Exercise 1

Let d be the discrete metric on 𝕂 and X : = ( 𝕂 , d ) .

(a)
Give explicit descriptions of 𝔹 X ( a , r ) and 𝔹 ¯ X ( a , r ) for a X and r > 0 .
(b)
Describe the cluster points of an arbitrary sequence in X .
(c)
For a X , decribe all sequences ( x n ) in X such that x n a .

Answers

Let a X and r > 0 , then we have three cases: if 0 < r < 1 , then 𝔹 X ( a , r ) = 𝔹 ¯ X ( a , r ) = { a } because d ( a , x ) < r only if d ( a , x ) = 0 , that is, if x = a ; when r = 1 , then 𝔹 X ( a , r ) = { a } , since for d ( x , a ) < 1 it is necessary that x = a , and 𝔹 ¯ X ( a , r ) = X since d ( x , a ) 1 independently of x ; lastly, if r > 1 , then 𝔹 X ( a , r ) = 𝔹 ¯ X ( a , r ) = X , it should evident that d ( a , x ) < r always.

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2023-10-19 02:54
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