“Homeomorphic'' is the standard equivalence relation in topology. To a topologist, spaces which are homeomorphic to each other aren't merely similar to each other, they are the same space. We study a class of functions which are homeomorphic at “most'' of the points of their domains and codomains, but which may fail to satisfy some of the properties required to be a homeomorphism at a “small'' portion of the points of these spaces. Such functions we call “almost homeomorphisms.'' One of the nice properties of almost homeomorphisms is the preservation of almost open sets, i.e. sets which are “close'' to being open, except for a “small'' set of points where the set is “defective.'' We also find a surprising result that all non-empty, perfect, Polish spaces are almost homeomorphic to each other.A standard technique in algebraic topology is to pass between a continuous map between topological spaces and the corresponding homomorphism of fundamental groups using the π1 functor. It is a non-trivial question to ask when a specific homomorphism is induced by a continuous map; that is, what is the image of the π1 functor on homomorphisms?We will call homomorphisms in the image of the π1 functor “tangible homomorphisms'' and call homomorphisms that are not induced by continuous functions “intangible homomorphisms.'' For example, Conner and Spencer used ultrafilters to prove there is a map from HEG to Z2 not induced by any continuous function f : HE→ Y , where Y is some topological space with π1(Y ) = Z2. However, in standard situations, such as when the domain is a simplicial complex, only tangible homomorphisms appear..Our job is to describe conditions when intangible homomorphisms exist and how easily these maps can be constructed. We use methods from Shelah and Pawlikowski to prove that Conner and Spencer could not have constructed these homomorphisms with a weak version of the Axiom of Choice. This leads us to define and examine a class of pathological objects that cannot be constructed without a strong version of the Axiom of Choice, which we call the class of inscrutable objects. Objects that do not need a strong version of the Axiom of Choice are scrutable. We show that the scrutable homomorphisms from the fundamental group of a Peano continuum are exactly the homomorphisms induced by a continuous function.



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Physical and Mathematical Sciences; Mathematics



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scrutability, inscrutability, tangible, intangible, almost homeomorphism, Polish, almost open, meager, nowhere dense, dense, Cantor, Godel, Shelah