Importance: High ✭✭✭
Author(s): Thomas, Robin
Keywords: connectivity
Recomm. for undergrads: no
Prize: none
Posted by: mdevos
on: March 10th, 2007
Problem   Is it true for all $ n \ge 0 $, that every sufficiently large $ n $-connected graph without a $ K_n $ minor has a set of $ n-5 $ vertices whose deletion results in a planar graph?

A famous conjecture of Jorgensen asserts that every 6-connected graph without a $ K_6 $-minor is apex (planar plus one vertex). If true, Jorgensen's conjecture does not generalize (naively) to higher connectivities, since for sufficiently large $ n $, there do exist $ n $-connected graphs which are not close to planar in the sense we are considering (many more than $ n-5 $ vertices must be deleted to leave a planar graph). This conjecture of Thomas asserts that all such graphs are small in size.

For $ n \le 6 $ this conjecture is true. For $ n \le 4 $ this conjecture is trivial, since any graph without a $ K_4 $-minor is planar. The $ n=5 $ case follows from a theorem of Wagner which gives a construction for all graphs without $ K_5 $-minors (and from which it follows that every 4-connected graph with no $ K_5 $ minor is planar). The $ n=6 $ case was recently resolved by DeVos, Hegde, Kawarabayashi, Norine, Thomas, and Wollan. The difficulties associated with finding $ K_n $ minors in graphs make this conjecture appear daunting, but if true, it would yield powerful insight into the structure of graphs.


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