Find the closure , Frontier and boundary of the following set











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Find the closure , Frontier and boundary of the following set in $ mathbb{R}^2$ given by



$$ S={(0,1) cup (2,0) } cup {(x,y): |x|+|y|< 1 }$$



Answer:



The closure is given by



$Cl (A) ={(0,1) cup (2,0) } cup {(x,y): |x|+|y| leq 1 }$



But I thing the closure can be



$ {(x,y):|x|+|y| leq 1 } cup {(0,2) }$ , because $ (0,1) in {(x,y):|x|+|y| leq 1 }$.



Am I right?



Also help me with the frontier and boundary of the set.










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  • 2




    You're definitely right when it comes to the closure. As far as the bondary/frontier goes, how do you distinguish between them? I'm under the impression that they're the same thing.
    – AlkaKadri
    Nov 20 at 0:06










  • @AlkaKadri, No there is a small difference between frontier of a set and boundary set as we can see from the example- "The frontier set of a closed unit ball is the empty set while the boundary of the unit closed ball is the unit circle". That is, frontier set does not include the boundary
    – M. A. SARKAR
    Nov 20 at 0:10






  • 1




    Okay, thanks for clarifying. Note that most people use 'frontier' and 'boundary' interchangeably, but I assume you're using frontier to mean the set of boundary points not included in the set itself? i.e., $partial S setminus S$?
    – AlkaKadri
    Nov 20 at 0:13






  • 2




    It's important to look at your definitions. Write down the definition of the exterior of a set. According to en.wikipedia.org/wiki/Interior_(topology), it's just the complement of the closure (which you've already obtained), but depending on what definition you're using there may be some more steps you have to show.
    – AlkaKadri
    Nov 20 at 0:18






  • 1




    Your notation for $S$ is weird: you cannot take a union of two points. You mean ${(0,1)} cup {(2,0)} = {(0,1),(2,0)}$ probably.
    – Henno Brandsma
    Nov 20 at 16:41















up vote
0
down vote

favorite
1












Find the closure , Frontier and boundary of the following set in $ mathbb{R}^2$ given by



$$ S={(0,1) cup (2,0) } cup {(x,y): |x|+|y|< 1 }$$



Answer:



The closure is given by



$Cl (A) ={(0,1) cup (2,0) } cup {(x,y): |x|+|y| leq 1 }$



But I thing the closure can be



$ {(x,y):|x|+|y| leq 1 } cup {(0,2) }$ , because $ (0,1) in {(x,y):|x|+|y| leq 1 }$.



Am I right?



Also help me with the frontier and boundary of the set.










share|cite|improve this question


















  • 2




    You're definitely right when it comes to the closure. As far as the bondary/frontier goes, how do you distinguish between them? I'm under the impression that they're the same thing.
    – AlkaKadri
    Nov 20 at 0:06










  • @AlkaKadri, No there is a small difference between frontier of a set and boundary set as we can see from the example- "The frontier set of a closed unit ball is the empty set while the boundary of the unit closed ball is the unit circle". That is, frontier set does not include the boundary
    – M. A. SARKAR
    Nov 20 at 0:10






  • 1




    Okay, thanks for clarifying. Note that most people use 'frontier' and 'boundary' interchangeably, but I assume you're using frontier to mean the set of boundary points not included in the set itself? i.e., $partial S setminus S$?
    – AlkaKadri
    Nov 20 at 0:13






  • 2




    It's important to look at your definitions. Write down the definition of the exterior of a set. According to en.wikipedia.org/wiki/Interior_(topology), it's just the complement of the closure (which you've already obtained), but depending on what definition you're using there may be some more steps you have to show.
    – AlkaKadri
    Nov 20 at 0:18






  • 1




    Your notation for $S$ is weird: you cannot take a union of two points. You mean ${(0,1)} cup {(2,0)} = {(0,1),(2,0)}$ probably.
    – Henno Brandsma
    Nov 20 at 16:41













up vote
0
down vote

favorite
1









up vote
0
down vote

favorite
1






1





Find the closure , Frontier and boundary of the following set in $ mathbb{R}^2$ given by



$$ S={(0,1) cup (2,0) } cup {(x,y): |x|+|y|< 1 }$$



Answer:



The closure is given by



$Cl (A) ={(0,1) cup (2,0) } cup {(x,y): |x|+|y| leq 1 }$



But I thing the closure can be



$ {(x,y):|x|+|y| leq 1 } cup {(0,2) }$ , because $ (0,1) in {(x,y):|x|+|y| leq 1 }$.



Am I right?



Also help me with the frontier and boundary of the set.










share|cite|improve this question













Find the closure , Frontier and boundary of the following set in $ mathbb{R}^2$ given by



$$ S={(0,1) cup (2,0) } cup {(x,y): |x|+|y|< 1 }$$



Answer:



The closure is given by



$Cl (A) ={(0,1) cup (2,0) } cup {(x,y): |x|+|y| leq 1 }$



But I thing the closure can be



$ {(x,y):|x|+|y| leq 1 } cup {(0,2) }$ , because $ (0,1) in {(x,y):|x|+|y| leq 1 }$.



Am I right?



Also help me with the frontier and boundary of the set.







real-analysis general-topology






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share|cite|improve this question











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asked Nov 20 at 0:02









M. A. SARKAR

2,1201619




2,1201619








  • 2




    You're definitely right when it comes to the closure. As far as the bondary/frontier goes, how do you distinguish between them? I'm under the impression that they're the same thing.
    – AlkaKadri
    Nov 20 at 0:06










  • @AlkaKadri, No there is a small difference between frontier of a set and boundary set as we can see from the example- "The frontier set of a closed unit ball is the empty set while the boundary of the unit closed ball is the unit circle". That is, frontier set does not include the boundary
    – M. A. SARKAR
    Nov 20 at 0:10






  • 1




    Okay, thanks for clarifying. Note that most people use 'frontier' and 'boundary' interchangeably, but I assume you're using frontier to mean the set of boundary points not included in the set itself? i.e., $partial S setminus S$?
    – AlkaKadri
    Nov 20 at 0:13






  • 2




    It's important to look at your definitions. Write down the definition of the exterior of a set. According to en.wikipedia.org/wiki/Interior_(topology), it's just the complement of the closure (which you've already obtained), but depending on what definition you're using there may be some more steps you have to show.
    – AlkaKadri
    Nov 20 at 0:18






  • 1




    Your notation for $S$ is weird: you cannot take a union of two points. You mean ${(0,1)} cup {(2,0)} = {(0,1),(2,0)}$ probably.
    – Henno Brandsma
    Nov 20 at 16:41














  • 2




    You're definitely right when it comes to the closure. As far as the bondary/frontier goes, how do you distinguish between them? I'm under the impression that they're the same thing.
    – AlkaKadri
    Nov 20 at 0:06










  • @AlkaKadri, No there is a small difference between frontier of a set and boundary set as we can see from the example- "The frontier set of a closed unit ball is the empty set while the boundary of the unit closed ball is the unit circle". That is, frontier set does not include the boundary
    – M. A. SARKAR
    Nov 20 at 0:10






  • 1




    Okay, thanks for clarifying. Note that most people use 'frontier' and 'boundary' interchangeably, but I assume you're using frontier to mean the set of boundary points not included in the set itself? i.e., $partial S setminus S$?
    – AlkaKadri
    Nov 20 at 0:13






  • 2




    It's important to look at your definitions. Write down the definition of the exterior of a set. According to en.wikipedia.org/wiki/Interior_(topology), it's just the complement of the closure (which you've already obtained), but depending on what definition you're using there may be some more steps you have to show.
    – AlkaKadri
    Nov 20 at 0:18






  • 1




    Your notation for $S$ is weird: you cannot take a union of two points. You mean ${(0,1)} cup {(2,0)} = {(0,1),(2,0)}$ probably.
    – Henno Brandsma
    Nov 20 at 16:41








2




2




You're definitely right when it comes to the closure. As far as the bondary/frontier goes, how do you distinguish between them? I'm under the impression that they're the same thing.
– AlkaKadri
Nov 20 at 0:06




You're definitely right when it comes to the closure. As far as the bondary/frontier goes, how do you distinguish between them? I'm under the impression that they're the same thing.
– AlkaKadri
Nov 20 at 0:06












@AlkaKadri, No there is a small difference between frontier of a set and boundary set as we can see from the example- "The frontier set of a closed unit ball is the empty set while the boundary of the unit closed ball is the unit circle". That is, frontier set does not include the boundary
– M. A. SARKAR
Nov 20 at 0:10




@AlkaKadri, No there is a small difference between frontier of a set and boundary set as we can see from the example- "The frontier set of a closed unit ball is the empty set while the boundary of the unit closed ball is the unit circle". That is, frontier set does not include the boundary
– M. A. SARKAR
Nov 20 at 0:10




1




1




Okay, thanks for clarifying. Note that most people use 'frontier' and 'boundary' interchangeably, but I assume you're using frontier to mean the set of boundary points not included in the set itself? i.e., $partial S setminus S$?
– AlkaKadri
Nov 20 at 0:13




Okay, thanks for clarifying. Note that most people use 'frontier' and 'boundary' interchangeably, but I assume you're using frontier to mean the set of boundary points not included in the set itself? i.e., $partial S setminus S$?
– AlkaKadri
Nov 20 at 0:13




2




2




It's important to look at your definitions. Write down the definition of the exterior of a set. According to en.wikipedia.org/wiki/Interior_(topology), it's just the complement of the closure (which you've already obtained), but depending on what definition you're using there may be some more steps you have to show.
– AlkaKadri
Nov 20 at 0:18




It's important to look at your definitions. Write down the definition of the exterior of a set. According to en.wikipedia.org/wiki/Interior_(topology), it's just the complement of the closure (which you've already obtained), but depending on what definition you're using there may be some more steps you have to show.
– AlkaKadri
Nov 20 at 0:18




1




1




Your notation for $S$ is weird: you cannot take a union of two points. You mean ${(0,1)} cup {(2,0)} = {(0,1),(2,0)}$ probably.
– Henno Brandsma
Nov 20 at 16:41




Your notation for $S$ is weird: you cannot take a union of two points. You mean ${(0,1)} cup {(2,0)} = {(0,1),(2,0)}$ probably.
– Henno Brandsma
Nov 20 at 16:41










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The frontier of $S$ is (probably, your example fits it, and it is a usage that occurs) $operatorname{Cl}{S}setminus S$ (all points in the closure that were not in the original set) so in this case $$operatorname{Fr}(S) = {(x,y): |x| + |y| =1}setminus {(0,1)}$$



clearly:
$$operatorname{Int}(S) = {(x,y): |x| + |y| < 1}$$



So $$operatorname{Bd}(S) =operatorname{Cl}(S) setminus operatorname{Int}(S) = {(2,0)} cup {(x,y): |x| + |y| = 1 }$$



So all these sets can be computed/determined one we know the closure and the interior.






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    The frontier of $S$ is (probably, your example fits it, and it is a usage that occurs) $operatorname{Cl}{S}setminus S$ (all points in the closure that were not in the original set) so in this case $$operatorname{Fr}(S) = {(x,y): |x| + |y| =1}setminus {(0,1)}$$



    clearly:
    $$operatorname{Int}(S) = {(x,y): |x| + |y| < 1}$$



    So $$operatorname{Bd}(S) =operatorname{Cl}(S) setminus operatorname{Int}(S) = {(2,0)} cup {(x,y): |x| + |y| = 1 }$$



    So all these sets can be computed/determined one we know the closure and the interior.






    share|cite|improve this answer

























      up vote
      2
      down vote













      The frontier of $S$ is (probably, your example fits it, and it is a usage that occurs) $operatorname{Cl}{S}setminus S$ (all points in the closure that were not in the original set) so in this case $$operatorname{Fr}(S) = {(x,y): |x| + |y| =1}setminus {(0,1)}$$



      clearly:
      $$operatorname{Int}(S) = {(x,y): |x| + |y| < 1}$$



      So $$operatorname{Bd}(S) =operatorname{Cl}(S) setminus operatorname{Int}(S) = {(2,0)} cup {(x,y): |x| + |y| = 1 }$$



      So all these sets can be computed/determined one we know the closure and the interior.






      share|cite|improve this answer























        up vote
        2
        down vote










        up vote
        2
        down vote









        The frontier of $S$ is (probably, your example fits it, and it is a usage that occurs) $operatorname{Cl}{S}setminus S$ (all points in the closure that were not in the original set) so in this case $$operatorname{Fr}(S) = {(x,y): |x| + |y| =1}setminus {(0,1)}$$



        clearly:
        $$operatorname{Int}(S) = {(x,y): |x| + |y| < 1}$$



        So $$operatorname{Bd}(S) =operatorname{Cl}(S) setminus operatorname{Int}(S) = {(2,0)} cup {(x,y): |x| + |y| = 1 }$$



        So all these sets can be computed/determined one we know the closure and the interior.






        share|cite|improve this answer












        The frontier of $S$ is (probably, your example fits it, and it is a usage that occurs) $operatorname{Cl}{S}setminus S$ (all points in the closure that were not in the original set) so in this case $$operatorname{Fr}(S) = {(x,y): |x| + |y| =1}setminus {(0,1)}$$



        clearly:
        $$operatorname{Int}(S) = {(x,y): |x| + |y| < 1}$$



        So $$operatorname{Bd}(S) =operatorname{Cl}(S) setminus operatorname{Int}(S) = {(2,0)} cup {(x,y): |x| + |y| = 1 }$$



        So all these sets can be computed/determined one we know the closure and the interior.







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered Nov 20 at 17:30









        Henno Brandsma

        103k345112




        103k345112






























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