There exists a point $y_0 in A $ with $d(y_0,x) = d(x,A)$ if choose the correct option












0














let A be a subset of $mathbb{R}^P$ and $ x in mathbb{R}^P$ denotes $d(x,A) = inf { d(x,y) : y in A}$.There exists a point $y_0 in A $ with $d(y_0,x) = d(x,A)$ if



choose the correct option



$a)$ $A$ is any closed non emepty subset of $mathbb{R}^P$



$b)$$ A$ is any non empty subset of $mathbb{R}^P$



$c)$$ A$ is any non empty compact subset of $mathbb{R}^P$



$d)$$ A$ is any non empty bounded subset of $mathbb{R}^P$



My attempt : i thinks option a) will correct because if A is closed then $x in bar A= A,$ that is $d(y_0,x) = d(x,A)=0$



I don't know that other option pliz help me










share|cite|improve this question






















  • Why do you claim that $xinoverline A$?
    – José Carlos Santos
    Nov 30 at 23:51










  • because A is closed @ Jose carlos sir
    – jasmine
    Nov 30 at 23:56






  • 1




    That explains why you claim that $overline A=A$. But why $xinoverline A$?
    – José Carlos Santos
    Nov 30 at 23:58










  • @JoséCarlosSantos.....soory sir that is my misunderstanding
    – jasmine
    Nov 30 at 23:59
















0














let A be a subset of $mathbb{R}^P$ and $ x in mathbb{R}^P$ denotes $d(x,A) = inf { d(x,y) : y in A}$.There exists a point $y_0 in A $ with $d(y_0,x) = d(x,A)$ if



choose the correct option



$a)$ $A$ is any closed non emepty subset of $mathbb{R}^P$



$b)$$ A$ is any non empty subset of $mathbb{R}^P$



$c)$$ A$ is any non empty compact subset of $mathbb{R}^P$



$d)$$ A$ is any non empty bounded subset of $mathbb{R}^P$



My attempt : i thinks option a) will correct because if A is closed then $x in bar A= A,$ that is $d(y_0,x) = d(x,A)=0$



I don't know that other option pliz help me










share|cite|improve this question






















  • Why do you claim that $xinoverline A$?
    – José Carlos Santos
    Nov 30 at 23:51










  • because A is closed @ Jose carlos sir
    – jasmine
    Nov 30 at 23:56






  • 1




    That explains why you claim that $overline A=A$. But why $xinoverline A$?
    – José Carlos Santos
    Nov 30 at 23:58










  • @JoséCarlosSantos.....soory sir that is my misunderstanding
    – jasmine
    Nov 30 at 23:59














0












0








0







let A be a subset of $mathbb{R}^P$ and $ x in mathbb{R}^P$ denotes $d(x,A) = inf { d(x,y) : y in A}$.There exists a point $y_0 in A $ with $d(y_0,x) = d(x,A)$ if



choose the correct option



$a)$ $A$ is any closed non emepty subset of $mathbb{R}^P$



$b)$$ A$ is any non empty subset of $mathbb{R}^P$



$c)$$ A$ is any non empty compact subset of $mathbb{R}^P$



$d)$$ A$ is any non empty bounded subset of $mathbb{R}^P$



My attempt : i thinks option a) will correct because if A is closed then $x in bar A= A,$ that is $d(y_0,x) = d(x,A)=0$



I don't know that other option pliz help me










share|cite|improve this question













let A be a subset of $mathbb{R}^P$ and $ x in mathbb{R}^P$ denotes $d(x,A) = inf { d(x,y) : y in A}$.There exists a point $y_0 in A $ with $d(y_0,x) = d(x,A)$ if



choose the correct option



$a)$ $A$ is any closed non emepty subset of $mathbb{R}^P$



$b)$$ A$ is any non empty subset of $mathbb{R}^P$



$c)$$ A$ is any non empty compact subset of $mathbb{R}^P$



$d)$$ A$ is any non empty bounded subset of $mathbb{R}^P$



My attempt : i thinks option a) will correct because if A is closed then $x in bar A= A,$ that is $d(y_0,x) = d(x,A)=0$



I don't know that other option pliz help me







general-topology






share|cite|improve this question













share|cite|improve this question











share|cite|improve this question




share|cite|improve this question










asked Nov 30 at 23:46









jasmine

1,520416




1,520416












  • Why do you claim that $xinoverline A$?
    – José Carlos Santos
    Nov 30 at 23:51










  • because A is closed @ Jose carlos sir
    – jasmine
    Nov 30 at 23:56






  • 1




    That explains why you claim that $overline A=A$. But why $xinoverline A$?
    – José Carlos Santos
    Nov 30 at 23:58










  • @JoséCarlosSantos.....soory sir that is my misunderstanding
    – jasmine
    Nov 30 at 23:59


















  • Why do you claim that $xinoverline A$?
    – José Carlos Santos
    Nov 30 at 23:51










  • because A is closed @ Jose carlos sir
    – jasmine
    Nov 30 at 23:56






  • 1




    That explains why you claim that $overline A=A$. But why $xinoverline A$?
    – José Carlos Santos
    Nov 30 at 23:58










  • @JoséCarlosSantos.....soory sir that is my misunderstanding
    – jasmine
    Nov 30 at 23:59
















Why do you claim that $xinoverline A$?
– José Carlos Santos
Nov 30 at 23:51




Why do you claim that $xinoverline A$?
– José Carlos Santos
Nov 30 at 23:51












because A is closed @ Jose carlos sir
– jasmine
Nov 30 at 23:56




because A is closed @ Jose carlos sir
– jasmine
Nov 30 at 23:56




1




1




That explains why you claim that $overline A=A$. But why $xinoverline A$?
– José Carlos Santos
Nov 30 at 23:58




That explains why you claim that $overline A=A$. But why $xinoverline A$?
– José Carlos Santos
Nov 30 at 23:58












@JoséCarlosSantos.....soory sir that is my misunderstanding
– jasmine
Nov 30 at 23:59




@JoséCarlosSantos.....soory sir that is my misunderstanding
– jasmine
Nov 30 at 23:59










2 Answers
2






active

oldest

votes


















2














a) It is correct. Take $ain A$ and consider the closed ball $overline{B_{d(x,a)}(x)}$. Then $ainoverline{B_{d(x,a)}(x)}cap A$ and, since $overline{B_{d(x,a)}(x)}cap A$ is closed and bounded, it is compact. So, there is a $a'inoverline{B_{d(x,a)}(x)}cap A$ such that$$d(x,a')=min{d(x,c),|,cinoverline{B_{d(x,a)}(x)}cap A}.$$And if $cin Asetminusleft(overline{B_{d(x,a)}(x)}cap Aright)$, then $d(x,c)>d(x,a)geqslant d(x,a')$. So, $d(x,a')=d(x,A)$.



b) It is false. take $x=(0,0)$ and $A=(0,1)times{0}$.



c) It is correct, since a) holds.



d) It is false. Consider the same example as inmy answer to b).






share|cite|improve this answer





















  • carlos sir what is $y_0$ in option b) how can i contradict im getting$ d(y_0,x) = d(x,A) = 0 = d( 0, (0,1))$?
    – jasmine
    Dec 1 at 0:48






  • 1




    Option b) is false, which means that there is no $y_0$. In my example, $d(x,A)=1$, but there is no $y_0in A$ such that $d(x,y_0)=1$.
    – José Carlos Santos
    Dec 1 at 7:33










  • Strictly speaking, the argument for a) depends on the correctness of c) and vie versa.
    – random
    Dec 1 at 12:02



















2














a) and c) are true and b) and d) are false. For a) choose ${x_k} subset A$ such that $d(x,x_k)$ converges to $d(x,A)$. Then ${x_k}$ is bouunded, so it has a convergent subsequence. If the limit of the subsequence is $y$ the $ y in A$ and $d(x,y)=d(x,A)$. C) follows from a). For b) and d) Take $x=0$ and $A=(0,1)$ in $mathbb R$. Thanks to Jose carlos Santos for pointing out an error in my earlier answer.






share|cite|improve this answer























  • thanks u @Kavi sir ,,,i need detail
    – jasmine
    Nov 30 at 23:59






  • 1




    This is not correct, since option a) is also true.
    – José Carlos Santos
    Dec 1 at 0:07






  • 1




    @JoséCarlosSantos Thanks. I have edited my answer using your comment. My answer is a bit different so I though of retaining it. I hope you have no objection to this.
    – Kavi Rama Murthy
    Dec 1 at 0:42






  • 1




    Of course I have no objection.
    – José Carlos Santos
    Dec 1 at 7:31











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2 Answers
2






active

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2 Answers
2






active

oldest

votes









active

oldest

votes






active

oldest

votes









2














a) It is correct. Take $ain A$ and consider the closed ball $overline{B_{d(x,a)}(x)}$. Then $ainoverline{B_{d(x,a)}(x)}cap A$ and, since $overline{B_{d(x,a)}(x)}cap A$ is closed and bounded, it is compact. So, there is a $a'inoverline{B_{d(x,a)}(x)}cap A$ such that$$d(x,a')=min{d(x,c),|,cinoverline{B_{d(x,a)}(x)}cap A}.$$And if $cin Asetminusleft(overline{B_{d(x,a)}(x)}cap Aright)$, then $d(x,c)>d(x,a)geqslant d(x,a')$. So, $d(x,a')=d(x,A)$.



b) It is false. take $x=(0,0)$ and $A=(0,1)times{0}$.



c) It is correct, since a) holds.



d) It is false. Consider the same example as inmy answer to b).






share|cite|improve this answer





















  • carlos sir what is $y_0$ in option b) how can i contradict im getting$ d(y_0,x) = d(x,A) = 0 = d( 0, (0,1))$?
    – jasmine
    Dec 1 at 0:48






  • 1




    Option b) is false, which means that there is no $y_0$. In my example, $d(x,A)=1$, but there is no $y_0in A$ such that $d(x,y_0)=1$.
    – José Carlos Santos
    Dec 1 at 7:33










  • Strictly speaking, the argument for a) depends on the correctness of c) and vie versa.
    – random
    Dec 1 at 12:02
















2














a) It is correct. Take $ain A$ and consider the closed ball $overline{B_{d(x,a)}(x)}$. Then $ainoverline{B_{d(x,a)}(x)}cap A$ and, since $overline{B_{d(x,a)}(x)}cap A$ is closed and bounded, it is compact. So, there is a $a'inoverline{B_{d(x,a)}(x)}cap A$ such that$$d(x,a')=min{d(x,c),|,cinoverline{B_{d(x,a)}(x)}cap A}.$$And if $cin Asetminusleft(overline{B_{d(x,a)}(x)}cap Aright)$, then $d(x,c)>d(x,a)geqslant d(x,a')$. So, $d(x,a')=d(x,A)$.



b) It is false. take $x=(0,0)$ and $A=(0,1)times{0}$.



c) It is correct, since a) holds.



d) It is false. Consider the same example as inmy answer to b).






share|cite|improve this answer





















  • carlos sir what is $y_0$ in option b) how can i contradict im getting$ d(y_0,x) = d(x,A) = 0 = d( 0, (0,1))$?
    – jasmine
    Dec 1 at 0:48






  • 1




    Option b) is false, which means that there is no $y_0$. In my example, $d(x,A)=1$, but there is no $y_0in A$ such that $d(x,y_0)=1$.
    – José Carlos Santos
    Dec 1 at 7:33










  • Strictly speaking, the argument for a) depends on the correctness of c) and vie versa.
    – random
    Dec 1 at 12:02














2












2








2






a) It is correct. Take $ain A$ and consider the closed ball $overline{B_{d(x,a)}(x)}$. Then $ainoverline{B_{d(x,a)}(x)}cap A$ and, since $overline{B_{d(x,a)}(x)}cap A$ is closed and bounded, it is compact. So, there is a $a'inoverline{B_{d(x,a)}(x)}cap A$ such that$$d(x,a')=min{d(x,c),|,cinoverline{B_{d(x,a)}(x)}cap A}.$$And if $cin Asetminusleft(overline{B_{d(x,a)}(x)}cap Aright)$, then $d(x,c)>d(x,a)geqslant d(x,a')$. So, $d(x,a')=d(x,A)$.



b) It is false. take $x=(0,0)$ and $A=(0,1)times{0}$.



c) It is correct, since a) holds.



d) It is false. Consider the same example as inmy answer to b).






share|cite|improve this answer












a) It is correct. Take $ain A$ and consider the closed ball $overline{B_{d(x,a)}(x)}$. Then $ainoverline{B_{d(x,a)}(x)}cap A$ and, since $overline{B_{d(x,a)}(x)}cap A$ is closed and bounded, it is compact. So, there is a $a'inoverline{B_{d(x,a)}(x)}cap A$ such that$$d(x,a')=min{d(x,c),|,cinoverline{B_{d(x,a)}(x)}cap A}.$$And if $cin Asetminusleft(overline{B_{d(x,a)}(x)}cap Aright)$, then $d(x,c)>d(x,a)geqslant d(x,a')$. So, $d(x,a')=d(x,A)$.



b) It is false. take $x=(0,0)$ and $A=(0,1)times{0}$.



c) It is correct, since a) holds.



d) It is false. Consider the same example as inmy answer to b).







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Dec 1 at 0:15









José Carlos Santos

148k22117219




148k22117219












  • carlos sir what is $y_0$ in option b) how can i contradict im getting$ d(y_0,x) = d(x,A) = 0 = d( 0, (0,1))$?
    – jasmine
    Dec 1 at 0:48






  • 1




    Option b) is false, which means that there is no $y_0$. In my example, $d(x,A)=1$, but there is no $y_0in A$ such that $d(x,y_0)=1$.
    – José Carlos Santos
    Dec 1 at 7:33










  • Strictly speaking, the argument for a) depends on the correctness of c) and vie versa.
    – random
    Dec 1 at 12:02


















  • carlos sir what is $y_0$ in option b) how can i contradict im getting$ d(y_0,x) = d(x,A) = 0 = d( 0, (0,1))$?
    – jasmine
    Dec 1 at 0:48






  • 1




    Option b) is false, which means that there is no $y_0$. In my example, $d(x,A)=1$, but there is no $y_0in A$ such that $d(x,y_0)=1$.
    – José Carlos Santos
    Dec 1 at 7:33










  • Strictly speaking, the argument for a) depends on the correctness of c) and vie versa.
    – random
    Dec 1 at 12:02
















carlos sir what is $y_0$ in option b) how can i contradict im getting$ d(y_0,x) = d(x,A) = 0 = d( 0, (0,1))$?
– jasmine
Dec 1 at 0:48




carlos sir what is $y_0$ in option b) how can i contradict im getting$ d(y_0,x) = d(x,A) = 0 = d( 0, (0,1))$?
– jasmine
Dec 1 at 0:48




1




1




Option b) is false, which means that there is no $y_0$. In my example, $d(x,A)=1$, but there is no $y_0in A$ such that $d(x,y_0)=1$.
– José Carlos Santos
Dec 1 at 7:33




Option b) is false, which means that there is no $y_0$. In my example, $d(x,A)=1$, but there is no $y_0in A$ such that $d(x,y_0)=1$.
– José Carlos Santos
Dec 1 at 7:33












Strictly speaking, the argument for a) depends on the correctness of c) and vie versa.
– random
Dec 1 at 12:02




Strictly speaking, the argument for a) depends on the correctness of c) and vie versa.
– random
Dec 1 at 12:02











2














a) and c) are true and b) and d) are false. For a) choose ${x_k} subset A$ such that $d(x,x_k)$ converges to $d(x,A)$. Then ${x_k}$ is bouunded, so it has a convergent subsequence. If the limit of the subsequence is $y$ the $ y in A$ and $d(x,y)=d(x,A)$. C) follows from a). For b) and d) Take $x=0$ and $A=(0,1)$ in $mathbb R$. Thanks to Jose carlos Santos for pointing out an error in my earlier answer.






share|cite|improve this answer























  • thanks u @Kavi sir ,,,i need detail
    – jasmine
    Nov 30 at 23:59






  • 1




    This is not correct, since option a) is also true.
    – José Carlos Santos
    Dec 1 at 0:07






  • 1




    @JoséCarlosSantos Thanks. I have edited my answer using your comment. My answer is a bit different so I though of retaining it. I hope you have no objection to this.
    – Kavi Rama Murthy
    Dec 1 at 0:42






  • 1




    Of course I have no objection.
    – José Carlos Santos
    Dec 1 at 7:31
















2














a) and c) are true and b) and d) are false. For a) choose ${x_k} subset A$ such that $d(x,x_k)$ converges to $d(x,A)$. Then ${x_k}$ is bouunded, so it has a convergent subsequence. If the limit of the subsequence is $y$ the $ y in A$ and $d(x,y)=d(x,A)$. C) follows from a). For b) and d) Take $x=0$ and $A=(0,1)$ in $mathbb R$. Thanks to Jose carlos Santos for pointing out an error in my earlier answer.






share|cite|improve this answer























  • thanks u @Kavi sir ,,,i need detail
    – jasmine
    Nov 30 at 23:59






  • 1




    This is not correct, since option a) is also true.
    – José Carlos Santos
    Dec 1 at 0:07






  • 1




    @JoséCarlosSantos Thanks. I have edited my answer using your comment. My answer is a bit different so I though of retaining it. I hope you have no objection to this.
    – Kavi Rama Murthy
    Dec 1 at 0:42






  • 1




    Of course I have no objection.
    – José Carlos Santos
    Dec 1 at 7:31














2












2








2






a) and c) are true and b) and d) are false. For a) choose ${x_k} subset A$ such that $d(x,x_k)$ converges to $d(x,A)$. Then ${x_k}$ is bouunded, so it has a convergent subsequence. If the limit of the subsequence is $y$ the $ y in A$ and $d(x,y)=d(x,A)$. C) follows from a). For b) and d) Take $x=0$ and $A=(0,1)$ in $mathbb R$. Thanks to Jose carlos Santos for pointing out an error in my earlier answer.






share|cite|improve this answer














a) and c) are true and b) and d) are false. For a) choose ${x_k} subset A$ such that $d(x,x_k)$ converges to $d(x,A)$. Then ${x_k}$ is bouunded, so it has a convergent subsequence. If the limit of the subsequence is $y$ the $ y in A$ and $d(x,y)=d(x,A)$. C) follows from a). For b) and d) Take $x=0$ and $A=(0,1)$ in $mathbb R$. Thanks to Jose carlos Santos for pointing out an error in my earlier answer.







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited Dec 1 at 1:51

























answered Nov 30 at 23:57









Kavi Rama Murthy

49.1k31854




49.1k31854












  • thanks u @Kavi sir ,,,i need detail
    – jasmine
    Nov 30 at 23:59






  • 1




    This is not correct, since option a) is also true.
    – José Carlos Santos
    Dec 1 at 0:07






  • 1




    @JoséCarlosSantos Thanks. I have edited my answer using your comment. My answer is a bit different so I though of retaining it. I hope you have no objection to this.
    – Kavi Rama Murthy
    Dec 1 at 0:42






  • 1




    Of course I have no objection.
    – José Carlos Santos
    Dec 1 at 7:31


















  • thanks u @Kavi sir ,,,i need detail
    – jasmine
    Nov 30 at 23:59






  • 1




    This is not correct, since option a) is also true.
    – José Carlos Santos
    Dec 1 at 0:07






  • 1




    @JoséCarlosSantos Thanks. I have edited my answer using your comment. My answer is a bit different so I though of retaining it. I hope you have no objection to this.
    – Kavi Rama Murthy
    Dec 1 at 0:42






  • 1




    Of course I have no objection.
    – José Carlos Santos
    Dec 1 at 7:31
















thanks u @Kavi sir ,,,i need detail
– jasmine
Nov 30 at 23:59




thanks u @Kavi sir ,,,i need detail
– jasmine
Nov 30 at 23:59




1




1




This is not correct, since option a) is also true.
– José Carlos Santos
Dec 1 at 0:07




This is not correct, since option a) is also true.
– José Carlos Santos
Dec 1 at 0:07




1




1




@JoséCarlosSantos Thanks. I have edited my answer using your comment. My answer is a bit different so I though of retaining it. I hope you have no objection to this.
– Kavi Rama Murthy
Dec 1 at 0:42




@JoséCarlosSantos Thanks. I have edited my answer using your comment. My answer is a bit different so I though of retaining it. I hope you have no objection to this.
– Kavi Rama Murthy
Dec 1 at 0:42




1




1




Of course I have no objection.
– José Carlos Santos
Dec 1 at 7:31




Of course I have no objection.
– José Carlos Santos
Dec 1 at 7:31


















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