598 research outputs found
On the Distinguishing Number of Cyclic Tournaments: Towards the Albertson-Collins Conjecture
A distinguishing -labeling of a digraph is a mapping from
the set of verticesof to the set of labels such that no
nontrivial automorphism of preserves all the labels.The distinguishing
number of is then the smallest for which admits a
distinguishing -labeling.From a result of Gluck (David Gluck, Trivial
set-stabilizers in finite permutation groups,{\em Can. J. Math.} 35(1) (1983),
59--67),it follows that for every cyclic tournament~ of (odd) order
.Let for every such tournament.Albertson and
Collins conjectured in 1999that the canonical 2-labeling given
by if and only if is distinguishing.We prove that
whenever one of the subtournaments of induced by vertices or
is rigid, satisfies Albertson-Collins Conjecture.Using
this property, we prove that several classes of cyclic tournaments satisfy
Albertson-Collins Conjecture.Moreover, we also prove that every Paley
tournament satisfies Albertson-Collins Conjecture
GrĂĄfszĂnezĂ©sek Ă©s grĂĄfok felbontĂĄsai = Colorings and decompositions of graphs
A nem-ismĂ©tlĆ szĂnezĂ©seket a vĂ©letlen mĂłdszer alkalmazhatĂłsĂĄga miatt kezdtĂ©k el vizsgĂĄlni. FelsĆ korlĂĄtot adtunk a szĂnek szĂĄmĂĄra, amely a maximum fok Ă©s a favastagsĂĄg lineĂĄris fĂŒggvĂ©nye. Olyan szĂnezĂ©seket is vizsgĂĄltunk, amelyek egy sĂkgrĂĄf oldalain nem-ismĂ©tlĆk. SejtĂ©s volt, hogy vĂ©ges sok szĂn elĂ©g. Ezt bizonyĂtottuk 24 szĂnnel. A kromatikus szĂĄmot Ă©s a metszĂ©si szĂĄmot algoritmikusan nehĂ©z meghatĂĄrozni. EzĂ©rt meglepĆ Albertson egy friss sejtĂ©se, amely kapcsolatot ĂĄllĂt fel közöttĂŒk: ha egy grĂĄf kromatikus szĂĄma r, akkor metszĂ©si szĂĄma legalĂĄbb annyi, mint a teljes r csĂșcsĂș grĂĄfĂ©. BizonyĂtottuk a sejtĂ©st, ha r<3.57n, valamint ha 12<r<17. Ez utĂłbbi azĂ©rt Ă©rdekes, mert a teljes r csĂșcsĂș grĂĄf metszĂ©si szĂĄma csak r<13 esetĂ©n ismert. A tĂ©makör legfontosabb nyitott kĂ©rdĂ©se a Hadwiger-sejtĂ©s, mely szerint minden r-kromatikus grĂĄf tartalmazza a teljes r csĂșcsĂș grĂĄfot minorkĂ©nt. Ennek ĂĄltalĂĄnosĂtĂĄsakĂ©nt fogalmaztĂĄk meg a lista szĂnezĂ©si Hadwiger sejtĂ©st: ha egy grĂĄf nem tartalmaz teljes r csĂșcsĂș grĂĄfot minorkĂ©nt, akkor az r-lista szĂnezhetĆ. Megmutattuk, hogy ez a sejtĂ©s hamis. LegalĂĄbb cr szĂnre szĂŒksĂ©gĂŒnk van bizonyos grĂĄfokra, ahol c=4/3. Thomassennel vetettĂŒk fel azt a kĂ©rdĂ©st, hogy milyen feltĂ©tel garantĂĄlja, hogy G Ă©lei felbonthatĂłk egy adott T fa pĂ©ldĂĄnyaira. Legyen Y az a fa, melynek fokszĂĄmsorozata (1,1,1,2,3). Megmutattuk, hogy minden 287-szeresen Ă©lösszefĂŒggĆ fa felbomlik Y-okra, ha az Ă©lszĂĄm oszthatĂł 4-gyel. | Nonrepetitive colorings often use the probabilistic method. We gave an upper bound as a linear function of the maximum degree and the tree-width. We also investigated colorings, which are nonrepetitive on faces of plane graphs. As conjectured, a finite number of colors suffice. We proved it by 24 colors. The chromatic and crossing numbers are both difficult to compute. The recent Albertson's conjecture is a surprising relation between the two: if the chromatic number is r, then the crossing number is at least the crossing number of the complete graph on r vertices. We proved this claim, if r<3.57n, or 12<r<17. The latter is remarkable, since the crossing number of the complete graph is only known for r<13. The most important open question of the field is Hadwiger's conjecture: every r-chromatic graph contains as a minor the complete graph on r vertices. As a generalisation, the following is the list coloring Hadwiger conjecture: if a graph does not contain as a minor the complete graph on r vertices , then the graph is r-list colorable. We proved the falsity of this claim. In our examples, at least cr colors are necessary, where c=4/3. Decomposition of graphs is well-studied. Thomassen and I posed the question of a sufficient connectivity condition, which guaranties a T-decomposition. Let Y be the tree with degree sequence (1,1,1,2,3). We proved every 287-edge connected graph has a Y-decomposition, if the size is divisible by four
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