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GridironCPJ
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Any ideas? I really can't think of any myself, as I'm quite the amatuer at topology.
Bacle2 said:lavinia:
If you're thinking 1-pt-compactification, then your resulting space would not be Hausdorff, since Q is not locally-compact (e.g., the sequence 1, 1.4, 1.414,... has no convergent subsequence).
Citan Uzuki said:It's possible to show a homeomorphism exists from [itex]\mathbb{Q}[/itex] to [itex]\mathbb{Q}[/itex] that is not monotone, but difficult to describe it explicitly (although one could create an explicit formula in principle if pressed). The key is in the following fact: any two nonempty countable densely ordered sets without endpoints are order-isomorphic. So you could let [itex]A = \{x\in \mathbb{Q} : x<\sqrt{2}\}[/itex], [itex]B = \{x \in \mathbb{Q} : x>\sqrt{2}\}[/itex], and then let [itex]f:A \rightarrow B[/itex] be an order-preserving bijection from A to B. Note that since the metric topology on the rationals agrees with the order topology, f is in fact a homeomorphism from A to B. So then let [itex]G:\mathbb{Q} \rightarrow \mathbb{Q}[/itex] be given by [itex]g(x) = f(x)[/itex] if [itex]x\in A[/itex] and [itex]g(x) = f^{-1}(x)[/itex] if [itex]x\in B[/itex]. Then the restriction of g to either A or B is continuous, and since A and B are both open in [itex]\mathbb{Q}[/itex], g is continuous. And we also have that [itex]g^{-1} = g[/itex], so g is in fact a homeomorphism, which is neither order-preserving nor order-reversing.
lavinia said:OK. I was just thinking of the image of the rationals in the circle under inverse stereographic projection then adding the point at infinity and taking the subset topology. That doesn't work?
A homeomorphism is a function that preserves topological properties between two spaces. In simpler terms, it is a continuous function that has a continuous inverse.
The domain and codomain of the given homeomorphism f: Q -> Q are both the set of rational numbers (Q).
Yes, one example is the function f(x) = 1/x, which maps the interval (0,1) to (1,∞). This function is not order-preserving or order-reversing because it does not preserve the order of the elements in the interval (0,1).
A homeomorphism is order-preserving if it preserves the order of elements in the domain and codomain. This means that for any two elements a and b in the domain, if a < b, then f(a) < f(b). A homeomorphism is order-reversing if it reverses the order of elements in the domain and codomain. This means that for any two elements a and b in the domain, if a < b, then f(a) > f(b).
The order of elements in the domain and codomain determines whether a homeomorphism is order-preserving or order-reversing. If the order is preserved, the homeomorphism will preserve the relative positions of elements in the two spaces. If the order is reversed, the homeomorphism will reverse the relative positions of elements in the two spaces.