Labeling Chordal Graphs: Distance Two Condition

  • Authors:
  • Denise Sakai

  • Affiliations:
  • -

  • Venue:
  • SIAM Journal on Discrete Mathematics
  • Year:
  • 1994

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Abstract

An $L(2,1)$-labelling of a graph $G$ is an assignment of nonnegative integers to the vertices of $G$ such that adjacent vertices get numbers at least two apart, and vertices at distance two get distinct numbers. The $L(2,1)$-labelling number of $G$, $\lambda(G)$, is the minimum range of labels over all such labellings. It is shown that, for chordal graphs $G$ with minimum degree $\Delta(G)m \lambda(G)\leq(\Delta(G)=3^{2}/4$; in particular, if $G$ is a unit interval graph with chromatic number $\chi(G), \lambda(G)\leq 2\chi(G)$, which is a better bound. As a consequence, it is shown that the conjecture $\lambda(G)\leq\Delta^{2}(G)$ by Griggs and Yeh [SIAM J. Discrete Math., 5(1992), pp. 586-595] is true for chordal graphs.