222 research outputs found

    Locating-dominating sets in twin-free graphs

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    A locating-dominating set of a graph GG is a dominating set DD of GG with the additional property that every two distinct vertices outside DD have distinct neighbors in DD; that is, for distinct vertices uu and vv outside DD, N(u)DN(v)DN(u) \cap D \ne N(v) \cap D where N(u)N(u) denotes the open neighborhood of uu. A graph is twin-free if every two distinct vertices have distinct open and closed neighborhoods. The location-domination number of GG, denoted γL(G)\gamma_L(G), is the minimum cardinality of a locating-dominating set in GG. It is conjectured [D. Garijo, A. Gonz\'alez and A. M\'arquez. The difference between the metric dimension and the determining number of a graph. Applied Mathematics and Computation 249 (2014), 487--501] that if GG is a twin-free graph of order nn without isolated vertices, then γL(G)n2\gamma_L(G)\le \frac{n}{2}. We prove the general bound γL(G)2n3\gamma_L(G)\le \frac{2n}{3}, slightly improving over the 2n3+1\lfloor\frac{2n}{3}\rfloor+1 bound of Garijo et al. We then provide constructions of graphs reaching the n2\frac{n}{2} bound, showing that if the conjecture is true, the family of extremal graphs is a very rich one. Moreover, we characterize the trees GG that are extremal for this bound. We finally prove the conjecture for split graphs and co-bipartite graphs.Comment: 11 pages; 4 figure

    Microbial Succession in Spontaneously Fermented Grape Must Before, During and After Stuck Fermentation

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    The microbial succession in spontaneously fermenting Riesling must was investigated from the beginning(pressing) until the end (sulphuring) of the fermentation in two harvest years (2008 and 2009) at a Mosellewinery (Germany). In both years, the fermentation was interrupted by a stuck period. The length of thestuck period varied considerably (20 weeks in 2008 and one week in 2009). Different yeasts (Candida,Debaryomyces, Pichia, Hanseniaspora, Saccharomyces, Metschnikowia, Cryptococcus, Filobasidium andRhodotorula) and bacteria (Gluconobacter, Asaia, Acetobacter, Oenococcus, Lactobacillus, Bacillus andPaenibacillus) were isolated successively by plating. The main fermenting organism was Saccharomycesuvarum. Specific primers were developed for S. uvarum, H. uvarum and C. boidinii, followed by thedetermination of the total cell counts with qPCR. The initial glucose concentration differed between thetwo years and was 116 g/L in 2008 and 85.4 g/L in 2009. Also, the fructose concentrations were differentin both years (114 g/L in 2008 and 77.8 g/L in 2009). The stuck period appeared when the glucose/fructoseratio was 0.34 and 0.12 respectively. The microbiota changed during the stuck period

    Remarks about disjoint dominating sets

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    We solve a number of problems posed by Hedetniemi, Hedetniemi, Laskar, Markus, and Slater concerning pairs of disjoint sets in graphs which are dominating or independent and dominating

    An independent dominating set in the complement of a minimum dominating set of a tree

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    We prove that for every tree TT of order at least 22 and every minimum dominating set DD of TT which contains at most one endvertex of TT, there is an independent dominating set II of TT which is disjoint from DD. This confirms a recent conjecture of Johnson, Prier, and Walsh

    Partitioning a graph into a dominating set, a total dominating set, and something else

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    A recent result of Henning and Southey (A note on graphs with disjoint dominating and total dominating set, {\it Ars Comb.} {\bf 89} (2008), 159--162) implies that every connected graph of minimum degree at least three has a dominating set DD and a total dominating set TT which are disjoint. We show that the Petersen graph is the only such graph for which DTD\cup T necessarily contains all vertices of the graph
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