a model for designing an assembly line with the possibility of sharing some tasks between two consecutive stations. The model identifies which tasks to share, looking for the best compromise between cycle time reduction and costs. After identifying the tasks to be shared, the model determines the workload on the stations. [PERSON] and [PERSON] [21] studied the sharing of tasks from the point of view of the skills of the operators and how to share the activities between them. The approach proposed focused on the skill of the operators to react to absenteism,turnover, or the replacement of inexperienced workers. Another recent work that focused on the workers was proposed by [PERSON] and [PERSON] [22]. In this case, the help policy of a worker towards the downstream stations is considered. [PERSON] and [PERSON] [23] studied the impact of changing the structure of the tasks of a product by splitting some tasks. The potential improvement depends strongly on the product structure. [PERSON] et al. [10] proposed mathematical models to design fractional assembly lines with three alternates: one to minimize the cycle time, and two other alternatives to minimize the storage of the buffers. The main findings of this research concern the higher productivity of the fractional assembly lines with higher robustness to the demand uncertainty, but the work in process inclines to increase. The redundancy of the production lines concerns the possibility of re-allocating an entire operation when the station goes into the failure state to a downstream station. The digital transformation of the production systems enables the development of redundancy policies. [PERSON] et al. [24] proposed a mixed-integer model to support the redundancy in an automotive body shop when the equipment fails. The simulation results highlight the greater robustness of the line to these failures. [PERSON] et al. [13; 14; 25] proposed several alternative models to design automatic production lines including the possibility of redundancy. The numerical results show how these approaches lead to a significant increase in productivity under stochastic conditions and failure events. [PERSON] [26] presented four alternatives to design automated flow lines: one to maximize the number of redundancies, one to allocate one redundancy for each operation, and these two models are reformulated considering the optimization of the cycle time. The simulation results show the improvements both for push and pull systems control and the benefits are relevant when the Mean Time To Repair increases. [PERSON] [27] integrated the design of mathematical models and the simulation to introduce redundancy in production lines with shared tasks. The first model minimizes the cycle time, while the second model provides several design alternatives, increasing the number of tasks shared with small increments of the optimized cycle time. Then, the simulations allow for identifying the best solution under particular conditions of failures or processing time variations.