Given a graph G, the Hadwiger number of G, denoted by h(G), is the
largest integer k such that G contains the complete graph Kkβ as a minor.
A hole in G is an induced cycle of length at least four. Hadwiger's
Conjecture from 1943 states that for every graph G, h(G)β₯Ο(G), where
Ο(G) denotes the chromatic number of G. In this paper we establish more
evidence for Hadwiger's conjecture by showing that if a graph G with
independence number Ξ±(G)β₯3 has no hole of length between 4 and
2Ξ±(G)β1, then h(G)β₯Ο(G). We also prove that if a graph G with
independence number Ξ±(G)β₯2 has no hole of length between 4 and
2Ξ±(G), then G contains an odd clique minor of size Ο(G), that is,
such a graph G satisfies the odd Hadwiger's conjecture
Given graphs H1β,β¦,Htβ, a graph G is (H1β,β¦,Htβ)-Ramsey-minimal if every t-coloring of the edges of G contains a
monochromatic Hiβ in color i for some iβ{1,β¦,t}, but any proper
subgraph of G does not possess this property. We define
Rminβ(H1β,β¦,Htβ) to be the family of (H1β,β¦,Htβ)-Ramsey-minimal graphs. A graph G is \dfn{Rminβ(H1β,β¦,Htβ)-saturated} if no element of Rminβ(H1β,β¦,Htβ)
is a subgraph of G, but for any edge e in G, some element of
Rminβ(H1β,β¦,Htβ) is a subgraph of G+e. We define
sat(n,Rminβ(H1β,β¦,Htβ)) to be the minimum number of edges
over all Rminβ(H1β,β¦,Htβ)-saturated graphs on n
vertices. In 1987, Hanson and Toft conjectured that sat(n,Rminβ(Kk1ββ,β¦,Kktββ))=(rβ2)(nβr+2)+(2rβ2β) for nβ₯r, where r=r(Kk1ββ,β¦,Kktββ) is the classical
Ramsey number for complete graphs. The first non-trivial case of Hanson and
Toft's conjecture for sufficiently large n was setteled in 2011, and is so
far the only settled case. Motivated by Hanson and Toft's conjecture, we study
the minimum number of edges over all Rminβ(K3β,Tkβ)-saturated graphs on n vertices, where Tkβ is the
family of all trees on k vertices. We show that for nβ₯18, sat(n,Rminβ(K3β,T4β))=β5n/2β. For kβ₯5 and nβ₯2k+(βk/2β+1)βk/2ββ2, we obtain an
asymptotic bound for sat(n,Rminβ(K3β,Tkβ)).Comment: to appear in Discrete Mathematic