Question #114849
Prove or disprove
a. Every topological space is metrizible
b. Any metric defined on X(#0) induces a topology on x
1
Expert's answer
2020-05-13T19:51:36-0400

1) Non-normal spaces cannot be metrizable. It is imply from statement that any metric space is normal (see [1]). Example of non-metrizable space: the topological vector space {f ⁣:RR}\{f\colon\mathbb R\to\mathbb R\} with the topology of pointwise convergence.


Particularly,  If a space is not Hausdorff it's not a metric space. Example of non Hausdorff space is infinite XX with topologyτ={UX ⁣:U= or XU is countable}\tau = \{ U \subset X\colon U = \emptyset \text{ or } X\setminus U \text{ is countable}\}


2) For any metric space (X,d)(X,d) the collection of subsets B(x,r)={yX ⁣:d(x,y)<r}B(x,r)=\{y\in X\colon d(x,y)<r\} form a basis for a topology on XX. It is obviously due to fact that an intersection of balls have another ball (see [2], first Lemma).


[1] https://math.stackexchange.com/questions/2872410/metric-space-is-normal-proof


[2] http://sites.millersville.edu/bikenaga/topology/notes/metric-spaces/metric-spaces.html


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