Abstract
Given an undirected graph G, an L(h,k)-labelling of G assigns colors to vertices from the integer set {0, ..., λ h,k }, such that any two vertices v i and v j receive colors c(v i ) and c(v j ) satisfying the following conditions: i) if v i and v j are adjacent then |c(v i ) – c(v j )| ≥ h; ii) if v i and v j are at distance two then |c(v i ) – c(v j )| ≥ k. The aim of the L(h,k)-labelling problem is to minimize λ h,k . In this paper we study the approximability of the L(h,k)-labelling problem on bipartite graphs and extend the results to s-partite and general graphs. Indeed, the decision version of this problem is known to be NP-complete in general and, to our knowledge, there are no polynomial solutions, either exact or approximate, for bipartite graphs.
Here, we state some results concerning the approximability of the L(h,k)-labelling problem for bipartite graphs, exploiting a novel technique, consisting in computing approximate vertex- and edge-colorings of auxiliary graphs to deduce an L(h,k)-labelling for the input bipartite graph. We derive an approximation algorithm with performance ratio bounded by \(\frac{4}{3} D^2\), where, D is equal to the minimum even value bounding the minimum of the maximum degrees of the two partitions.
One of the above coloring algorithms is in fact an approximating edge-coloring algorithm for hypergraphs of maximum dimension d, i.e. the maximum edge cardinality, with performance ratio d.
Furthermore, we consider a different approximation technique based on the reduction of the L(h,k)-labelling problem to the vertex-coloring of the square of a graph. Using this approach we derive an approximation algorithm with performance ratio bounded by min(h, 2k)\(\sqrt{n} + o(k + \sqrt{n})\), assuming h ≥ k. Hence, the first technique is competitive when D = O(n 1/4).
These algorithms match with a result in [2] stating that L(1,1)-labelling n-vertex bipartite graphs is hard to approximate within n 1/2 − ε, for any ε > 0, unless NP = ZPP.
We then extend the latter approximation strategy to s-partite graphs, obtaining a (min(h, sk)\(\sqrt{n} + o(sk \sqrt{n})\))-approximation ratio, and to general graphs deriving an \((h\sqrt{n} + o(h\sqrt{n}))\)-approximation algorithm, assuming h ≥ k.
Finally, we prove that the L(h,k) – labelling problem is not easier than coloring the square of a graph.
Partially supported by the Italian Research Project PRIN 2003 ”Optimization, simulation and complexity of the design and management of communication networks”.
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Calamoneri, T., Vocca, P. (2005). On the Approximability of the L(h,k)-Labelling Problem on Bipartite Graphs (Extended Abstract). In: Pelc, A., Raynal, M. (eds) Structural Information and Communication Complexity. SIROCCO 2005. Lecture Notes in Computer Science, vol 3499. Springer, Berlin, Heidelberg. https://doi.org/10.1007/11429647_7
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DOI: https://doi.org/10.1007/11429647_7
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