Locally Convex tvs closure of ${0}$












0












$begingroup$


Let $E$ be a topological vector space locally convex, defined by the family of seminorms $mathcal{F}=(p_j)_{jin J}$.



I can't prove that $underset{jin J}bigcap Ker(p_j)=overline{{0}}$






functional-analysis topological-vector-spaces locally-convex-spaces







share|cite|improve this question









$endgroup$












  • $begingroup$
    This condition is equivalent to your topology being Hausdorff. If you didn't start with enough seminorms, then your TVS won't have this property. However, if you require your TVS to be Hausdorff, distinct points can be separated by seminorms. What would that tell you about the intersection of the kernels of the seminorms?
    $endgroup$
    – Ashwin Trisal
    Dec 16 '18 at 8:13






  • 1




    $begingroup$
    @AshwinTrisal No, this is not true, this identity always holds. If $X$ is $T_1$ we get that the right hand closure is just ${0}$ and we do get the separation property.
    $endgroup$
    – Henno Brandsma
    Dec 16 '18 at 9:29
















0












$begingroup$


Let $E$ be a topological vector space locally convex, defined by the family of seminorms $mathcal{F}=(p_j)_{jin J}$.



I can't prove that $underset{jin J}bigcap Ker(p_j)=overline{{0}}$






functional-analysis topological-vector-spaces locally-convex-spaces







share|cite|improve this question









$endgroup$












  • $begingroup$
    This condition is equivalent to your topology being Hausdorff. If you didn't start with enough seminorms, then your TVS won't have this property. However, if you require your TVS to be Hausdorff, distinct points can be separated by seminorms. What would that tell you about the intersection of the kernels of the seminorms?
    $endgroup$
    – Ashwin Trisal
    Dec 16 '18 at 8:13






  • 1




    $begingroup$
    @AshwinTrisal No, this is not true, this identity always holds. If $X$ is $T_1$ we get that the right hand closure is just ${0}$ and we do get the separation property.
    $endgroup$
    – Henno Brandsma
    Dec 16 '18 at 9:29














0












0








0


1



$begingroup$


Let $E$ be a topological vector space locally convex, defined by the family of seminorms $mathcal{F}=(p_j)_{jin J}$.



I can't prove that $underset{jin J}bigcap Ker(p_j)=overline{{0}}$






functional-analysis topological-vector-spaces locally-convex-spaces







share|cite|improve this question









$endgroup$




Let $E$ be a topological vector space locally convex, defined by the family of seminorms $mathcal{F}=(p_j)_{jin J}$.



I can't prove that $underset{jin J}bigcap Ker(p_j)=overline{{0}}$






functional-analysis topological-vector-spaces locally-convex-spaces




functional-analysis topological-vector-spaces locally-convex-spaces






share|cite|improve this question













share|cite|improve this question











share|cite|improve this question




share|cite|improve this question










asked Dec 16 '18 at 8:10









Anas BOUALIIAnas BOUALII

1397




1397












  • $begingroup$
    This condition is equivalent to your topology being Hausdorff. If you didn't start with enough seminorms, then your TVS won't have this property. However, if you require your TVS to be Hausdorff, distinct points can be separated by seminorms. What would that tell you about the intersection of the kernels of the seminorms?
    $endgroup$
    – Ashwin Trisal
    Dec 16 '18 at 8:13






  • 1




    $begingroup$
    @AshwinTrisal No, this is not true, this identity always holds. If $X$ is $T_1$ we get that the right hand closure is just ${0}$ and we do get the separation property.
    $endgroup$
    – Henno Brandsma
    Dec 16 '18 at 9:29


















  • $begingroup$
    This condition is equivalent to your topology being Hausdorff. If you didn't start with enough seminorms, then your TVS won't have this property. However, if you require your TVS to be Hausdorff, distinct points can be separated by seminorms. What would that tell you about the intersection of the kernels of the seminorms?
    $endgroup$
    – Ashwin Trisal
    Dec 16 '18 at 8:13






  • 1




    $begingroup$
    @AshwinTrisal No, this is not true, this identity always holds. If $X$ is $T_1$ we get that the right hand closure is just ${0}$ and we do get the separation property.
    $endgroup$
    – Henno Brandsma
    Dec 16 '18 at 9:29
















$begingroup$
This condition is equivalent to your topology being Hausdorff. If you didn't start with enough seminorms, then your TVS won't have this property. However, if you require your TVS to be Hausdorff, distinct points can be separated by seminorms. What would that tell you about the intersection of the kernels of the seminorms?
$endgroup$
– Ashwin Trisal
Dec 16 '18 at 8:13




$begingroup$
This condition is equivalent to your topology being Hausdorff. If you didn't start with enough seminorms, then your TVS won't have this property. However, if you require your TVS to be Hausdorff, distinct points can be separated by seminorms. What would that tell you about the intersection of the kernels of the seminorms?
$endgroup$
– Ashwin Trisal
Dec 16 '18 at 8:13




1




1




$begingroup$
@AshwinTrisal No, this is not true, this identity always holds. If $X$ is $T_1$ we get that the right hand closure is just ${0}$ and we do get the separation property.
$endgroup$
– Henno Brandsma
Dec 16 '18 at 9:29




$begingroup$
@AshwinTrisal No, this is not true, this identity always holds. If $X$ is $T_1$ we get that the right hand closure is just ${0}$ and we do get the separation property.
$endgroup$
– Henno Brandsma
Dec 16 '18 at 9:29










1 Answer
1






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oldest

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2












$begingroup$

Suppose $x in overline{{0}}$.



We know that all $p_j$ are by definition continuous in the topology generated by the semininorms, so for any $j in J$: $$p_j(x) in p_j[overline{{0}}] subseteq overline{p_j[{0}]} = overline{{0}} = {0}$$ as the last closure is taken in the reals (or complex numbers) where singletons are closed, and so $x in operatorname{Ker}(p_j)$ for all $j$, also using that $p_j(0)=0$ of course.



This shows $$overline{{0}} subseteq bigcap_{j in J} operatorname{Ker}(p_j)$$



To see the other inclusion, let $x$ be in $bigcap_{j in J} operatorname{Ker}(p_j)$ and let $O$ be a basic neighbourhood of $x$. This means that there are finitely many $p_{j_1},ldots,p_{j_n}$ and an $epsilon >0$ such that $O = bigcap_{i=1}^n B_{j_i}(x,varepsilon)$, where $B_{j_i}(x,varepsilon)= {y in X: p_{j_i}(x - y) < varepsilon}$ is the open ball around $x$ induced by the seminorm $p_{j_i}$.



The fact that $x in operatorname{Ker}(p_{j_i})$ implies that $0 in B_{j_i}(x,varepsilon)$, and so $0 in O$. As $O$ is an arbitary base element containing $x$ we have shown that $x in overline{{0}}$ as required.






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    1 Answer
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    active

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    2












    $begingroup$

    Suppose $x in overline{{0}}$.



    We know that all $p_j$ are by definition continuous in the topology generated by the semininorms, so for any $j in J$: $$p_j(x) in p_j[overline{{0}}] subseteq overline{p_j[{0}]} = overline{{0}} = {0}$$ as the last closure is taken in the reals (or complex numbers) where singletons are closed, and so $x in operatorname{Ker}(p_j)$ for all $j$, also using that $p_j(0)=0$ of course.



    This shows $$overline{{0}} subseteq bigcap_{j in J} operatorname{Ker}(p_j)$$



    To see the other inclusion, let $x$ be in $bigcap_{j in J} operatorname{Ker}(p_j)$ and let $O$ be a basic neighbourhood of $x$. This means that there are finitely many $p_{j_1},ldots,p_{j_n}$ and an $epsilon >0$ such that $O = bigcap_{i=1}^n B_{j_i}(x,varepsilon)$, where $B_{j_i}(x,varepsilon)= {y in X: p_{j_i}(x - y) < varepsilon}$ is the open ball around $x$ induced by the seminorm $p_{j_i}$.



    The fact that $x in operatorname{Ker}(p_{j_i})$ implies that $0 in B_{j_i}(x,varepsilon)$, and so $0 in O$. As $O$ is an arbitary base element containing $x$ we have shown that $x in overline{{0}}$ as required.






    share|cite|improve this answer









    $endgroup$


















      2












      $begingroup$

      Suppose $x in overline{{0}}$.



      We know that all $p_j$ are by definition continuous in the topology generated by the semininorms, so for any $j in J$: $$p_j(x) in p_j[overline{{0}}] subseteq overline{p_j[{0}]} = overline{{0}} = {0}$$ as the last closure is taken in the reals (or complex numbers) where singletons are closed, and so $x in operatorname{Ker}(p_j)$ for all $j$, also using that $p_j(0)=0$ of course.



      This shows $$overline{{0}} subseteq bigcap_{j in J} operatorname{Ker}(p_j)$$



      To see the other inclusion, let $x$ be in $bigcap_{j in J} operatorname{Ker}(p_j)$ and let $O$ be a basic neighbourhood of $x$. This means that there are finitely many $p_{j_1},ldots,p_{j_n}$ and an $epsilon >0$ such that $O = bigcap_{i=1}^n B_{j_i}(x,varepsilon)$, where $B_{j_i}(x,varepsilon)= {y in X: p_{j_i}(x - y) < varepsilon}$ is the open ball around $x$ induced by the seminorm $p_{j_i}$.



      The fact that $x in operatorname{Ker}(p_{j_i})$ implies that $0 in B_{j_i}(x,varepsilon)$, and so $0 in O$. As $O$ is an arbitary base element containing $x$ we have shown that $x in overline{{0}}$ as required.






      share|cite|improve this answer









      $endgroup$
















        2












        2








        2





        $begingroup$

        Suppose $x in overline{{0}}$.



        We know that all $p_j$ are by definition continuous in the topology generated by the semininorms, so for any $j in J$: $$p_j(x) in p_j[overline{{0}}] subseteq overline{p_j[{0}]} = overline{{0}} = {0}$$ as the last closure is taken in the reals (or complex numbers) where singletons are closed, and so $x in operatorname{Ker}(p_j)$ for all $j$, also using that $p_j(0)=0$ of course.



        This shows $$overline{{0}} subseteq bigcap_{j in J} operatorname{Ker}(p_j)$$



        To see the other inclusion, let $x$ be in $bigcap_{j in J} operatorname{Ker}(p_j)$ and let $O$ be a basic neighbourhood of $x$. This means that there are finitely many $p_{j_1},ldots,p_{j_n}$ and an $epsilon >0$ such that $O = bigcap_{i=1}^n B_{j_i}(x,varepsilon)$, where $B_{j_i}(x,varepsilon)= {y in X: p_{j_i}(x - y) < varepsilon}$ is the open ball around $x$ induced by the seminorm $p_{j_i}$.



        The fact that $x in operatorname{Ker}(p_{j_i})$ implies that $0 in B_{j_i}(x,varepsilon)$, and so $0 in O$. As $O$ is an arbitary base element containing $x$ we have shown that $x in overline{{0}}$ as required.






        share|cite|improve this answer









        $endgroup$



        Suppose $x in overline{{0}}$.



        We know that all $p_j$ are by definition continuous in the topology generated by the semininorms, so for any $j in J$: $$p_j(x) in p_j[overline{{0}}] subseteq overline{p_j[{0}]} = overline{{0}} = {0}$$ as the last closure is taken in the reals (or complex numbers) where singletons are closed, and so $x in operatorname{Ker}(p_j)$ for all $j$, also using that $p_j(0)=0$ of course.



        This shows $$overline{{0}} subseteq bigcap_{j in J} operatorname{Ker}(p_j)$$



        To see the other inclusion, let $x$ be in $bigcap_{j in J} operatorname{Ker}(p_j)$ and let $O$ be a basic neighbourhood of $x$. This means that there are finitely many $p_{j_1},ldots,p_{j_n}$ and an $epsilon >0$ such that $O = bigcap_{i=1}^n B_{j_i}(x,varepsilon)$, where $B_{j_i}(x,varepsilon)= {y in X: p_{j_i}(x - y) < varepsilon}$ is the open ball around $x$ induced by the seminorm $p_{j_i}$.



        The fact that $x in operatorname{Ker}(p_{j_i})$ implies that $0 in B_{j_i}(x,varepsilon)$, and so $0 in O$. As $O$ is an arbitary base element containing $x$ we have shown that $x in overline{{0}}$ as required.







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered Dec 16 '18 at 9:28









        Henno BrandsmaHenno Brandsma

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