50 research outputs found
Odd circuits in dense binary matroids
We show that, for each real number and odd integer
there is an integer such that, if is a simple binary matroid with and with no -element circuit, then has critical
number at most . The result is an easy application of a regularity lemma for
finite abelian groups due to Green
Cycles with consecutive odd lengths
It is proved that there exists an absolute constant c > 0 such that for every
natural number k, every non-bipartite 2-connected graph with average degree at
least ck contains k cycles with consecutive odd lengths. This implies the
existence of the absolute constant d > 0 that every non-bipartite 2-connected
graph with minimum degree at least dk contains cycles of all lengths modulo k,
thus providing an answer (in a strong form) to a question of Thomassen. Both
results are sharp up to the constant factors.Comment: 7 page
Dense H-free graphs are almost (Χ(H)-1)-partite
By using the Szemeredi Regularity Lemma, Alon and Sudakov recently
extended the classical Andrasfai-Erdos-Sos theorem to cover general graphs. We
prove, without using the Regularity Lemma, that the following stronger statement
is true.
Given any (r+1)-partite graph H whose smallest part has t vertices, there exists
a constant C such that for any given ε>0 and sufficiently large n the following is
true. Whenever G is an n-vertex graph with minimum degree
δ(G)≥(1 −
3/3r−1 + ε)n,
either G contains H, or we can delete f(n,H)≤Cn2−1/t edges from G to obtain an
r-partite graph. Further, we are able to determine the correct order of magnitude
of f(n,H) in terms of the Zarankiewicz extremal function
Bipartite induced density in triangle-free graphs
We prove that any triangle-free graph on vertices with minimum degree at
least contains a bipartite induced subgraph of minimum degree at least
. This is sharp up to a logarithmic factor in . Relatedly, we show
that the fractional chromatic number of any such triangle-free graph is at most
the minimum of and as . This is
sharp up to constant factors. Similarly, we show that the list chromatic number
of any such triangle-free graph is at most as
.
Relatedly, we also make two conjectures. First, any triangle-free graph on
vertices has fractional chromatic number at most
as . Second, any triangle-free
graph on vertices has list chromatic number at most as
.Comment: 20 pages; in v2 added note of concurrent work and one reference; in
v3 added more notes of ensuing work and a result towards one of the
conjectures (for list colouring