Exact algorithms for $L(2,1)$-labeling of graphs

Abstract : The notion of distance constrained graph labelings, motivated by the Frequency Assignment Problem, reads as follows: A mapping from the vertex set of a graph $G=(V,E)$ into an interval of integers $\{0, \dots ,k\}$ is an $L(2,1)$-labeling of $G$ of span $k$ if any two adjacent vertices are mapped onto integers that are at least 2 apart, and every two vertices with a common neighbor are mapped onto distinct integers. It is known that for any fixed $k\ge 4$, deciding the existence of such a labeling is an NP-complete problem. We present exact exponential time algorithms that are faster than the naive $O((k+1)^n)$ algorithm that would try all possible mappings. The improvement is best seen in the first NP-complete case of $k=4$ -- here the running time of our algorithm is $O(1.3006^n)$. % $O(1.3161^n)$. Furthermore we show that dynamic programming can be used to establish %an $O(c^n)$ algorithm to compute an optimal $L(2,1)$-labeling, for a constant $c< 4$. an $O(3.8730^n)$ algorithm to compute an optimal $L(2,1)$-labeling.
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[Research Report] RR-6587, INRIA. 2008
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Frédéric Havet, Martin Klazar, Jan Kratochvil, Dieter Kratsch, Matthieu Liedloff. Exact algorithms for $L(2,1)$-labeling of graphs. [Research Report] RR-6587, INRIA. 2008. 〈inria-00303330〉

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