I'm having some problems with this problem concerning VC dimensions ( http://en.wikipedia.org/wiki/VC_dimension ), I hope for some helping input.
Given a set $L$ of $n$ lines in the plane, define a hypergraph $H=(L,S)$ such that its vertices are the lines and a subset $l$ of $L$ belongs to $S$ iff there exist points $p$ and $q$ such that all lines in $l$ lie between $p$ and $q$, what is the VC dimension of $H$?
Now, I've managed to show the VC dimension is at least 5, I basically found a group of 5 lines with 4 of them being parrlell to each other with differing lengths and the fifth being perpendicular to the four. but I can't think of a solid proof why it shouldn't be more than 5 (it might be more, its a bit confusing to me).
Thanks a lot.