Hadwiger's conjecture for K6-free graphs

Neil Robertson, Paul Seymour, Robin Thomas

Research output: Contribution to journalArticlepeer-review

190 Scopus citations


In 1943, Hadwiger made the conjecture that every loopless graph not contractible to the complete graph on t+1 vertices is t-colourable. When t≤3 this is easy, and when t=4, Wagner's theorem of 1937 shows the conjecture to be equivalent to the four-colour conjecture (the 4CC). However, when t≥5 it has remained open. Here we show that when t=5 it is also equivalent to the 4CC. More precisely, we show (without assuming the 4CC) that every minimal counterexample to Hadwiger's conjecture when t=5 is "apex", that is, it consists of a planar graph with one additional vertex. Consequently, the 4CC implies Hadwiger's conjecture when t=5, because it implies that apex graphs are 5-colourable.

Original languageEnglish (US)
Pages (from-to)279-361
Number of pages83
Issue number3
StatePublished - Sep 1993
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Computational Mathematics
  • Discrete Mathematics and Combinatorics


  • AMS subject classification code (1991): 05C15, 05C75


Dive into the research topics of 'Hadwiger's conjecture for K6-free graphs'. Together they form a unique fingerprint.

Cite this