1,458 research outputs found
Computing Minimum Rainbow and Strong Rainbow Colorings of Block Graphs
A path in an edge-colored graph is rainbow if no two edges of it are
colored the same. The graph is rainbow-connected if there is a rainbow path
between every pair of vertices. If there is a rainbow shortest path between
every pair of vertices, the graph is strongly rainbow-connected. The
minimum number of colors needed to make rainbow-connected is known as the
rainbow connection number of , and is denoted by . Similarly,
the minimum number of colors needed to make strongly rainbow-connected is
known as the strong rainbow connection number of , and is denoted by
. We prove that for every , deciding whether
is NP-complete for split graphs, which form a subclass
of chordal graphs. Furthermore, there exists no polynomial-time algorithm for
approximating the strong rainbow connection number of an -vertex split graph
with a factor of for any unless P = NP. We
then turn our attention to block graphs, which also form a subclass of chordal
graphs. We determine the strong rainbow connection number of block graphs, and
show it can be computed in linear time. Finally, we provide a polynomial-time
characterization of bridgeless block graphs with rainbow connection number at
most 4.Comment: 13 pages, 3 figure
The strong rainbow vertex-connection of graphs
A vertex-colored graph is said to be rainbow vertex-connected if every
two vertices of are connected by a path whose internal vertices have
distinct colors, such a path is called a rainbow path. The rainbow
vertex-connection number of a connected graph , denoted by , is the
smallest number of colors that are needed in order to make rainbow
vertex-connected. If for every pair of distinct vertices, contains a
rainbow geodesic, then is strong rainbow vertex-connected. The
minimum number for which there exists a -vertex-coloring of that
results in a strongly rainbow vertex-connected graph is called the strong
rainbow vertex-connection number of , denoted by . Observe that
for any nontrivial connected graph . In this paper,
sharp upper and lower bounds of are given for a connected graph
of order , that is, . Graphs of order such that
are characterized, respectively. It is also shown that,
for each pair of integers with and , there
exists a connected graph such that and .Comment: 10 page
The rainbow vertex-index of complementary graphs
A vertex-colored graph is \emph{rainbow vertex-connected} if two vertices are connected by a path whose internal vertices have distinct colors. The \emph{rainbow vertex-connection number} of a connected graph , denoted by , is the smallest number of colors that are needed in order to make rainbow vertex-connected. If for every pair of distinct vertices, contains a vertex-rainbow geodesic, then is \emph{strongly rainbow vertex-connected}. The minimum for which there exists a -coloring of that results in a strongly rainbow-vertex-connected graph is called the \emph{strong rainbow vertex number} of . Thus for every nontrivial connected graph . A tree in is called a \emph{rainbow vertex tree} if the internal vertices of receive different colors. For a graph and a set of at least two vertices, \emph{an -Steiner tree} or \emph{a Steiner tree connecting } (or simply, \emph{an -tree}) is a such subgraph of that is a tree with . For and , an -Steiner tree is said to be a \emph{rainbow vertex -tree} if the internal vertices of receive distinct colors. The minimum number of colors that are needed in a vertex-coloring of such that there is a rainbow vertex -tree for every -set of is called the {\it -rainbow vertex-index} of , denoted by . In this paper, we first investigate the strong rainbow vertex-connection of complementary graphs. The -rainbow vertex-index of complementary graphs are also studied
Total Rainbow Connection Number Of Shackle Product Of Antiprism Graph (〖AP〗_3)
Function if is said to be k total rainbows in , for each pair of vertex there is a path called with each edge and each vertex on the path will have a different color. The total connection number is denoted by trc defined as the minimum number of colors needed to make graph to be total rainbow connected. Total rainbow connection numbers can also be applied to graphs that are the result of operations. The denoted shackle graph is a graph resulting from the denoted graph where t is number of copies of G. This research discusses rainbow connection numbers rc and total rainbow connection trc(G) using the shackle operation, where is the antiprism graph . Based on this research, rainbow connection numbers rc shack , and total rainbow connection trc shack for .Fungsi jika c : G → {1,2,. . . , k} dikatakan k total pelangi pada G, untuk setiap pasang titik terdapat lintasan disebut x-y dengan setiap sisi dan setiap titik pada lintasan akan memiliki warna berbeda. Bilangan terhubung total pelangi dilambangkan dengan trc(G), didefinisikan sebagai jumlah minimum warna yang diperlukan untuk membuat graf G menjadi terhubung-total pelangi. Bilangan terhubung total pelangi juga dapat diterapkan pada graf yang merupakan hasil operasi. Graf shackle yang dilambangkan (G1,G2,…,Gt) adalah graf yang dihasilkan dari graf G yang dilambangkan (G,t) dengan t adalah jumlah salinan dari Penelitian ini membahas mengenai bilangan terhubung pelangi rc dan bilangan terhubung total pelangi trc(G)menggunakan operasi shackle, dimana G adalah graf Antiprisma (AP3)Berdasarkan penelitian ini, diperoleh bilangan terhubung pelangi rc(shack AP3,t )= t+2, dan total pelangi trc(shack AP3,t)=2t+3 untuk t ≥2
Rainbow Connection Number of Prism and Product of Two Graphs
An edge-colouring of a graph is rainbow connected if, for any two vertices of , there are internally vertex-disjoint paths joining them, each of which is rainbow and then a minimal numbers of color is required to make rainbow connected. The rainbow connection numbers of a connected graph , denoted . In this paper we will discuss the rainbow connection number for some special graphs and its operations, namely prism graph , antiprism graph , tensor product of , joint graph +
Chasing the Rainbow Connection: Hardness, Algorithms, and Bounds
We study rainbow connectivity of graphs from the algorithmic and graph-theoretic points of view. The study is divided into three parts. First, we study the complexity of deciding whether a given edge-colored graph is rainbow-connected. That is, we seek to verify whether the graph has a path on which no color repeats between each pair of its vertices. We obtain a comprehensive map of the hardness landscape of the problem. While the problem is NP-complete in general, we identify several structural properties that render the problem tractable. At the same time, we strengthen the known NP-completeness results for the problem. We pinpoint various parameters for which the problem is fixed-parameter tractable, including dichotomy results for popular width parameters, such as treewidth and pathwidth. The study extends to variants of the problem that consider vertex-colored graphs and/or rainbow shortest paths. We also consider upper and lower bounds for exact parameterized algorithms. In particular, we show that when parameterized by the number of colors k, the existence of a rainbow s-t path can be decided in O∗ (2k ) time and polynomial space. For the highly related problem of finding a path on which all the k colors appear, i.e., a colorful path, we explain the modest progress over the last twenty years. Namely, we prove that the existence of an algorithm for finding a colorful path in (2 − ε)k nO(1) time for some ε > 0 disproves the so-called Set Cover Conjecture.Second, we focus on the problem of finding a rainbow coloring. The minimum number of colors for which a graph G is rainbow-connected is known as its rainbow connection number, denoted by rc(G). Likewise, the minimum number of colors required to establish a rainbow shortest path between each pair of vertices in G is known as its strong rainbow connection number, denoted by src(G). We give new hardness results for computing rc(G) and src(G), including their vertex variants. The hardness results exclude polynomial-time algorithms for restricted graph classes and also fast exact exponential-time algorithms (under reasonable complexity assumptions). For positive results, we show that rainbow coloring is tractable for e.g., graphs of bounded treewidth. In addition, we give positive parameterized results for certain variants and relaxations of the problems in which the goal is to save k colors from a trivial upper bound, or to rainbow connect only a certain number of vertex pairs.Third, we take a more graph-theoretic view on rainbow coloring. We observe upper bounds on the rainbow connection numbers in terms of other well-known graph parameters. Furthermore, despite the interest, there have been few results on the strong rainbow connection number of a graph. We give improved bounds and determine exactly the rainbow and strong rainbow connection numbers for some subclasses of chordal graphs. Finally, we pose open problems and conjectures arising from our work
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