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The 1Factorization Problem and some related Conjectures
, 2004
"... E dalla crisalide sbucó una farfalla meravigliosa, tanto che tutti i fiori si aprirono ad essa. Out of the chrysalis there came a beautiful butterfly. It was so beautiful that all the flowers opened up to it. Daniela Rigato (19511996) The Classification Problem is the problem of determining whether ..."
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E dalla crisalide sbucó una farfalla meravigliosa, tanto che tutti i fiori si aprirono ad essa. Out of the chrysalis there came a beautiful butterfly. It was so beautiful that all the flowers opened up to it. Daniela Rigato (19511996) The Classification Problem is the problem of determining whether or not a given graph is ∆edge colourable, where ∆ is the maximum degree. This problem is known to be NPhard, even when restricted to the class of cubic simple graphs. A theorem of Chetwynd and Hilton states that all regular graphs of order 2n and degree at least ( √ 7−1 2
Chromatic Index Critical Graphs of Orders 11 and 12
, 1997
"... A chromaticindexcritical graph G on n vertices is nontrivial if it has at most \Deltab n 2 c edges. We prove that there is no chromaticindexcritical graph of order 12, and that there are precisely two nontrivial chromatic index critical graphs on 11 vertices. Together with known results thi ..."
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A chromaticindexcritical graph G on n vertices is nontrivial if it has at most \Deltab n 2 c edges. We prove that there is no chromaticindexcritical graph of order 12, and that there are precisely two nontrivial chromatic index critical graphs on 11 vertices. Together with known results this implies that there are precisely three nontrivial chromaticindex critical graphs of order 12. 1 Introduction A famous theorem of Vizing [20] states that the chromatic index Ø 0 (G) of a simple graph G is \Delta(G) or \Delta(G) + 1, where \Delta(G) denotes the maximum vertex degree in G. A graph G is class 1 if Ø 0 (G) = \Delta(G) and it is class 2 otherwise. A class 2 graph G is (chromatic index) critical if Ø 0 (G \Gamma e) ! Ø 0 (G) for each edge e of G. If we want to stress the maximum vertex degree of a critical graph G we say G is \Delta(G)critical. Critical graphs of odd order are easy to construct while not much is known about critical graphs of even order. One reas...
Chromaticindexcritical graphs of order 13 and 14
, 2003
"... A graph is chromaticindexcritical if it cannot be edgecoloured with ∆ colours (with ∆ the maximal degree of the graph), and if the removal of any edge decreases its chromatic index. The Critical Graph Conjecture stated that any such graph has odd order. It has been proved false and the smallest k ..."
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A graph is chromaticindexcritical if it cannot be edgecoloured with ∆ colours (with ∆ the maximal degree of the graph), and if the removal of any edge decreases its chromatic index. The Critical Graph Conjecture stated that any such graph has odd order. It has been proved false and the smallest known counterexample has order 18 [18, 31]. In this paper we show that there are no chromaticindexcritical graphs of order 14. Our result extends that of [5] and leaves order 16 as the only case to be checked in order to decide on the minimality of the counterexample given by Chetwynd and Fiol. In addition we list all nontrivial critical graphs of order 13.
ChromaticIndex Critical Graphs of Even Order
, 1997
"... A kcritrical graph G has maximum degree k 0, chromatic index Ø 0 (G) = k + 1 and Ø 0 (G \Gamma e) ! k + 1 for each edge e of G. The Critical Graph Conjecture, Jakobsen [8] and Beineke, Wilson [1], claims that every kcritical graph is of odd order. Fiorini and Wilson [6] conjectured that ev ..."
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A kcritrical graph G has maximum degree k 0, chromatic index Ø 0 (G) = k + 1 and Ø 0 (G \Gamma e) ! k + 1 for each edge e of G. The Critical Graph Conjecture, Jakobsen [8] and Beineke, Wilson [1], claims that every kcritical graph is of odd order. Fiorini and Wilson [6] conjectured that every kcritical graph of even order has a 1factor. Chetwynd and Yap [4] stated the problem whether it is true that if G is a kcritical graph of odd order, then G \Gamma v has a 1factor for every vertex v of minimum degree. These conjectures are disproved and the problem is answered in the negative for k 2 f3; 4g. We disprove these conjectures and answer the problem in the negative for all k 3. We also construct kcritical graphs on n vertices with degree sequence 23 2 4 n\Gamma3 , answering a question of Yap [11]. 1 Introduction We consider connected multigraphs M = (V (M); E(M)) without loops, where V (M) (E(M)) denotes the set of vertices (edges) of M . The degree dM (v) of a v...
International Journal of
, 2014
"... Abstract. In this paper, we study the structure of 5critical graphs in terms of their size. In particular, we have obtained bounds for the number of major vertices in several classes of 5critical graphs, that are stronger than the existing bounds. ..."
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Abstract. In this paper, we study the structure of 5critical graphs in terms of their size. In particular, we have obtained bounds for the number of major vertices in several classes of 5critical graphs, that are stronger than the existing bounds.