 A particular homeomorphism is given by stereographically projecting the topological torus into R3 from the north pole of "S"3.The torus can also be described as a quotient of the Cartesian plane under the identifications

or, equivalently, as the quotient of the unit square by pasting the opposite edges together, described as a fundamental polygon "ABA"−1"B"−1.The fundamental group of the torus is just the direct product of the fundamental group of the circle with itself