1,176 research outputs found
Star Colouring of Bounded Degree Graphs and Regular Graphs
A -star colouring of a graph is a function
such that for every edge of
, and every bicoloured connected subgraph of is a star. The star
chromatic number of , , is the least integer such that is
-star colourable. We prove that for
every -regular graph with . We reveal the structure and
properties of even-degree regular graphs that attain this lower bound. The
structure of such graphs is linked with a certain type of Eulerian
orientations of . Moreover, this structure can be expressed in the LC-VSP
framework of Telle and Proskurowski (SIDMA, 1997), and hence can be tested by
an FPT algorithm with the parameter either treewidth, cliquewidth, or
rankwidth. We prove that for , a -regular graph is
-star colourable only if is divisible by . For
each and divisible by , we construct a -regular
Hamiltonian graph on vertices which is -star colourable.
The problem -STAR COLOURABILITY takes a graph as input and asks
whether is -star colourable. We prove that 3-STAR COLOURABILITY is
NP-complete for planar bipartite graphs of maximum degree three and arbitrarily
large girth. Besides, it is coNP-hard to test whether a bipartite graph of
maximum degree eight has a unique 3-star colouring up to colour swaps. For
, -STAR COLOURABILITY of bipartite graphs of maximum degree is
NP-complete, and does not even admit a -time algorithm unless ETH
fails
Few Long Lists for Edge Choosability of Planar Cubic Graphs
It is known that every loopless cubic graph is 4-edge choosable. We prove the
following strengthened result.
Let G be a planar cubic graph having b cut-edges. There exists a set F of at
most 5b/2 edges of G with the following property. For any function L which
assigns to each edge of F a set of 4 colours and which assigns to each edge in
E(G)-F a set of 3 colours, the graph G has a proper edge colouring where the
colour of each edge e belongs to L(e).Comment: 14 pages, 1 figur
Hamiltonian cycles and 1-factors in 5-regular graphs
It is proven that for any integer and ,
there exist infinitely many 5-regular graphs of genus containing a
1-factorisation with exactly pairs of 1-factors that are perfect, i.e. form
a hamiltonian cycle. For , this settles a problem of Kotzig from 1964.
Motivated by Kotzig and Labelle's "marriage" operation, we discuss two gluing
techniques aimed at producing graphs of high cyclic edge-connectivity. We prove
that there exist infinitely many planar 5-connected 5-regular graphs in which
every 1-factorisation has zero perfect pairs. On the other hand, by the Four
Colour Theorem and a result of Brinkmann and the first author, every planar
4-connected 5-regular graph satisfying a condition on its hamiltonian cycles
has a linear number of 1-factorisations each containing at least one perfect
pair. We also prove that every planar 5-connected 5-regular graph satisfying a
stronger condition contains a 1-factorisation with at most nine perfect pairs,
whence, every such graph admitting a 1-factorisation with ten perfect pairs has
at least two edge-Kempe equivalence classes. The paper concludes with further
results on edge-Kempe equivalence classes in planar 5-regular graphs.Comment: 27 pages, 13 figures; corrected figure
Filling the complexity gaps for colouring planar and bounded degree graphs.
We consider a natural restriction of the List Colouring problem, k-Regular List Colouring, which corresponds to the List Colouring problem where every list has size exactly k. We give a complete classification of the complexity of k-Regular List Colouring restricted to planar graphs, planar bipartite graphs, planar triangle-free graphs and to planar graphs with no 4-cycles and no 5-cycles. We also give a complete classification of the complexity of this problem and a number of related colouring problems for graphs with bounded maximum degree
Filling the complexity gaps for colouring planar and bounded degree graphs
We consider a natural restriction of the List Colouring problem, k-Regular List Colouring, which corresponds to the List Colouring problem where every list has size exactly k. We give a complete classification of the complexity of k-Regular List Colouring restricted to planar graphs, planar bipartite graphs, planar triangle-free graphs and to planar graphs with no 4-cycles and no 5-cycles. We also give a complete classification of the complexity of this problem and a number of related colouring problems for graphs with bounded maximum degree
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