I have a discrete optimization problem in which there are some predetermined nodes in the set A. a vehicle must visit them as the traveling salesman problem. there are some other nodes in the set B that they must be covered by a node in set A or a new node that is chosen from a set of candidate nodes. we define a node in B is covered if there is a node at a distance of R from it. I want to choose the minimum nodes from candidate nodes that must be added to set A to cover all nodes in B. the objective function is minimizing the tour length visiting nodes in A and the new nodes added. I want to model and solve this problem. Could anyone introduce articles in the literature that are somewhat related to this issue?