To: mathgeeks@plover.com Subject: Difficult geometry problem Organization: Plover Systems Date: Sun, 05 Jun 2005 14:57:27 -0400 From: Mark Jason Dominus Let 0 < r < 1 be given. For each lattice point except (0,0), plant a circular tree of radius r with center at the lattice point. Stand at (0,0). How many trees are visible, as a function or r? This was unsolved as of 1985, which was the last I heard from it.