Planes, Spheres and Pseudospheres — Greg EganPlanes, Spheres and Pseudospheres by Greg Egan A sheet of paper or a piece of fabric can lie perfectly flat on a table, but it can also be rolled up into a cylinder or a cone, and twisted in certain ways, without being stretched or sheared. Figures drawn on the material when it lies flat retain their shape, in the sense that distances measured within the material itself are unchanged by the way it
