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Pré-Publication, Document De Travail Année : 2014

On cliques of signed and switchable signed graphs

Résumé

Vertex coloring of a graph G with n-colors can be equivalently thought to be a graph homomorphism (edge preserving vertex mapping) of G to the complete graph Kn of order n. So, in that sense, the chromatic number χ(G) of G will be the order of the smallest complete graph to which G admits a homomorphism to. As every graph, which is not a complete graph, admits a homomorphism to a smaller complete graph, we can redefine the chromatic number χ(G) of G to be the order of the smallest graph to which G admits a homomorphism to. Of course, such a smallest graph must be a complete graph as they are the only graphs with chromatic number equal to their order. The concept of vertex coloring can be generalize for other types of graphs, namely, oriented graphs (directed graphs with no cycle of length 1 or 2), 2-edge-colored or signed graphs (graphs with positive or negative signs assigned to each edge) and switchable signed graphs (equivalence class of signed graph with respect to switching signs of edges incident to the same vertex) using the notion of graph homomorphism. Naturally, the chromatic number is defined to be the order of the smallest graph (of the same type) to which a graph admits homomorphism to. For the above mentioned type of graphs, the graphs with smallest order, that is, the graphs with order equal to their (so defined) "chromatic number" are called ocliques, scliques and [s]-cliques respectively. These "cliques" turns out to be much more complicated than their undirected counterpart and are interesting objects of study. In this article, we mainly study different aspects of "cliques" for signed and switchable signed graphs. In particular, we show that it is NP-hard to decide if edges of a given undirected graph can be assigned positive and negative signatures such that it becomes an sclique or an [s]-clique. We also show that, asymptotically, almost all signed graphs are scliques or [s]-cliques. Furthermore, we prove a sufficient and necessary condition for a signed graph (or switchable signed graph) to be an sclique (or [s]-clique). We study the number of vertices that an sclique (or [s]-clique) can have when their underlying graph is planar and prove a tight upper bound of 15. We also study the same for outerplanar graphs and planar graphs with given girth (length of the smallest cycle). Finally, we generalize the concept of "cliques" for n-edge-colored graphs (graphs with one among n different colors assigned to each of its edge) and do a similar study for outerplanar and planar graphs.
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Dates et versions

hal-01078218 , version 1 (28-10-2014)
hal-01078218 , version 2 (20-05-2015)
hal-01078218 , version 3 (07-11-2017)

Identifiants

  • HAL Id : hal-01078218 , version 1

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Julien Bensmail, Christopher Duffy, Sagnik Sen. On cliques of signed and switchable signed graphs. 2014. ⟨hal-01078218v1⟩
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