52 research outputs found
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
Intersecting and -intersecting hypergraphs with maximal covering number: the Erd\H{o}s-Lov\'asz theme revisited
Erd\H{o}s and Lov\'asz noticed that an -uniform intersecting hypergraph
with maximal covering number, that is , must have at least
edges. There has been no improvement on this lower bound for
45 years. We try to understand the reason by studying some small cases to see
whether the truth lies very close to this simple bound. Let denote the
minimum number of edges in an intersecting -uniform hypergraph. It was known
that and . We obtain the following new results: The extremal
example for uniformity 4 is unique. Somewhat surprisingly it is not symmetric
by any means. For uniformity 5, , and we found 3 examples, none of
them being some known graph. We use both theoretical arguments and computer
searches. In the footsteps of Erd\H{o}s and Lov\'asz, we also consider the
special case, when the hypergraph is part of a finite projective plane. We
determine the exact answer for . For uniformity 6, there is a
unique extremal example.
In a related question, we try to find -intersecting -uniform
hypergraphs with maximal covering number, that is . An infinite
family of examples is to take all possible -sets of a -vertex set.
There is also a geometric candidate: biplanes. These are symmetric 2-designs
with . We determined that only 3 biplanes of the 18 known examples
are extremal
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