2 research outputs found
Labeling outerplanar graphs with maximum degree three
An L(2, 1)-labeling of a graph G is an assignment of a nonnegative integer to each vertex of G such that adjacent vertices receive integers that differ by at least two and vertices at distance two receive distinct integers. The span of such a labeling is the difference between the largest and smallest integers used. The Ξ»-number of G, denoted by Ξ»(G), is the minimum span over all L(2, 1)-labelings of G. Bodlaender et al. conjectured that if G is an outerplanar graph of maximum degree β, then Ξ»(G) β€ β + 2. Calamoneri and Petreschi proved that this conjecture is true when β β₯ 8 but false when β = 3. Meanwhile, they proved that Ξ»(G) β€ β + 5 for any outerplanar graph G with β = 3 and asked whether or not this bound is sharp. In this paper we answer this question by proving that Ξ»(G) β€ β + 3 for every outerplanar graph with maximum degree β = 3. We also show that this bound β + 3 can be achieved by infinitely many outerplanar graphs with β = 3