Griggs and Yeh's Conjecture and $L(p,1)$-labelings

International audienceAn L(p,1)-labeling of a graph is a function f from the vertex set to the positive integers such that |f(x) − f(y)| ≥ p if dist(x, y) = 1 and |f(x) − f(y)| ≥ 1 if dist(x, y) = 2, where dist(x,y) is the distance between the two vertices x and y in the graph. The span of an L(p,1)- labeling f is the difference between the largest and the smallest labels used by f. In 1992, Griggs and Yeh conjectured that every graph with maximum degree Δ ≥ 2 has an L(2, 1)-labeling with span at most Δ^2. We settle this conjecture for Δ sufficiently large. More generally, we show that for any positive integer p there exists a constant Δ_p such that every graph with maximum degree Δ ≥ Δ_p has an L(p,1)-labeling with span at most Δ^2. This yields that for each positive integer p, there is an integer C_p such that every graph with maximum degree Δ has an L(p,1)-labeling with span at most Δ^2 + C_p