Locating Two Facilities on a Tree Subject to Distance Constraints

We wish to locate two new facilities on a tree network, where demands occur at vertices. We minimize any convex function which is nondecreasing in two related single facility convex objective functions, and in the distance between the facilities, subject to a distance constraint imposing an upper bound on the distance between the facilities, as well as to distance constraints imposing upper bounds on the distances between the new facilities and the demand points. We consider both vertex-restricted and vertex-unrestricted problem versions, and, for the latter problem, reduce the location problem when the constraint is tight to a problem of locating a single new facility on a path.