Problem 3.1 Prove that in two-dimensional Bravais lattices an n-fold rotation axis with n = 5 or n > 7 does not exist. Show first that the axis can be chosen to pass through a lattice point. Then argue by "reductio ad absurdum" using the set of points into which the nearest neighbor of the fixed point is taken by the n rotations to construct a point closer to the fixed point than its nearest neighbor.