If $A, B in [m,n] subseteq mathbb{R}$ are sets of measure zero, then is $AB = {ab | a in A, b in B}$ measure...











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If $A, B in [m,n]$, where $[m,n] subseteq mathbb{R}$ are sets of measure zero, then is $AB = {ab ; | ; a in A, b in B}$ measure zero?










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closed as off-topic by Brahadeesh, supinf, Christopher, Gibbs, Eckhard Nov 23 at 21:00


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    If $A, B in [m,n]$, where $[m,n] subseteq mathbb{R}$ are sets of measure zero, then is $AB = {ab ; | ; a in A, b in B}$ measure zero?










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    closed as off-topic by Brahadeesh, supinf, Christopher, Gibbs, Eckhard Nov 23 at 21:00


    This question appears to be off-topic. The users who voted to close gave this specific reason:


    • "This question is missing context or other details: Please improve the question by providing additional context, which ideally includes your thoughts on the problem and any attempts you have made to solve it. This information helps others identify where you have difficulties and helps them write answers appropriate to your experience level." – Brahadeesh, supinf, Christopher, Gibbs

    If this question can be reworded to fit the rules in the help center, please edit the question.















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      If $A, B in [m,n]$, where $[m,n] subseteq mathbb{R}$ are sets of measure zero, then is $AB = {ab ; | ; a in A, b in B}$ measure zero?










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      If $A, B in [m,n]$, where $[m,n] subseteq mathbb{R}$ are sets of measure zero, then is $AB = {ab ; | ; a in A, b in B}$ measure zero?







      real-analysis analysis measure-theory lebesgue-measure outer-measure






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      asked Nov 23 at 8:24









      Ralph Xu

      566




      566




      closed as off-topic by Brahadeesh, supinf, Christopher, Gibbs, Eckhard Nov 23 at 21:00


      This question appears to be off-topic. The users who voted to close gave this specific reason:


      • "This question is missing context or other details: Please improve the question by providing additional context, which ideally includes your thoughts on the problem and any attempts you have made to solve it. This information helps others identify where you have difficulties and helps them write answers appropriate to your experience level." – Brahadeesh, supinf, Christopher, Gibbs

      If this question can be reworded to fit the rules in the help center, please edit the question.




      closed as off-topic by Brahadeesh, supinf, Christopher, Gibbs, Eckhard Nov 23 at 21:00


      This question appears to be off-topic. The users who voted to close gave this specific reason:


      • "This question is missing context or other details: Please improve the question by providing additional context, which ideally includes your thoughts on the problem and any attempts you have made to solve it. This information helps others identify where you have difficulties and helps them write answers appropriate to your experience level." – Brahadeesh, supinf, Christopher, Gibbs

      If this question can be reworded to fit the rules in the help center, please edit the question.






















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          If $C$ is the Cantor set and $A=B={e^{x}:xin C}$ then $A$ and $B$ have measure $0$ but $AB$ contains all numbers of the form $e^{x+y}$ with $x,y in C$. Since $C+C=[0,2]$ it follows that $AB$ does not have measure $0$. [I have used the following facts: $e^{x}$ is absolutely continuous on $[0,1]$ and any absolutely continuous function maps sets of measure $0$ to sets of measure $0$. Hence $A$ and $B$ have measure $0$]. Note also that $AB$ contains the entire interval $[e^{0},e^{2}]$ so it has positive measure.






          share|cite|improve this answer























          • I followed your argument, but I was thinking about the following, but I'm not able to figure out where went wrong: Let $E = A cup B$. Then $AB subseteq E^2$. We know $E$ has measure zero. Since the function $f(x) = x^2$ is Lipschitz, it maps sets of measure zero to sets of measure zero, so $E^2$ has measure zero, which means its subset $AB$ is also measure zero.
            – Ralph Xu
            Nov 23 at 8:58










          • $AB$ is the image of $C+C=[0,1]$ under the absolutely continuous function $e^x$. We know that every absolutely continuous function sends a set of measure zero to a set of measure zero. But can we say that it sends a set of positive measure to a set of positive measure?
            – Anupam
            Nov 23 at 9:03












          • @Anupam No you cannot say that any absolutely continuous function maps a set of positive measure to a set of positive measure. (An obvious counterexample is given by a constant function). But in my argument $AB$ contains all the numbers from $e^{0}$ to $e^{2}$ so it has positive measure.
            – Kavi Rama Murthy
            Nov 23 at 9:12












          • Ok. Sorry, I got your argument. May be $C+C=[0,2]$ and $AB$ contains all elements from $e^0$ to $e^4$.
            – Anupam
            Nov 23 at 9:16








          • 1




            @RalphXu You are getting confused with notations. It is not true that $AB$ is contained in the image of $E$ under $f(x)=x^{2}$. The image consists of the points $t^{2}$, with $t in E$ not products of any two elements of $E$.
            – Kavi Rama Murthy
            Nov 23 at 9:16


















          1 Answer
          1






          active

          oldest

          votes








          1 Answer
          1






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes








          up vote
          4
          down vote



          accepted










          If $C$ is the Cantor set and $A=B={e^{x}:xin C}$ then $A$ and $B$ have measure $0$ but $AB$ contains all numbers of the form $e^{x+y}$ with $x,y in C$. Since $C+C=[0,2]$ it follows that $AB$ does not have measure $0$. [I have used the following facts: $e^{x}$ is absolutely continuous on $[0,1]$ and any absolutely continuous function maps sets of measure $0$ to sets of measure $0$. Hence $A$ and $B$ have measure $0$]. Note also that $AB$ contains the entire interval $[e^{0},e^{2}]$ so it has positive measure.






          share|cite|improve this answer























          • I followed your argument, but I was thinking about the following, but I'm not able to figure out where went wrong: Let $E = A cup B$. Then $AB subseteq E^2$. We know $E$ has measure zero. Since the function $f(x) = x^2$ is Lipschitz, it maps sets of measure zero to sets of measure zero, so $E^2$ has measure zero, which means its subset $AB$ is also measure zero.
            – Ralph Xu
            Nov 23 at 8:58










          • $AB$ is the image of $C+C=[0,1]$ under the absolutely continuous function $e^x$. We know that every absolutely continuous function sends a set of measure zero to a set of measure zero. But can we say that it sends a set of positive measure to a set of positive measure?
            – Anupam
            Nov 23 at 9:03












          • @Anupam No you cannot say that any absolutely continuous function maps a set of positive measure to a set of positive measure. (An obvious counterexample is given by a constant function). But in my argument $AB$ contains all the numbers from $e^{0}$ to $e^{2}$ so it has positive measure.
            – Kavi Rama Murthy
            Nov 23 at 9:12












          • Ok. Sorry, I got your argument. May be $C+C=[0,2]$ and $AB$ contains all elements from $e^0$ to $e^4$.
            – Anupam
            Nov 23 at 9:16








          • 1




            @RalphXu You are getting confused with notations. It is not true that $AB$ is contained in the image of $E$ under $f(x)=x^{2}$. The image consists of the points $t^{2}$, with $t in E$ not products of any two elements of $E$.
            – Kavi Rama Murthy
            Nov 23 at 9:16















          up vote
          4
          down vote



          accepted










          If $C$ is the Cantor set and $A=B={e^{x}:xin C}$ then $A$ and $B$ have measure $0$ but $AB$ contains all numbers of the form $e^{x+y}$ with $x,y in C$. Since $C+C=[0,2]$ it follows that $AB$ does not have measure $0$. [I have used the following facts: $e^{x}$ is absolutely continuous on $[0,1]$ and any absolutely continuous function maps sets of measure $0$ to sets of measure $0$. Hence $A$ and $B$ have measure $0$]. Note also that $AB$ contains the entire interval $[e^{0},e^{2}]$ so it has positive measure.






          share|cite|improve this answer























          • I followed your argument, but I was thinking about the following, but I'm not able to figure out where went wrong: Let $E = A cup B$. Then $AB subseteq E^2$. We know $E$ has measure zero. Since the function $f(x) = x^2$ is Lipschitz, it maps sets of measure zero to sets of measure zero, so $E^2$ has measure zero, which means its subset $AB$ is also measure zero.
            – Ralph Xu
            Nov 23 at 8:58










          • $AB$ is the image of $C+C=[0,1]$ under the absolutely continuous function $e^x$. We know that every absolutely continuous function sends a set of measure zero to a set of measure zero. But can we say that it sends a set of positive measure to a set of positive measure?
            – Anupam
            Nov 23 at 9:03












          • @Anupam No you cannot say that any absolutely continuous function maps a set of positive measure to a set of positive measure. (An obvious counterexample is given by a constant function). But in my argument $AB$ contains all the numbers from $e^{0}$ to $e^{2}$ so it has positive measure.
            – Kavi Rama Murthy
            Nov 23 at 9:12












          • Ok. Sorry, I got your argument. May be $C+C=[0,2]$ and $AB$ contains all elements from $e^0$ to $e^4$.
            – Anupam
            Nov 23 at 9:16








          • 1




            @RalphXu You are getting confused with notations. It is not true that $AB$ is contained in the image of $E$ under $f(x)=x^{2}$. The image consists of the points $t^{2}$, with $t in E$ not products of any two elements of $E$.
            – Kavi Rama Murthy
            Nov 23 at 9:16













          up vote
          4
          down vote



          accepted







          up vote
          4
          down vote



          accepted






          If $C$ is the Cantor set and $A=B={e^{x}:xin C}$ then $A$ and $B$ have measure $0$ but $AB$ contains all numbers of the form $e^{x+y}$ with $x,y in C$. Since $C+C=[0,2]$ it follows that $AB$ does not have measure $0$. [I have used the following facts: $e^{x}$ is absolutely continuous on $[0,1]$ and any absolutely continuous function maps sets of measure $0$ to sets of measure $0$. Hence $A$ and $B$ have measure $0$]. Note also that $AB$ contains the entire interval $[e^{0},e^{2}]$ so it has positive measure.






          share|cite|improve this answer














          If $C$ is the Cantor set and $A=B={e^{x}:xin C}$ then $A$ and $B$ have measure $0$ but $AB$ contains all numbers of the form $e^{x+y}$ with $x,y in C$. Since $C+C=[0,2]$ it follows that $AB$ does not have measure $0$. [I have used the following facts: $e^{x}$ is absolutely continuous on $[0,1]$ and any absolutely continuous function maps sets of measure $0$ to sets of measure $0$. Hence $A$ and $B$ have measure $0$]. Note also that $AB$ contains the entire interval $[e^{0},e^{2}]$ so it has positive measure.







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited Nov 23 at 9:32

























          answered Nov 23 at 8:37









          Kavi Rama Murthy

          43k31751




          43k31751












          • I followed your argument, but I was thinking about the following, but I'm not able to figure out where went wrong: Let $E = A cup B$. Then $AB subseteq E^2$. We know $E$ has measure zero. Since the function $f(x) = x^2$ is Lipschitz, it maps sets of measure zero to sets of measure zero, so $E^2$ has measure zero, which means its subset $AB$ is also measure zero.
            – Ralph Xu
            Nov 23 at 8:58










          • $AB$ is the image of $C+C=[0,1]$ under the absolutely continuous function $e^x$. We know that every absolutely continuous function sends a set of measure zero to a set of measure zero. But can we say that it sends a set of positive measure to a set of positive measure?
            – Anupam
            Nov 23 at 9:03












          • @Anupam No you cannot say that any absolutely continuous function maps a set of positive measure to a set of positive measure. (An obvious counterexample is given by a constant function). But in my argument $AB$ contains all the numbers from $e^{0}$ to $e^{2}$ so it has positive measure.
            – Kavi Rama Murthy
            Nov 23 at 9:12












          • Ok. Sorry, I got your argument. May be $C+C=[0,2]$ and $AB$ contains all elements from $e^0$ to $e^4$.
            – Anupam
            Nov 23 at 9:16








          • 1




            @RalphXu You are getting confused with notations. It is not true that $AB$ is contained in the image of $E$ under $f(x)=x^{2}$. The image consists of the points $t^{2}$, with $t in E$ not products of any two elements of $E$.
            – Kavi Rama Murthy
            Nov 23 at 9:16


















          • I followed your argument, but I was thinking about the following, but I'm not able to figure out where went wrong: Let $E = A cup B$. Then $AB subseteq E^2$. We know $E$ has measure zero. Since the function $f(x) = x^2$ is Lipschitz, it maps sets of measure zero to sets of measure zero, so $E^2$ has measure zero, which means its subset $AB$ is also measure zero.
            – Ralph Xu
            Nov 23 at 8:58










          • $AB$ is the image of $C+C=[0,1]$ under the absolutely continuous function $e^x$. We know that every absolutely continuous function sends a set of measure zero to a set of measure zero. But can we say that it sends a set of positive measure to a set of positive measure?
            – Anupam
            Nov 23 at 9:03












          • @Anupam No you cannot say that any absolutely continuous function maps a set of positive measure to a set of positive measure. (An obvious counterexample is given by a constant function). But in my argument $AB$ contains all the numbers from $e^{0}$ to $e^{2}$ so it has positive measure.
            – Kavi Rama Murthy
            Nov 23 at 9:12












          • Ok. Sorry, I got your argument. May be $C+C=[0,2]$ and $AB$ contains all elements from $e^0$ to $e^4$.
            – Anupam
            Nov 23 at 9:16








          • 1




            @RalphXu You are getting confused with notations. It is not true that $AB$ is contained in the image of $E$ under $f(x)=x^{2}$. The image consists of the points $t^{2}$, with $t in E$ not products of any two elements of $E$.
            – Kavi Rama Murthy
            Nov 23 at 9:16
















          I followed your argument, but I was thinking about the following, but I'm not able to figure out where went wrong: Let $E = A cup B$. Then $AB subseteq E^2$. We know $E$ has measure zero. Since the function $f(x) = x^2$ is Lipschitz, it maps sets of measure zero to sets of measure zero, so $E^2$ has measure zero, which means its subset $AB$ is also measure zero.
          – Ralph Xu
          Nov 23 at 8:58




          I followed your argument, but I was thinking about the following, but I'm not able to figure out where went wrong: Let $E = A cup B$. Then $AB subseteq E^2$. We know $E$ has measure zero. Since the function $f(x) = x^2$ is Lipschitz, it maps sets of measure zero to sets of measure zero, so $E^2$ has measure zero, which means its subset $AB$ is also measure zero.
          – Ralph Xu
          Nov 23 at 8:58












          $AB$ is the image of $C+C=[0,1]$ under the absolutely continuous function $e^x$. We know that every absolutely continuous function sends a set of measure zero to a set of measure zero. But can we say that it sends a set of positive measure to a set of positive measure?
          – Anupam
          Nov 23 at 9:03






          $AB$ is the image of $C+C=[0,1]$ under the absolutely continuous function $e^x$. We know that every absolutely continuous function sends a set of measure zero to a set of measure zero. But can we say that it sends a set of positive measure to a set of positive measure?
          – Anupam
          Nov 23 at 9:03














          @Anupam No you cannot say that any absolutely continuous function maps a set of positive measure to a set of positive measure. (An obvious counterexample is given by a constant function). But in my argument $AB$ contains all the numbers from $e^{0}$ to $e^{2}$ so it has positive measure.
          – Kavi Rama Murthy
          Nov 23 at 9:12






          @Anupam No you cannot say that any absolutely continuous function maps a set of positive measure to a set of positive measure. (An obvious counterexample is given by a constant function). But in my argument $AB$ contains all the numbers from $e^{0}$ to $e^{2}$ so it has positive measure.
          – Kavi Rama Murthy
          Nov 23 at 9:12














          Ok. Sorry, I got your argument. May be $C+C=[0,2]$ and $AB$ contains all elements from $e^0$ to $e^4$.
          – Anupam
          Nov 23 at 9:16






          Ok. Sorry, I got your argument. May be $C+C=[0,2]$ and $AB$ contains all elements from $e^0$ to $e^4$.
          – Anupam
          Nov 23 at 9:16






          1




          1




          @RalphXu You are getting confused with notations. It is not true that $AB$ is contained in the image of $E$ under $f(x)=x^{2}$. The image consists of the points $t^{2}$, with $t in E$ not products of any two elements of $E$.
          – Kavi Rama Murthy
          Nov 23 at 9:16




          @RalphXu You are getting confused with notations. It is not true that $AB$ is contained in the image of $E$ under $f(x)=x^{2}$. The image consists of the points $t^{2}$, with $t in E$ not products of any two elements of $E$.
          – Kavi Rama Murthy
          Nov 23 at 9:16



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