if ${x| f(x) neq g(x))}$ has zero measure then $int f = int g$.












2












$begingroup$


Let $f,g$ be integrable functions in a closed box $B subset mathbb R^n$.
Suppose that the set $A = {x in B| f(x) neq g(x)}$ is negligible. Show that $int_Bf =int_B g$



What I tried:



Notice that $B = A cup (A^c cap B)$ and that $A cap (A^c cap B) = emptyset$ obviously.



Then $int_Bf - int_B g = int_B f-g = int_{A}f-g+int_{A^c cap B}f-g = int_{A}f-g$



Now see that $0 = v(A)inf_{x in A}(f-g)(x) leq int_{A}f-g leq v(A) sup_{x in A}(f-g)(x) =0$ because $v(A) = 0$ since it's negligible, and $f,g$ are integrable on $B$ so we know they are bounded.



Where is my issue:



I'm unsure about



$int_B f-g = int_{A}f-g+int_{A^c cap B}f-g$, I realize it's true when $A,A^c cap B$ are boxes. If I split a big box into two smaller non intersecting boxes, then the integral on the big box is equal to the sum of the integrals of the two smaller boxes, but is it also true for general shapes?



Is there a way to prove this without resorting to integrals over non boxes?










share|cite|improve this question









$endgroup$












  • $begingroup$
    Not sure about your tags... I would not tag calculus and I would tag Lebesgue integration instead of Riemann Integration.
    $endgroup$
    – Zuriel
    Dec 13 '18 at 21:23
















2












$begingroup$


Let $f,g$ be integrable functions in a closed box $B subset mathbb R^n$.
Suppose that the set $A = {x in B| f(x) neq g(x)}$ is negligible. Show that $int_Bf =int_B g$



What I tried:



Notice that $B = A cup (A^c cap B)$ and that $A cap (A^c cap B) = emptyset$ obviously.



Then $int_Bf - int_B g = int_B f-g = int_{A}f-g+int_{A^c cap B}f-g = int_{A}f-g$



Now see that $0 = v(A)inf_{x in A}(f-g)(x) leq int_{A}f-g leq v(A) sup_{x in A}(f-g)(x) =0$ because $v(A) = 0$ since it's negligible, and $f,g$ are integrable on $B$ so we know they are bounded.



Where is my issue:



I'm unsure about



$int_B f-g = int_{A}f-g+int_{A^c cap B}f-g$, I realize it's true when $A,A^c cap B$ are boxes. If I split a big box into two smaller non intersecting boxes, then the integral on the big box is equal to the sum of the integrals of the two smaller boxes, but is it also true for general shapes?



Is there a way to prove this without resorting to integrals over non boxes?










share|cite|improve this question









$endgroup$












  • $begingroup$
    Not sure about your tags... I would not tag calculus and I would tag Lebesgue integration instead of Riemann Integration.
    $endgroup$
    – Zuriel
    Dec 13 '18 at 21:23














2












2








2


1



$begingroup$


Let $f,g$ be integrable functions in a closed box $B subset mathbb R^n$.
Suppose that the set $A = {x in B| f(x) neq g(x)}$ is negligible. Show that $int_Bf =int_B g$



What I tried:



Notice that $B = A cup (A^c cap B)$ and that $A cap (A^c cap B) = emptyset$ obviously.



Then $int_Bf - int_B g = int_B f-g = int_{A}f-g+int_{A^c cap B}f-g = int_{A}f-g$



Now see that $0 = v(A)inf_{x in A}(f-g)(x) leq int_{A}f-g leq v(A) sup_{x in A}(f-g)(x) =0$ because $v(A) = 0$ since it's negligible, and $f,g$ are integrable on $B$ so we know they are bounded.



Where is my issue:



I'm unsure about



$int_B f-g = int_{A}f-g+int_{A^c cap B}f-g$, I realize it's true when $A,A^c cap B$ are boxes. If I split a big box into two smaller non intersecting boxes, then the integral on the big box is equal to the sum of the integrals of the two smaller boxes, but is it also true for general shapes?



Is there a way to prove this without resorting to integrals over non boxes?










share|cite|improve this question









$endgroup$




Let $f,g$ be integrable functions in a closed box $B subset mathbb R^n$.
Suppose that the set $A = {x in B| f(x) neq g(x)}$ is negligible. Show that $int_Bf =int_B g$



What I tried:



Notice that $B = A cup (A^c cap B)$ and that $A cap (A^c cap B) = emptyset$ obviously.



Then $int_Bf - int_B g = int_B f-g = int_{A}f-g+int_{A^c cap B}f-g = int_{A}f-g$



Now see that $0 = v(A)inf_{x in A}(f-g)(x) leq int_{A}f-g leq v(A) sup_{x in A}(f-g)(x) =0$ because $v(A) = 0$ since it's negligible, and $f,g$ are integrable on $B$ so we know they are bounded.



Where is my issue:



I'm unsure about



$int_B f-g = int_{A}f-g+int_{A^c cap B}f-g$, I realize it's true when $A,A^c cap B$ are boxes. If I split a big box into two smaller non intersecting boxes, then the integral on the big box is equal to the sum of the integrals of the two smaller boxes, but is it also true for general shapes?



Is there a way to prove this without resorting to integrals over non boxes?







calculus measure-theory riemann-integration






share|cite|improve this question













share|cite|improve this question











share|cite|improve this question




share|cite|improve this question










asked Dec 13 '18 at 20:58









Oria GruberOria Gruber

6,48032460




6,48032460












  • $begingroup$
    Not sure about your tags... I would not tag calculus and I would tag Lebesgue integration instead of Riemann Integration.
    $endgroup$
    – Zuriel
    Dec 13 '18 at 21:23


















  • $begingroup$
    Not sure about your tags... I would not tag calculus and I would tag Lebesgue integration instead of Riemann Integration.
    $endgroup$
    – Zuriel
    Dec 13 '18 at 21:23
















$begingroup$
Not sure about your tags... I would not tag calculus and I would tag Lebesgue integration instead of Riemann Integration.
$endgroup$
– Zuriel
Dec 13 '18 at 21:23




$begingroup$
Not sure about your tags... I would not tag calculus and I would tag Lebesgue integration instead of Riemann Integration.
$endgroup$
– Zuriel
Dec 13 '18 at 21:23










2 Answers
2






active

oldest

votes


















2












$begingroup$

We can also prove this in the framework of Riemann integration.



Take $h = f- g$ and



$$A = {x in B ,|, f(x) neq g(x) } = { x in B ,|, h(x) neq 0}$$ where $A$
has zero measure.



If $P$ is any partition of $B$ into sub-rectangles (boxes) then any such sub-rectangle $R$ has non-zero content and so must include points $y$ not in A such that $h(y) = 0$.



Thus, $inf_{x in R}h(x) leqslant 0 leqslant sup_{x in R} h(x)$ for each $R in P$ and the lower and upper Darboux sums satisfy



$$L(P,h) leqslant 0 leqslant U(P,h), \ implies L(P,h) leqslant sup_P L(P,h) leqslant 0 leqslant inf_P ,U(P,h) leqslant U(P,h)$$



However, we are given that $f$ and $g$, and , hence, $h$ are integrable.



Thus,



$$int_B h = sup_P L(P,h) geqslant 0 , quad int_B h = inf_P ,L(P,h) leqslant 0,$$



which implies



$$int_Bf - int_B g = int_Bh = 0$$






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Why must any such sub rectangle include points not from $A$? How did you use the fact that $A$ has zero measure?
    $endgroup$
    – Oria Gruber
    Dec 13 '18 at 22:27










  • $begingroup$
    In Riemann integration over boxes in $mathbb{R}^n$ we always use partitions where the sub-boxes are of the form $R = [a_1,b_1] times [a_2,b_2] times ldots times [a_n,b_n]$ and each interval is non-empty. Otherwise $vol(R) = 0$ and it contributes nothing to a Darboux or Rieman sum. It is impossible for $R subset A$ since the measure of $R$ is non-zero. Every partition element has to include points where $h$ is $0$. This is the easiest proof of this theorem requiring the least amount of measure theory.
    $endgroup$
    – RRL
    Dec 13 '18 at 22:45





















1












$begingroup$

Since $f$ and $g$ are integrable, so is $f-g$, so $A=(f-g)^{-1}(mathbb{R}setminus{0})$ is a measurable set. Hence also $A^ccap B$ is measurable.



Then use the fact that for any integrable function $f$ and partitioning measurable sets $A$ and $B$ we have $f=chi_Af+chi_Bf$ and hence $int f=intchi_Af+chi_Bf=intchi_Af+intchi_Bf=int_Af+int_Bf$.



Here $chi_A$ denotes the indicator function, which is $1$ in $A$ and $0$ elsewhere.






share|cite|improve this answer











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    2 Answers
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    2 Answers
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    oldest

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    active

    oldest

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    2












    $begingroup$

    We can also prove this in the framework of Riemann integration.



    Take $h = f- g$ and



    $$A = {x in B ,|, f(x) neq g(x) } = { x in B ,|, h(x) neq 0}$$ where $A$
    has zero measure.



    If $P$ is any partition of $B$ into sub-rectangles (boxes) then any such sub-rectangle $R$ has non-zero content and so must include points $y$ not in A such that $h(y) = 0$.



    Thus, $inf_{x in R}h(x) leqslant 0 leqslant sup_{x in R} h(x)$ for each $R in P$ and the lower and upper Darboux sums satisfy



    $$L(P,h) leqslant 0 leqslant U(P,h), \ implies L(P,h) leqslant sup_P L(P,h) leqslant 0 leqslant inf_P ,U(P,h) leqslant U(P,h)$$



    However, we are given that $f$ and $g$, and , hence, $h$ are integrable.



    Thus,



    $$int_B h = sup_P L(P,h) geqslant 0 , quad int_B h = inf_P ,L(P,h) leqslant 0,$$



    which implies



    $$int_Bf - int_B g = int_Bh = 0$$






    share|cite|improve this answer











    $endgroup$













    • $begingroup$
      Why must any such sub rectangle include points not from $A$? How did you use the fact that $A$ has zero measure?
      $endgroup$
      – Oria Gruber
      Dec 13 '18 at 22:27










    • $begingroup$
      In Riemann integration over boxes in $mathbb{R}^n$ we always use partitions where the sub-boxes are of the form $R = [a_1,b_1] times [a_2,b_2] times ldots times [a_n,b_n]$ and each interval is non-empty. Otherwise $vol(R) = 0$ and it contributes nothing to a Darboux or Rieman sum. It is impossible for $R subset A$ since the measure of $R$ is non-zero. Every partition element has to include points where $h$ is $0$. This is the easiest proof of this theorem requiring the least amount of measure theory.
      $endgroup$
      – RRL
      Dec 13 '18 at 22:45


















    2












    $begingroup$

    We can also prove this in the framework of Riemann integration.



    Take $h = f- g$ and



    $$A = {x in B ,|, f(x) neq g(x) } = { x in B ,|, h(x) neq 0}$$ where $A$
    has zero measure.



    If $P$ is any partition of $B$ into sub-rectangles (boxes) then any such sub-rectangle $R$ has non-zero content and so must include points $y$ not in A such that $h(y) = 0$.



    Thus, $inf_{x in R}h(x) leqslant 0 leqslant sup_{x in R} h(x)$ for each $R in P$ and the lower and upper Darboux sums satisfy



    $$L(P,h) leqslant 0 leqslant U(P,h), \ implies L(P,h) leqslant sup_P L(P,h) leqslant 0 leqslant inf_P ,U(P,h) leqslant U(P,h)$$



    However, we are given that $f$ and $g$, and , hence, $h$ are integrable.



    Thus,



    $$int_B h = sup_P L(P,h) geqslant 0 , quad int_B h = inf_P ,L(P,h) leqslant 0,$$



    which implies



    $$int_Bf - int_B g = int_Bh = 0$$






    share|cite|improve this answer











    $endgroup$













    • $begingroup$
      Why must any such sub rectangle include points not from $A$? How did you use the fact that $A$ has zero measure?
      $endgroup$
      – Oria Gruber
      Dec 13 '18 at 22:27










    • $begingroup$
      In Riemann integration over boxes in $mathbb{R}^n$ we always use partitions where the sub-boxes are of the form $R = [a_1,b_1] times [a_2,b_2] times ldots times [a_n,b_n]$ and each interval is non-empty. Otherwise $vol(R) = 0$ and it contributes nothing to a Darboux or Rieman sum. It is impossible for $R subset A$ since the measure of $R$ is non-zero. Every partition element has to include points where $h$ is $0$. This is the easiest proof of this theorem requiring the least amount of measure theory.
      $endgroup$
      – RRL
      Dec 13 '18 at 22:45
















    2












    2








    2





    $begingroup$

    We can also prove this in the framework of Riemann integration.



    Take $h = f- g$ and



    $$A = {x in B ,|, f(x) neq g(x) } = { x in B ,|, h(x) neq 0}$$ where $A$
    has zero measure.



    If $P$ is any partition of $B$ into sub-rectangles (boxes) then any such sub-rectangle $R$ has non-zero content and so must include points $y$ not in A such that $h(y) = 0$.



    Thus, $inf_{x in R}h(x) leqslant 0 leqslant sup_{x in R} h(x)$ for each $R in P$ and the lower and upper Darboux sums satisfy



    $$L(P,h) leqslant 0 leqslant U(P,h), \ implies L(P,h) leqslant sup_P L(P,h) leqslant 0 leqslant inf_P ,U(P,h) leqslant U(P,h)$$



    However, we are given that $f$ and $g$, and , hence, $h$ are integrable.



    Thus,



    $$int_B h = sup_P L(P,h) geqslant 0 , quad int_B h = inf_P ,L(P,h) leqslant 0,$$



    which implies



    $$int_Bf - int_B g = int_Bh = 0$$






    share|cite|improve this answer











    $endgroup$



    We can also prove this in the framework of Riemann integration.



    Take $h = f- g$ and



    $$A = {x in B ,|, f(x) neq g(x) } = { x in B ,|, h(x) neq 0}$$ where $A$
    has zero measure.



    If $P$ is any partition of $B$ into sub-rectangles (boxes) then any such sub-rectangle $R$ has non-zero content and so must include points $y$ not in A such that $h(y) = 0$.



    Thus, $inf_{x in R}h(x) leqslant 0 leqslant sup_{x in R} h(x)$ for each $R in P$ and the lower and upper Darboux sums satisfy



    $$L(P,h) leqslant 0 leqslant U(P,h), \ implies L(P,h) leqslant sup_P L(P,h) leqslant 0 leqslant inf_P ,U(P,h) leqslant U(P,h)$$



    However, we are given that $f$ and $g$, and , hence, $h$ are integrable.



    Thus,



    $$int_B h = sup_P L(P,h) geqslant 0 , quad int_B h = inf_P ,L(P,h) leqslant 0,$$



    which implies



    $$int_Bf - int_B g = int_Bh = 0$$







    share|cite|improve this answer














    share|cite|improve this answer



    share|cite|improve this answer








    edited Dec 13 '18 at 22:12

























    answered Dec 13 '18 at 22:03









    RRLRRL

    51.4k42573




    51.4k42573












    • $begingroup$
      Why must any such sub rectangle include points not from $A$? How did you use the fact that $A$ has zero measure?
      $endgroup$
      – Oria Gruber
      Dec 13 '18 at 22:27










    • $begingroup$
      In Riemann integration over boxes in $mathbb{R}^n$ we always use partitions where the sub-boxes are of the form $R = [a_1,b_1] times [a_2,b_2] times ldots times [a_n,b_n]$ and each interval is non-empty. Otherwise $vol(R) = 0$ and it contributes nothing to a Darboux or Rieman sum. It is impossible for $R subset A$ since the measure of $R$ is non-zero. Every partition element has to include points where $h$ is $0$. This is the easiest proof of this theorem requiring the least amount of measure theory.
      $endgroup$
      – RRL
      Dec 13 '18 at 22:45




















    • $begingroup$
      Why must any such sub rectangle include points not from $A$? How did you use the fact that $A$ has zero measure?
      $endgroup$
      – Oria Gruber
      Dec 13 '18 at 22:27










    • $begingroup$
      In Riemann integration over boxes in $mathbb{R}^n$ we always use partitions where the sub-boxes are of the form $R = [a_1,b_1] times [a_2,b_2] times ldots times [a_n,b_n]$ and each interval is non-empty. Otherwise $vol(R) = 0$ and it contributes nothing to a Darboux or Rieman sum. It is impossible for $R subset A$ since the measure of $R$ is non-zero. Every partition element has to include points where $h$ is $0$. This is the easiest proof of this theorem requiring the least amount of measure theory.
      $endgroup$
      – RRL
      Dec 13 '18 at 22:45


















    $begingroup$
    Why must any such sub rectangle include points not from $A$? How did you use the fact that $A$ has zero measure?
    $endgroup$
    – Oria Gruber
    Dec 13 '18 at 22:27




    $begingroup$
    Why must any such sub rectangle include points not from $A$? How did you use the fact that $A$ has zero measure?
    $endgroup$
    – Oria Gruber
    Dec 13 '18 at 22:27












    $begingroup$
    In Riemann integration over boxes in $mathbb{R}^n$ we always use partitions where the sub-boxes are of the form $R = [a_1,b_1] times [a_2,b_2] times ldots times [a_n,b_n]$ and each interval is non-empty. Otherwise $vol(R) = 0$ and it contributes nothing to a Darboux or Rieman sum. It is impossible for $R subset A$ since the measure of $R$ is non-zero. Every partition element has to include points where $h$ is $0$. This is the easiest proof of this theorem requiring the least amount of measure theory.
    $endgroup$
    – RRL
    Dec 13 '18 at 22:45






    $begingroup$
    In Riemann integration over boxes in $mathbb{R}^n$ we always use partitions where the sub-boxes are of the form $R = [a_1,b_1] times [a_2,b_2] times ldots times [a_n,b_n]$ and each interval is non-empty. Otherwise $vol(R) = 0$ and it contributes nothing to a Darboux or Rieman sum. It is impossible for $R subset A$ since the measure of $R$ is non-zero. Every partition element has to include points where $h$ is $0$. This is the easiest proof of this theorem requiring the least amount of measure theory.
    $endgroup$
    – RRL
    Dec 13 '18 at 22:45













    1












    $begingroup$

    Since $f$ and $g$ are integrable, so is $f-g$, so $A=(f-g)^{-1}(mathbb{R}setminus{0})$ is a measurable set. Hence also $A^ccap B$ is measurable.



    Then use the fact that for any integrable function $f$ and partitioning measurable sets $A$ and $B$ we have $f=chi_Af+chi_Bf$ and hence $int f=intchi_Af+chi_Bf=intchi_Af+intchi_Bf=int_Af+int_Bf$.



    Here $chi_A$ denotes the indicator function, which is $1$ in $A$ and $0$ elsewhere.






    share|cite|improve this answer











    $endgroup$


















      1












      $begingroup$

      Since $f$ and $g$ are integrable, so is $f-g$, so $A=(f-g)^{-1}(mathbb{R}setminus{0})$ is a measurable set. Hence also $A^ccap B$ is measurable.



      Then use the fact that for any integrable function $f$ and partitioning measurable sets $A$ and $B$ we have $f=chi_Af+chi_Bf$ and hence $int f=intchi_Af+chi_Bf=intchi_Af+intchi_Bf=int_Af+int_Bf$.



      Here $chi_A$ denotes the indicator function, which is $1$ in $A$ and $0$ elsewhere.






      share|cite|improve this answer











      $endgroup$
















        1












        1








        1





        $begingroup$

        Since $f$ and $g$ are integrable, so is $f-g$, so $A=(f-g)^{-1}(mathbb{R}setminus{0})$ is a measurable set. Hence also $A^ccap B$ is measurable.



        Then use the fact that for any integrable function $f$ and partitioning measurable sets $A$ and $B$ we have $f=chi_Af+chi_Bf$ and hence $int f=intchi_Af+chi_Bf=intchi_Af+intchi_Bf=int_Af+int_Bf$.



        Here $chi_A$ denotes the indicator function, which is $1$ in $A$ and $0$ elsewhere.






        share|cite|improve this answer











        $endgroup$



        Since $f$ and $g$ are integrable, so is $f-g$, so $A=(f-g)^{-1}(mathbb{R}setminus{0})$ is a measurable set. Hence also $A^ccap B$ is measurable.



        Then use the fact that for any integrable function $f$ and partitioning measurable sets $A$ and $B$ we have $f=chi_Af+chi_Bf$ and hence $int f=intchi_Af+chi_Bf=intchi_Af+intchi_Bf=int_Af+int_Bf$.



        Here $chi_A$ denotes the indicator function, which is $1$ in $A$ and $0$ elsewhere.







        share|cite|improve this answer














        share|cite|improve this answer



        share|cite|improve this answer








        edited Dec 13 '18 at 21:31

























        answered Dec 13 '18 at 21:20









        SmileyCraftSmileyCraft

        3,591517




        3,591517






























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