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Fixed-parameter complexity of $\lambda$-labelings. (English) Zbl 0982.05085
A $\lambda (p,q)$ labelling of a graph $G=(V,E)$ is an assignment of non-negative integers to the vertices in $V$, such that adjacent vertices get labels that differ by at least $p$, and vertices of distance two in $G$ get labels that differ by at least $q$. $L(G;p,q)$ is the minimum possible maximum value in a $\lambda (p,q)$ labelling of $G$. The standard $\lambda$-coloring problem is to determine $L(G;2,1)$ of a graph. This paper shows that this problem is solvable in polynomial time on almost $k$-trees (graphs that can be formed by adding $k$ edges to a tree), and is NP-hard for every fixed maximum label value. It is also shown for all values $p>q\geq 1$ that there are several $\lambda$ such that deciding if $L(G;p,q) \leq \lambda$ is NP-complete (taking $\lambda$ here as fixed part of the problem description). Some other related results are also shown.

##### MSC:
 05C78 Graph labelling 68R10 Graph theory in connection with computer science (including graph drawing) 05C90 Applications of graph theory 05C15 Coloring of graphs and hypergraphs
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##### References:
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