We can now say precisely what we mean by a continuous function and by a homeomorphism. Let X and Y be topological spacest. A functionf:X ---+ Y is continuous if for each point x of X and each neighbourhood N off(x) in Y the setf -1(N) is a neighbourhood of x in X. A function h:X ---+ Y is called a homeo-morphism if it is one-one, onto, continuous, and has a continuous inverse. When such a function exists, X and Yare called homeomorphic (or topologically equivalent) spaces.