9,322 research outputs found
On the Connectivity of Token Graphs of Trees
Let and be integers such that , and let be a
simple graph of order . The -token graph of is the graph
whose vertices are the -subsets of , where two vertices are adjacent
in whenever their symmetric difference is an edge of . In this
paper we show that if is a tree, then the connectivity of is equal
to the minimum degree of
On the spectra of token graphs of cycles and other graphs
The -token graph of a graph is the graph whose vertices are
the -subsets of vertices from , two of which being adjacent whenever
their symmetric difference is a pair of adjacent vertices in . It is a known
result that the algebraic connectivity (or second Laplacian eigenvalue) of
equals the algebraic connectivity of .
In this paper, we first give results that relate the algebraic connectivities
of a token graph and the same graph after removing a vertex. Then, we prove the
result on the algebraic connectivity of 2-token graphs for two infinite
families: the odd graphs for all , and the multipartite complete
graphs for all In the case of
cycles, we present a new method that allows us to compute the whole spectrum of
. This method also allows us to obtain closed formulas that give
asymptotically exact approximations for most of the eigenvalues of
On the spectra of token graphs of cycles and other graphs
© 2023 Elsevier. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/The k-token graph of a graph G is the graph whose vertices are the k-subsets of vertices from G, two of which being adjacent whenever their symmetric difference is a pair of adjacent vertices in G. It is a known result that the algebraic connectivity (or second Laplacian eigenvalue) of equals the algebraic connectivity of G. In this paper, we first give results that relate the algebraic connectivities of a token graph and the same graph after removing a vertex. Then, we prove the result on the algebraic connectivity of 2-token graphs for two infinite families: the odd graphs for all r, and the multipartite complete graphs for all In the case of cycles, we present a new method that allows us to compute the whole spectrum of . This method also allows us to obtain closed formulas that give asymptotically exact approximations for most of the eigenvalues of .Peer ReviewedPostprint (author's final draft
Connectivity properties of some transformation graphs
Many combinatorial problems can be formulated as \can we transform configuration 1 into configuration 2 if certain transformations are allowed?" In order to study such questions, we introduce a so-called transformation graph. This graph has the set of all possible configurations as its vertex set, and there is an edge between two configurations if one configuration can be obtained from the other by one of the allowed transformations. Then a question like \can we go from one configuration to another one" becomes a question about connectivity properties of transformation graphs.
In this thesis, we study the following types of transformation graphs in particular:
Labelled Token Graphs: Here configurations are arrangements of labelled tokens on a given graph, and we can go from one arrangement to another one by moving one token at a time along an edge of the given graph. We classify all cases when labelled token graphs are connected, and classify all pairs of arrangements that are in the same component. We also look at the problem how hard it is to determine the length of the shortest path between two arrangements.
Strong k-Colour Graphs: For this transformation graph, the configurations are the proper vertex-colourings of a given graph with k colours, in which all k colours are actually used. We call such a colouring a strong k-colouring. We study the problem when we can transform any strong k-colouring into any other one by recolouring one vertex at a time, always maintaining a strong k-colouring. For certain classes of graphs, we can completely determine when the transformation graph of strong k-colourings is connected
Approximating Minimum-Cost k-Node Connected Subgraphs via Independence-Free Graphs
We present a 6-approximation algorithm for the minimum-cost -node
connected spanning subgraph problem, assuming that the number of nodes is at
least . We apply a combinatorial preprocessing, based on the
Frank-Tardos algorithm for -outconnectivity, to transform any input into an
instance such that the iterative rounding method gives a 2-approximation
guarantee. This is the first constant-factor approximation algorithm even in
the asymptotic setting of the problem, that is, the restriction to instances
where the number of nodes is lower bounded by a function of .Comment: 20 pages, 1 figure, 28 reference
The Complexity of Change
Many combinatorial problems can be formulated as "Can I transform
configuration 1 into configuration 2, if certain transformations only are
allowed?". An example of such a question is: given two k-colourings of a graph,
can I transform the first k-colouring into the second one, by recolouring one
vertex at a time, and always maintaining a proper k-colouring? Another example
is: given two solutions of a SAT-instance, can I transform the first solution
into the second one, by changing the truth value one variable at a time, and
always maintaining a solution of the SAT-instance? Other examples can be found
in many classical puzzles, such as the 15-Puzzle and Rubik's Cube.
In this survey we shall give an overview of some older and more recent work
on this type of problem. The emphasis will be on the computational complexity
of the problems: how hard is it to decide if a certain transformation is
possible or not?Comment: 28 pages, 6 figure
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