Cyclic labelling with constraints at two distances

Motivated by problems in radio channel assignment, we consider the vertex-labelling of graphs with nonnegative integers. The objective is to minimize the span of the labelling, subject to constraints imposed at graph distances one and two. We show that the minimum span is (up to rounding) a piecewise linear function of the constraints, and give a complete specification, together with the associated optimal assignments, for trees and cycles