10 research outputs found
Supersaturation Problem for Color-Critical Graphs
The \emph{Tur\'an function} \ex(n,F) of a graph is the maximum number
of edges in an -free graph with vertices. The classical results of
Tur\'an and Rademacher from 1941 led to the study of supersaturated graphs
where the key question is to determine , the minimum number of copies
of that a graph with vertices and \ex(n,F)+q edges can have.
We determine asymptotically when is \emph{color-critical}
(that is, contains an edge whose deletion reduces its chromatic number) and
.
Determining the exact value of seems rather difficult. For
example, let be the limit superior of for which the extremal
structures are obtained by adding some edges to a maximum -free graph.
The problem of determining for cliques was a well-known question of Erd\H
os that was solved only decades later by Lov\'asz and Simonovits. Here we prove
that for every {color-critical}~. Our approach also allows us to
determine for a number of graphs, including odd cycles, cliques with one
edge removed, and complete bipartite graphs plus an edge.Comment: 27 pages, 2 figure
Supersaturation Problem for the Bowtie
The Tur\'an function denotes the maximal number of edges in an
-free graph on vertices. We consider the function , the
minimal number of copies of in a graph on vertices with
edges. The value of has been extensively studied when is
bipartite or colour-critical. In this paper we investigate the simplest
remaining graph , namely, two triangles sharing a vertex, and establish the
asymptotic value of for .Comment: 23 pages, 1 figur
Structure and Supersaturation for Intersecting Families
The extremal problems regarding the maximum possible size of intersecting
families of various combinatorial objects have been extensively studied. In
this paper, we investigate supersaturation extensions, which in this context
ask for the minimum number of disjoint pairs that must appear in families
larger than the extremal threshold. We study the minimum number of disjoint
pairs in families of permutations and in -uniform set families, and
determine the structure of the optimal families. Our main tool is a removal
lemma for disjoint pairs. We also determine the typical structure of
-uniform set families without matchings of size when , and show that almost all -uniform intersecting families on vertex
set are trivial when .Comment: 23 pages + appendi
The exact minimum number of triangles in graphs of given order and size
What is the minimum number of triangles in a graph of given order and size?
Motivated by earlier results of Mantel and Tur\'an, Rademacher solved the first
non-trivial case of this problem in 1941. The problem was revived by Erd\H{o}s
in 1955; it is now known as the Erd\H{o}s-Rademacher problem. After attracting
much attention, it was solved asymptotically in a major breakthrough by
Razborov in 2008. In this paper, we provide an exact solution for all large
graphs whose edge density is bounded away from~, which in this range
confirms a conjecture of Lov\'asz and Simonovits from 1975. Furthermore, we
give a description of the extremal graphs.Comment: Published in Forum of Mathematics, Pi, Volume 8, e8 (2020
Supersaturation problem for color-critical graphs
The \emph{Tur\'an function} \ex(n,F) of a graph is the maximum number of edges in an -free graph with vertices. The classical results of Tur\'an and Rademacher from 1941 led to the study of supersaturated graphs where the key question is to determine , the minimum number of copies of that a graph with vertices and \ex(n,F)+q edges can have.
We determine asymptotically when is \emph{color-critical} (that is, contains an edge whose deletion reduces its chromatic number) and .
Determining the exact value of seems rather difficult. For example, let be the limit superior of for which the extremal structures are obtained by adding some edges to a maximal -free graph. The problem of determining for cliques was a well-known question of Erd\H os that was solved only decades later by Lov\'asz and Simonovits. Here we prove that for every {color-critical} . Our approach also allows us to determine for a number of graphs, including odd cycles, cliques with one edge removed, and complete bipartite graphs plus an edge