Zhang, P., Zhao, W., & Zhu, D. (2013). Complexity and Approximation Results for the Min-Sum and Min-Max Disjoint Paths Problems. Computing and Informatics, 32(1), 23–45. https://cai.type.sk/content/2013/1/complexity-and-approximation-results-for-the-min-sum-and-min-max-disjoint-paths-problems/