Abstract. In this paper we continue the study of paired-domination in graphs. A paired-dominating set, abbreviated PDS, of a graph G with no isolated vertex is a dominating set of vertices whose induced subgraph has a perfect matching. The paired--domination number of G, denoted by γp(G), is the minimum cardinality of a PDS of G. The upper paired-domination number of G, denoted by Γp(G), is the maximum cardinality of a minimal PDS of G. Let G be a connected graph of order n ≥ 3. Haynes and Slater in [Paired-domination in graphs, Networks 32 (1998), [199][200][201][202][203][204][205][206], showed that γp(G) ≤ n − 1 and they determine the extremal graphs G achieving this bound. In this paper we obtain analogous results for Γp(G). Dorbec, Henning and McCoy in [Upper total domination versus upper paired-domination, Questiones Mathematicae 30 (2007), 1-12] determine Γp(Pn), instead in this paper we determine Γp(Cn). Moreover, we describe some families of graphs G for which the equality γp(G) = Γp(G) holds.

Let \(G=(V,E)\) be a graph with no isolated vertices. A set \(S\subseteq V\) is a paired-dominating set of \(G\) if every vertex not in \(S\) is adjacent with some vertex in \(S\) and the subgraph induced by \(S\) contains a perfect matching. The paired-domination number \(\gamma_{p}(G)\) of \(G\) is defined to be the minimum cardinality of a paired-dominating set of \(G\). Let \(G\) be a graph of order \(n\). In [Paired-domination in graphs, Networks 32 (1998), 199-206] Haynes and Slater described graphs \(G\) with \(\gamma_{p}(G)=n\) and also graphs with \(\gamma_{p}(G)=n-1\). In this paper we show all graphs for which \(\gamma_{p}(G)=n-2\)

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In this paper we show upper bounds for the sum and the product of the lower domination parameters and the chromatic index of a graph. We also present some families of graphs for which these upper bounds are achieved. Next, we give a lower bound for the sum of the upper domination parameters and the chromatic index. This lower bound is a function of the number of vertices of a graph and a new graph parameter which is defined here. In this case we also characterize graphs for which a respective equality holds.

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