20 research outputs found
An asymptotically tight bound on the Q-index of graphs with forbidden cycles
Let G be a graph of order n and let q(G) be that largest eigenvalue of the
signless Laplacian of G. In this note it is shown that if k>1 and q(G)>=n+2k-2,
then G contains cycles of length l whenever 2<l<2k+3. This bound is
asymptotically tight. It implies an asymptotic solution to a recent conjecture
about the maximum q(G) of a graph G with no cycle of a specified length.Comment: 10 pages. Version 2 takes care of some mistakes in version
Maxima of the Q-index: graphs without long paths
This paper gives tight upper bound on the largest eigenvalue q(G) of the
signless Laplacian of graphs with no paths of given order. The main ingredient
of our proof is a stability result of its own interest, about graphs with large
minimum degree and with no long paths. This result extends previous work of Ali
and Staton.Comment: 10 page
Tur\`an numbers of Multiple Paths and Equibipartite Trees
The Tur\'an number of a graph H, ex(n;H), is the maximum number of edges in
any graph on n vertices which does not contain H as a subgraph. Let P_l denote
a path on l vertices, and kP_l denote k vertex-disjoint copies of P_l. We
determine ex(n, kP_3) for n appropriately large, answering in the positive a
conjecture of Gorgol. Further, we determine ex (n, kP_l) for arbitrary l, and n
appropriately large relative to k and l. We provide some background on the
famous Erd\H{o}s-S\'os conjecture, and conditional on its truth we determine
ex(n;H) when H is an equibipartite forest, for appropriately large n.Comment: 17 pages, 13 figures; Updated to incorporate referee's suggestions;
minor structural change
Improved bounds on the multicolor Ramsey numbers of paths and even cycles
We study the multicolor Ramsey numbers for paths and even cycles,
and , which are the smallest integers such that every coloring of
the complete graph has a monochromatic copy of or
respectively. For a long time, has only been known to lie between
and . A recent breakthrough by S\'ark\"ozy and later
improvement by Davies, Jenssen and Roberts give an upper bound of . We improve the upper bound to . Our approach uses structural insights in connected graphs without a
large matching. These insights may be of independent interest
The maximum number of copies in -free graphs
Generalizing Tur\'an's classical extremal problem, Alon and Shikhelman
investigated the problem of maximizing the number of copies in an -free
graph, for a pair of graphs and . Whereas Alon and Shikhelman were
primarily interested in determining the order of magnitude for large classes of
graphs , we focus on the case when and are paths, where we find
asymptotic and in some cases exact results. We also consider other structures
like stars and the set of cycles of length at least , where we derive
asymptotically sharp estimates. Our results generalize well-known extremal
theorems of Erd\H{o}s and Gallai
Spectral radius of graphs forbidden or
Let be the graph obtained from a cycle by adding a
new vertex connecting two adjacent vertices in . In this note, we obtain
the graph maximizing the spectral radius among all graphs with size and
containing no subgraph isomorphic to . As a byproduct, we will
show that if the spectral radius , then must
contains all the cycles for unless .Comment: 11 pages, 1 figur
Extremal graphs without long paths and a given graph
For a family of graphs , the Tur\'{a}n number
is the maximum number of edges in an -vertex graph
containing no member of as a subgraph. The maximum number of
edges in an -vertex connected graph containing no member of as
a subgraph is denoted by . Let be the path on
vertices and be a graph with chromatic number more than . Katona and
Xiao [Extremal graphs without long paths and large cliques, European J.
Combin., 2023 103807] posed the following conjecture: Suppose that the
chromatic number of is more than . Then
. In this paper, we
determine the exact value of for sufficiently
large . Moreover, we obtain asymptotical result for
, which solves the conjecture proposed by Katona and
Xiao.Comment: 16 pages, 6 conference