A Basic Elementary Extension of the Duchet-Meyniel Theorem

Abstract

The Conjecture of Hadwiger implies that the Hadwiger number $h$ times the independence number $α$ of a graph is at least the number of vertices $n$ of the graph. In 1982 Duchet and Meyniel proved a weak version of the inequality, replacing the independence number $α$ by $2α-1$, that is, $$(2α-1)\cdot h \geq n.$$ In 2005 Kawarabayashi, Plummer and the second author published an improvement of the theorem, replacing $2α- 1$ by $2α- 3/2$ when $α$ is at least 3. Since then a further improvement by Kawarabayashi and Song has been obtained, replacing $2α- 1$ by $2α- 2$ when $α$ is at least 3. In this paper a basic elementary extension of the Theorem of Duchet and Meyniel is presented. This may be of help to avoid dealing with basic cases when looking for more substantial improvements. The main unsolved problem (due to Seymour) is to improve, even just slightly, the theorem of Duchet and Meyniel in the case when the independence number $α$ is equal to 2. The case $α= 2$ of Hadwiger's Conjecture was first pointed out by Mader as an interesting special case.