2014 IMO Problems/Problem 6

Revision as of 05:40, 9 October 2014 by Timneh (talk | contribs) (Created page with "==Problem== A set of lines in the plane is in \textit{general position} if no two are parallel and no three pass through the same point. A set of lines in general position cuts t...")
(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)

Problem

A set of lines in the plane is in \textit{general position} if no two are parallel and no three pass through the same point. A set of lines in general position cuts the plane into regions, some of which have finite are; we call these its \textit{finite regions}. Prove that for all sufficiently large $n$, in any set of $n$ lines in general position it is possible to colour at least $\sqrt{n}$ of the lines blue in such a way that none of its finite regions has a completely blue boundary.

Solution

Alternate solutions are always welcome. If you have a different, elegant solution to this problem, please add it to this page.

See Also

2014 IMO (Problems) • Resources
Preceded by
Problem 5
1 2 3 4 5 6 Followed by
Problem 6
All IMO Problems and Solutions