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1
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On light subgraphs in plane graphs of minimum degree five

100%
Jendrol' S., Madaras T.
Discussiones Mathematicae Graph Theory
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1996
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tom 16
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nr 2
207-217
EN
A subgraph of a plane graph is light if the sum of the degrees of the vertices of the subgraph in the graph is small. It is well known that a plane graph of minimum degree five contains light edges and light triangles. In this paper we show that every plane graph of minimum degree five contains also light stars $K_{1,3}$ and $K_{1,4}$ and a light 4-path P₄. The results obtained for $K_{1,3}$ and P₄ are best possible.
2
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On Super (a, d)-H-Antimagic Total Covering of Star Related Graphs

100%
Kathiresan K. M., Laurence S. D.
Discussiones Mathematicae Graph Theory
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2015
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tom 35
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nr 4
755-764
EN
Let G = (V (G),E(G)) be a simple graph and H be a subgraph of G. G admits an H-covering, if every edge in E(G) belongs to at least one subgraph of G that is isomorphic to H. An (a, d)-H-antimagic total labeling of G is a bijection λ: V (G) ∪ E(G) → {1, 2, 3, . . . , |V (G)| + |E(G)|} such that for all subgraphs H′ isomorphic to H, the H′ weights [...] constitute an arithmetic progression a, a+d, a+2d, . . . , a+(n−1)d where a and d are positive integers and n is the number of subgraphs of G isomorphic to H. Additionally, the labeling λ is called a super (a, d)-H-antimagic total labeling if λ(V (G)) = {1, 2, 3, . . . , |V (G)|}. In this paper we study super (a, d)-H-antimagic total labelings of star related graphs Gu[Sn] and caterpillars.
3
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Backbone colorings along stars and matchings in split graphs: their span is close to the chromatic number

100%
Broersma H., Marchal B., Paulusma D., Salman A.
Discussiones Mathematicae Graph Theory
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2009
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tom 29
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nr 1
143-162
EN
We continue the study on backbone colorings, a variation on classical vertex colorings that was introduced at WG2003. Given a graph G = (V,E) and a spanning subgraph H of G (the backbone of G), a λ-backbone coloring for G and H is a proper vertex coloring V→ {1,2,...} of G in which the colors assigned to adjacent vertices in H differ by at least λ. The algorithmic and combinatorial properties of backbone colorings have been studied for various types of backbones in a number of papers. The main outcome of earlier studies is that the minimum number l of colors, for which such colorings V→ {1,2,...,l} exist, in the worst case is a factor times the chromatic number (for path, tree, matching and star backbones). We show here that for split graphs and matching or star backbones, l is at most a small additive constant (depending on λ) higher than the chromatic number. Our proofs combine algorithmic and combinatorial arguments. We also indicate other graph classes for which our results imply better upper bounds on l than the previously known bounds.
4
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Wiener index of generalized stars and their quadratic line graphs

100%
Dobrynin A., Mel'nikov L.
Discussiones Mathematicae Graph Theory
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2006
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tom 26
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nr 1
161-175
EN
The Wiener index, W, is the sum of distances between all pairs of vertices in a graph G. The quadratic line graph is defined as L(L(G)), where L(G) is the line graph of G. A generalized star S is a tree consisting of Δ ≥ 3 paths with the unique common endvertex. A relation between the Wiener index of S and of its quadratic graph is presented. It is shown that generalized stars having the property W(S) = W(L(L(S)) exist only for 4 ≤ Δ ≤ 6. Infinite families of generalized stars with this property are constructed.
5
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On local structure of 1-planar graphs of minimum degree 5 and girth 4

100%
Hudák D., Madaras T.
Discussiones Mathematicae Graph Theory
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2009
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tom 29
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nr 2
385-400
EN
A graph is 1-planar if it can be embedded in the plane so that each edge is crossed by at most one other edge. We prove that each 1-planar graph of minimum degree 5 and girth 4 contains (1) a 5-vertex adjacent to an ≤ 6-vertex, (2) a 4-cycle whose every vertex has degree at most 9, (3) a $K_{1,4}$ with all vertices having degree at most 11.
6
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On the Crossing Numbers of Cartesian Products of Stars and Graphs of Order Six

75%
Klešč M., Schrötter Š.
Discussiones Mathematicae Graph Theory
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2013
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tom 33
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nr 3
583-597
EN
The crossing number cr(G) of a graph G is the minimal number of crossings over all drawings of G in the plane. According to their special structure, the class of Cartesian products of two graphs is one of few graph classes for which some exact values of crossing numbers were obtained. The crossing numbers of Cartesian products of paths, cycles or stars with all graphs of order at most four are known. Moreover, except of six graphs, the crossing numbers of Cartesian products G⃞K1,n for all other connected graphs G on five vertices are known. In this paper we are dealing with the Cartesian products of stars with graphs on six vertices. We give the exact values of crossing numbers for some of these graphs and we summarise all known results concerning crossing numbers of these graphs. Moreover, we give the crossing number of G1⃞T for the special graph G1 on six vertices and for any tree T with no vertex of degree two as well as the crossing number of K1,n⃞T for any tree T with maximum degree five.
7
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Light classes of generalized stars in polyhedral maps on surfaces

75%
Jendrol' S., Voss H.-J.
Discussiones Mathematicae Graph Theory
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2004
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tom 24
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nr 1
85-107
EN
A generalized s-star, s ≥ 1, is a tree with a root Z of degree s; all other vertices have degree ≤ 2. $S_i$ denotes a generalized 3-star, all three maximal paths starting in Z have exactly i+1 vertices (including Z). Let 𝕄 be a surface of Euler characteristic χ(𝕄) ≤ 0, and m(𝕄):= ⎣(5 + √{49-24χ(𝕄 )})/2⎦. We prove: (1) Let k ≥ 1, d ≥ m(𝕄) be integers. Each polyhedral map G on 𝕄 with a k-path (on k vertices) contains a k-path of maximum degree ≤ d in G or a generalized s-star T, s ≤ m(𝕄), on d + 2- m(𝕄) vertices with root Z, where Z has degree ≤ k·m(𝕄) and the maximum degree of T∖{Z} is ≤ d in G. Similar results are obtained for the plane and for large polyhedral maps on 𝕄.. (2) Let k and i be integers with k ≥ 3, 1 ≤ i ≤ [k/2]. If a polyhedral map G on 𝕄 with a large enough number of vertices contains a k-path then G contains a k-path or a 3-star $S_i$ of maximum degree ≤ 4(k+i) in G. This bound is tight. Similar results hold for plane graphs.
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