Consider the following transformation of the Cartesian plane: choose a lattice point and rotate the plane $90^\circ$ counterclockwise about that lattice point. Is it possible, through a sequence of such transformations, to take the triangle with vertices $(0,0)$, $(1,0)$ and $(0,1)$ to the triangle with vertices $(0,0)$, $(1,0)$ and $(1,1)$?
Problem
Source: XI OlimpĂada Matemática del Cono Sur (2000)
Tags: geometry, geometric transformation, rotation, analytic geometry, invariant, geometry unsolved