We leave the reader to investigate the other possibilities: we note that the relation of topological equivalence is clearly an equivalence relation, so that proving each of spaces (a) and (d) homeomorphic to space (c) will suffice. In topology these four are considered to be the 'same space'. The sphere with three points removed is different (not homeomorphic to the above). Why? Can you describe a subset of the complex plane homeomorphic to a sphere with three points removed? Returning to the proof of Euler's theorem, thickening the trees T and r gave a decomposition of P into two discs with a common boundary and there-fore, by sending the points of one disc into the northern hemisphere and sending the points of the other south, a way of defining a homeomorphism from the poly-hedron P to the sphere.1t is possible to produce an argument in the opposite direc-tion (we shall do so in Chapter 7) and show that if Pis topologically equivalent to the sphere then P satisfies hypotheses (a) and