Does partition of unity implies second countable?












2












$begingroup$


Reading the definition of partition of unity:




Let $Asubset Bbb R^n$ and let $mathcal{O}$ be an open cover of $A$. Then there is a collection $Phi$ of $C^infty$ functions $varphi$ defined in an open set containing $A$, with the following properties:




  1. For each $x in A$ we have $0 leq varphi(x) leq 1$.


  2. For each $x in A$ there is an open set $V$ containing $x$ such that all but finitely many $varphi in Phi$ are $0$ on $V$.


  3. For each $x in A$ we have $sum_{varphi in Phi}varphi(x)=1$ (by 2 for each $x$ their sum is finite in some open set containing $x$).


  4. For each $varphi in Phi$ there is an open set $U$ in $mathcal{O}$ such that $varphi = 0$ outside of some closed set contained in $U$.





Make me feel that implies second countable because of condition (2), but I am no quite sure if this hold.






real-analysis manifolds second-countable







share|cite|improve this question









$endgroup$

















    2












    $begingroup$


    Reading the definition of partition of unity:




    Let $Asubset Bbb R^n$ and let $mathcal{O}$ be an open cover of $A$. Then there is a collection $Phi$ of $C^infty$ functions $varphi$ defined in an open set containing $A$, with the following properties:




    1. For each $x in A$ we have $0 leq varphi(x) leq 1$.


    2. For each $x in A$ there is an open set $V$ containing $x$ such that all but finitely many $varphi in Phi$ are $0$ on $V$.


    3. For each $x in A$ we have $sum_{varphi in Phi}varphi(x)=1$ (by 2 for each $x$ their sum is finite in some open set containing $x$).


    4. For each $varphi in Phi$ there is an open set $U$ in $mathcal{O}$ such that $varphi = 0$ outside of some closed set contained in $U$.





    Make me feel that implies second countable because of condition (2), but I am no quite sure if this hold.






    real-analysis manifolds second-countable







    share|cite|improve this question









    $endgroup$















      2












      2








      2





      $begingroup$


      Reading the definition of partition of unity:




      Let $Asubset Bbb R^n$ and let $mathcal{O}$ be an open cover of $A$. Then there is a collection $Phi$ of $C^infty$ functions $varphi$ defined in an open set containing $A$, with the following properties:




      1. For each $x in A$ we have $0 leq varphi(x) leq 1$.


      2. For each $x in A$ there is an open set $V$ containing $x$ such that all but finitely many $varphi in Phi$ are $0$ on $V$.


      3. For each $x in A$ we have $sum_{varphi in Phi}varphi(x)=1$ (by 2 for each $x$ their sum is finite in some open set containing $x$).


      4. For each $varphi in Phi$ there is an open set $U$ in $mathcal{O}$ such that $varphi = 0$ outside of some closed set contained in $U$.





      Make me feel that implies second countable because of condition (2), but I am no quite sure if this hold.






      real-analysis manifolds second-countable







      share|cite|improve this question









      $endgroup$




      Reading the definition of partition of unity:




      Let $Asubset Bbb R^n$ and let $mathcal{O}$ be an open cover of $A$. Then there is a collection $Phi$ of $C^infty$ functions $varphi$ defined in an open set containing $A$, with the following properties:




      1. For each $x in A$ we have $0 leq varphi(x) leq 1$.


      2. For each $x in A$ there is an open set $V$ containing $x$ such that all but finitely many $varphi in Phi$ are $0$ on $V$.


      3. For each $x in A$ we have $sum_{varphi in Phi}varphi(x)=1$ (by 2 for each $x$ their sum is finite in some open set containing $x$).


      4. For each $varphi in Phi$ there is an open set $U$ in $mathcal{O}$ such that $varphi = 0$ outside of some closed set contained in $U$.





      Make me feel that implies second countable because of condition (2), but I am no quite sure if this hold.






      real-analysis manifolds second-countable




      real-analysis manifolds second-countable






      share|cite|improve this question













      share|cite|improve this question











      share|cite|improve this question




      share|cite|improve this question










      asked Apr 13 '18 at 8:46







      user97512





























          2 Answers
          2






          active

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          1












          $begingroup$

          There is a property called paracompactness stating that every open cover has an open locally finite refinement. So to a given open cover $mathcal{U}$ there is a refining open cover $mathcal{V}$ such that every point $x$ has a neighborhood $W$ that intersects only finitely many members of $mathcal{V}$.



          The existence of partition of unity is equivalent to paracompactness. And paracompactness does not imply second countability. Also, every metrizable space is paracompact. Note that second countability is a global property, but paracompactness and metrizability is not. Any topological sum of paracompact or metrizable spaces is paracompact or metrizable, but if you have uncountably many nontrivial summands, then the sum won't be second countable.






          share|cite|improve this answer









          $endgroup$









          • 3




            $begingroup$
            As a specific example, the Sorgenfrey line (the real line under the lower limit topology, where open sets are $[a,b)$, $[a, infty)$, or $(-infty, b)$) is paracompact Hausdorff, but not second countable.
            $endgroup$
            – Fargle
            Apr 13 '18 at 9:03



















          0












          $begingroup$

          Remark: If a topological space $X$ is connected and has a $C^infty$ atlas, then:




          $$X text{ admits partitions of unity}Leftrightarrow X text{ is Hausdorff and second-countable}$$




          The proof of $(Leftarrow)$ can be found in any book about smooth manifolds (for example, in Lee's Introduction to Smooth Manifolds, theorem 2.23).



          Here are an outline for $(Rightarrow)$:



          (a) Hausdorff: Let $p,qin X$ be distinct. Take a partition of unity ${varphi_U,varphi_V}$ subordinate to the open cover ${U:=M-{p},V:=M-{q}}$. In that case, $varphi_U(p)=varphi_V(q)=0$ and $varphi_U(q)=varphi_V(p)=1$. The idea is to take small neighbourhood $W_p$ such that $varphi_U(x)$ is close to $0$ and $varphi_V(x)$ is close to $1$ for $xin W_p$. Similarly, take a small $W_q$ at which $varphi_U(y)$ is close to $1$ and $varphi_V(y)$ is close to $0$ for $yin W_q$. Then $W_p,W_q$ separate $p,q$.



          (b) Second-countable: Discussed here.






          share|cite|improve this answer











          $endgroup$









          • 1




            $begingroup$
            Your claim is not true, unless you also assume $X$ has at most countably many connected components. For example, a disjoint union of uncountably many copies of $mathbb R$ admits partitions of unity, but is not second-countable.
            $endgroup$
            – Jack Lee
            Jan 20 at 16:30










          • $begingroup$
            Thanks for that, @JackLee. I was implicitly assuming $X$ to be connected, but forgot to mention it. I'll correct it.
            $endgroup$
            – rmdmc89
            Jan 20 at 23:32













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          2 Answers
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          active

          oldest

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          2 Answers
          2






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          1












          $begingroup$

          There is a property called paracompactness stating that every open cover has an open locally finite refinement. So to a given open cover $mathcal{U}$ there is a refining open cover $mathcal{V}$ such that every point $x$ has a neighborhood $W$ that intersects only finitely many members of $mathcal{V}$.



          The existence of partition of unity is equivalent to paracompactness. And paracompactness does not imply second countability. Also, every metrizable space is paracompact. Note that second countability is a global property, but paracompactness and metrizability is not. Any topological sum of paracompact or metrizable spaces is paracompact or metrizable, but if you have uncountably many nontrivial summands, then the sum won't be second countable.






          share|cite|improve this answer









          $endgroup$









          • 3




            $begingroup$
            As a specific example, the Sorgenfrey line (the real line under the lower limit topology, where open sets are $[a,b)$, $[a, infty)$, or $(-infty, b)$) is paracompact Hausdorff, but not second countable.
            $endgroup$
            – Fargle
            Apr 13 '18 at 9:03
















          1












          $begingroup$

          There is a property called paracompactness stating that every open cover has an open locally finite refinement. So to a given open cover $mathcal{U}$ there is a refining open cover $mathcal{V}$ such that every point $x$ has a neighborhood $W$ that intersects only finitely many members of $mathcal{V}$.



          The existence of partition of unity is equivalent to paracompactness. And paracompactness does not imply second countability. Also, every metrizable space is paracompact. Note that second countability is a global property, but paracompactness and metrizability is not. Any topological sum of paracompact or metrizable spaces is paracompact or metrizable, but if you have uncountably many nontrivial summands, then the sum won't be second countable.






          share|cite|improve this answer









          $endgroup$









          • 3




            $begingroup$
            As a specific example, the Sorgenfrey line (the real line under the lower limit topology, where open sets are $[a,b)$, $[a, infty)$, or $(-infty, b)$) is paracompact Hausdorff, but not second countable.
            $endgroup$
            – Fargle
            Apr 13 '18 at 9:03














          1












          1








          1





          $begingroup$

          There is a property called paracompactness stating that every open cover has an open locally finite refinement. So to a given open cover $mathcal{U}$ there is a refining open cover $mathcal{V}$ such that every point $x$ has a neighborhood $W$ that intersects only finitely many members of $mathcal{V}$.



          The existence of partition of unity is equivalent to paracompactness. And paracompactness does not imply second countability. Also, every metrizable space is paracompact. Note that second countability is a global property, but paracompactness and metrizability is not. Any topological sum of paracompact or metrizable spaces is paracompact or metrizable, but if you have uncountably many nontrivial summands, then the sum won't be second countable.






          share|cite|improve this answer









          $endgroup$



          There is a property called paracompactness stating that every open cover has an open locally finite refinement. So to a given open cover $mathcal{U}$ there is a refining open cover $mathcal{V}$ such that every point $x$ has a neighborhood $W$ that intersects only finitely many members of $mathcal{V}$.



          The existence of partition of unity is equivalent to paracompactness. And paracompactness does not imply second countability. Also, every metrizable space is paracompact. Note that second countability is a global property, but paracompactness and metrizability is not. Any topological sum of paracompact or metrizable spaces is paracompact or metrizable, but if you have uncountably many nontrivial summands, then the sum won't be second countable.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Apr 13 '18 at 8:59









          user87690user87690

          6,5761825




          6,5761825








          • 3




            $begingroup$
            As a specific example, the Sorgenfrey line (the real line under the lower limit topology, where open sets are $[a,b)$, $[a, infty)$, or $(-infty, b)$) is paracompact Hausdorff, but not second countable.
            $endgroup$
            – Fargle
            Apr 13 '18 at 9:03














          • 3




            $begingroup$
            As a specific example, the Sorgenfrey line (the real line under the lower limit topology, where open sets are $[a,b)$, $[a, infty)$, or $(-infty, b)$) is paracompact Hausdorff, but not second countable.
            $endgroup$
            – Fargle
            Apr 13 '18 at 9:03








          3




          3




          $begingroup$
          As a specific example, the Sorgenfrey line (the real line under the lower limit topology, where open sets are $[a,b)$, $[a, infty)$, or $(-infty, b)$) is paracompact Hausdorff, but not second countable.
          $endgroup$
          – Fargle
          Apr 13 '18 at 9:03




          $begingroup$
          As a specific example, the Sorgenfrey line (the real line under the lower limit topology, where open sets are $[a,b)$, $[a, infty)$, or $(-infty, b)$) is paracompact Hausdorff, but not second countable.
          $endgroup$
          – Fargle
          Apr 13 '18 at 9:03











          0












          $begingroup$

          Remark: If a topological space $X$ is connected and has a $C^infty$ atlas, then:




          $$X text{ admits partitions of unity}Leftrightarrow X text{ is Hausdorff and second-countable}$$




          The proof of $(Leftarrow)$ can be found in any book about smooth manifolds (for example, in Lee's Introduction to Smooth Manifolds, theorem 2.23).



          Here are an outline for $(Rightarrow)$:



          (a) Hausdorff: Let $p,qin X$ be distinct. Take a partition of unity ${varphi_U,varphi_V}$ subordinate to the open cover ${U:=M-{p},V:=M-{q}}$. In that case, $varphi_U(p)=varphi_V(q)=0$ and $varphi_U(q)=varphi_V(p)=1$. The idea is to take small neighbourhood $W_p$ such that $varphi_U(x)$ is close to $0$ and $varphi_V(x)$ is close to $1$ for $xin W_p$. Similarly, take a small $W_q$ at which $varphi_U(y)$ is close to $1$ and $varphi_V(y)$ is close to $0$ for $yin W_q$. Then $W_p,W_q$ separate $p,q$.



          (b) Second-countable: Discussed here.






          share|cite|improve this answer











          $endgroup$









          • 1




            $begingroup$
            Your claim is not true, unless you also assume $X$ has at most countably many connected components. For example, a disjoint union of uncountably many copies of $mathbb R$ admits partitions of unity, but is not second-countable.
            $endgroup$
            – Jack Lee
            Jan 20 at 16:30










          • $begingroup$
            Thanks for that, @JackLee. I was implicitly assuming $X$ to be connected, but forgot to mention it. I'll correct it.
            $endgroup$
            – rmdmc89
            Jan 20 at 23:32


















          0












          $begingroup$

          Remark: If a topological space $X$ is connected and has a $C^infty$ atlas, then:




          $$X text{ admits partitions of unity}Leftrightarrow X text{ is Hausdorff and second-countable}$$




          The proof of $(Leftarrow)$ can be found in any book about smooth manifolds (for example, in Lee's Introduction to Smooth Manifolds, theorem 2.23).



          Here are an outline for $(Rightarrow)$:



          (a) Hausdorff: Let $p,qin X$ be distinct. Take a partition of unity ${varphi_U,varphi_V}$ subordinate to the open cover ${U:=M-{p},V:=M-{q}}$. In that case, $varphi_U(p)=varphi_V(q)=0$ and $varphi_U(q)=varphi_V(p)=1$. The idea is to take small neighbourhood $W_p$ such that $varphi_U(x)$ is close to $0$ and $varphi_V(x)$ is close to $1$ for $xin W_p$. Similarly, take a small $W_q$ at which $varphi_U(y)$ is close to $1$ and $varphi_V(y)$ is close to $0$ for $yin W_q$. Then $W_p,W_q$ separate $p,q$.



          (b) Second-countable: Discussed here.






          share|cite|improve this answer











          $endgroup$









          • 1




            $begingroup$
            Your claim is not true, unless you also assume $X$ has at most countably many connected components. For example, a disjoint union of uncountably many copies of $mathbb R$ admits partitions of unity, but is not second-countable.
            $endgroup$
            – Jack Lee
            Jan 20 at 16:30










          • $begingroup$
            Thanks for that, @JackLee. I was implicitly assuming $X$ to be connected, but forgot to mention it. I'll correct it.
            $endgroup$
            – rmdmc89
            Jan 20 at 23:32
















          0












          0








          0





          $begingroup$

          Remark: If a topological space $X$ is connected and has a $C^infty$ atlas, then:




          $$X text{ admits partitions of unity}Leftrightarrow X text{ is Hausdorff and second-countable}$$




          The proof of $(Leftarrow)$ can be found in any book about smooth manifolds (for example, in Lee's Introduction to Smooth Manifolds, theorem 2.23).



          Here are an outline for $(Rightarrow)$:



          (a) Hausdorff: Let $p,qin X$ be distinct. Take a partition of unity ${varphi_U,varphi_V}$ subordinate to the open cover ${U:=M-{p},V:=M-{q}}$. In that case, $varphi_U(p)=varphi_V(q)=0$ and $varphi_U(q)=varphi_V(p)=1$. The idea is to take small neighbourhood $W_p$ such that $varphi_U(x)$ is close to $0$ and $varphi_V(x)$ is close to $1$ for $xin W_p$. Similarly, take a small $W_q$ at which $varphi_U(y)$ is close to $1$ and $varphi_V(y)$ is close to $0$ for $yin W_q$. Then $W_p,W_q$ separate $p,q$.



          (b) Second-countable: Discussed here.






          share|cite|improve this answer











          $endgroup$



          Remark: If a topological space $X$ is connected and has a $C^infty$ atlas, then:




          $$X text{ admits partitions of unity}Leftrightarrow X text{ is Hausdorff and second-countable}$$




          The proof of $(Leftarrow)$ can be found in any book about smooth manifolds (for example, in Lee's Introduction to Smooth Manifolds, theorem 2.23).



          Here are an outline for $(Rightarrow)$:



          (a) Hausdorff: Let $p,qin X$ be distinct. Take a partition of unity ${varphi_U,varphi_V}$ subordinate to the open cover ${U:=M-{p},V:=M-{q}}$. In that case, $varphi_U(p)=varphi_V(q)=0$ and $varphi_U(q)=varphi_V(p)=1$. The idea is to take small neighbourhood $W_p$ such that $varphi_U(x)$ is close to $0$ and $varphi_V(x)$ is close to $1$ for $xin W_p$. Similarly, take a small $W_q$ at which $varphi_U(y)$ is close to $1$ and $varphi_V(y)$ is close to $0$ for $yin W_q$. Then $W_p,W_q$ separate $p,q$.



          (b) Second-countable: Discussed here.







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited Jan 20 at 23:33

























          answered Jan 17 at 14:52









          rmdmc89rmdmc89

          2,1971923




          2,1971923








          • 1




            $begingroup$
            Your claim is not true, unless you also assume $X$ has at most countably many connected components. For example, a disjoint union of uncountably many copies of $mathbb R$ admits partitions of unity, but is not second-countable.
            $endgroup$
            – Jack Lee
            Jan 20 at 16:30










          • $begingroup$
            Thanks for that, @JackLee. I was implicitly assuming $X$ to be connected, but forgot to mention it. I'll correct it.
            $endgroup$
            – rmdmc89
            Jan 20 at 23:32
















          • 1




            $begingroup$
            Your claim is not true, unless you also assume $X$ has at most countably many connected components. For example, a disjoint union of uncountably many copies of $mathbb R$ admits partitions of unity, but is not second-countable.
            $endgroup$
            – Jack Lee
            Jan 20 at 16:30










          • $begingroup$
            Thanks for that, @JackLee. I was implicitly assuming $X$ to be connected, but forgot to mention it. I'll correct it.
            $endgroup$
            – rmdmc89
            Jan 20 at 23:32










          1




          1




          $begingroup$
          Your claim is not true, unless you also assume $X$ has at most countably many connected components. For example, a disjoint union of uncountably many copies of $mathbb R$ admits partitions of unity, but is not second-countable.
          $endgroup$
          – Jack Lee
          Jan 20 at 16:30




          $begingroup$
          Your claim is not true, unless you also assume $X$ has at most countably many connected components. For example, a disjoint union of uncountably many copies of $mathbb R$ admits partitions of unity, but is not second-countable.
          $endgroup$
          – Jack Lee
          Jan 20 at 16:30












          $begingroup$
          Thanks for that, @JackLee. I was implicitly assuming $X$ to be connected, but forgot to mention it. I'll correct it.
          $endgroup$
          – rmdmc89
          Jan 20 at 23:32






          $begingroup$
          Thanks for that, @JackLee. I was implicitly assuming $X$ to be connected, but forgot to mention it. I'll correct it.
          $endgroup$
          – rmdmc89
          Jan 20 at 23:32




















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