A tree is planted at each lattice point in a circular orchard which has center at the origin and radius r. If the radius of trees exceeds 1/r units, one is unable to see out of the orchard in any direction. However, if the radii of the trees are <1/sqrt(r^2 + 1), one can see out at certain angles.

Euclid's orchard | lattice point | orchard-planting problem | visible point