Prove that if $f=g$ for almost all $xin B$ then $int_Bf=int_Bg$

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Let $BsubseteqmathbbR^n$ be a closed generalized rectangle
and let $f,gcolon BtomathbbR$ be two integrable functions. Prove
that if $B'=left xin Bmid fleft(xright)neq gleft(xright)right $
has measure zero then $int_Bf=int_Bg$.



My attempt:



$f,g$ are integrable then so is $f-g$ and $f-g=0$ for almost all
$xin B$. Moreover there are generalized rectangles $R_1,R_2,ldotssubseteq B$
where $R_i^intcap R_j^int=varnothing$ such that $B'subseteqbigcup R_i$
and $sum_iVleft(R_iright)<varepsilon$ for any $varepsilon$.



Now I thought if I could cover $B'$ by a finite number of rectangles
in $B$ I could then complete them also by a finite number of rectangles
to a partition $Pi$ of $B$. Then by splitting a Riemann
Sum over that partition I think I can finish.



So is there a way to cover $B'$ by a finite number of rectangles?



and/or Is there other way proving this?



EDIT:



Definitions for integrable function:




$fcolon BtomathbbR$ is integrable if for any $varepsilon>0$
there is a partition $Pi=left B_1,ldots,B_Nright $ of
$B$ such that $sum_i=1^NVleft(B_iright)undersetB_itextOSCf<varepsilon$.



OR



$fcolon BtomathbbR$ is integrable if for any $varepsilon>0$
there is $delta>0$ such that for any partition $Pi=left B_1,ldots,B_Nright $
of $B$ with $textdiamPi<delta$: $sum_i=1^NVleft(B_iright)undersetB_itextOSCf<varepsilon$




Definition for the integral of integrable function:




If $fcolon BtomathbbR$ is integrable then $int_Bf=sup_PiunderlineSleft(f,Piright)$
where $underlineS$ is the lower darboux sum



OR



If $fcolon BtomathbbR$ is integrable then $int_Bf=lim_textdiamPito0Sleft(f,Pi,Xright)$
where $Sleft(f,Pi,Xright)$ is a riemann sum








share|cite|improve this question





















  • What definition of integration are you using? Riemann integration?
    – Eric Wofsey
    Aug 2 at 15:45










  • @EricWofsey Edited in the question
    – Jon
    Aug 2 at 16:00














up vote
1
down vote

favorite
1












Let $BsubseteqmathbbR^n$ be a closed generalized rectangle
and let $f,gcolon BtomathbbR$ be two integrable functions. Prove
that if $B'=left xin Bmid fleft(xright)neq gleft(xright)right $
has measure zero then $int_Bf=int_Bg$.



My attempt:



$f,g$ are integrable then so is $f-g$ and $f-g=0$ for almost all
$xin B$. Moreover there are generalized rectangles $R_1,R_2,ldotssubseteq B$
where $R_i^intcap R_j^int=varnothing$ such that $B'subseteqbigcup R_i$
and $sum_iVleft(R_iright)<varepsilon$ for any $varepsilon$.



Now I thought if I could cover $B'$ by a finite number of rectangles
in $B$ I could then complete them also by a finite number of rectangles
to a partition $Pi$ of $B$. Then by splitting a Riemann
Sum over that partition I think I can finish.



So is there a way to cover $B'$ by a finite number of rectangles?



and/or Is there other way proving this?



EDIT:



Definitions for integrable function:




$fcolon BtomathbbR$ is integrable if for any $varepsilon>0$
there is a partition $Pi=left B_1,ldots,B_Nright $ of
$B$ such that $sum_i=1^NVleft(B_iright)undersetB_itextOSCf<varepsilon$.



OR



$fcolon BtomathbbR$ is integrable if for any $varepsilon>0$
there is $delta>0$ such that for any partition $Pi=left B_1,ldots,B_Nright $
of $B$ with $textdiamPi<delta$: $sum_i=1^NVleft(B_iright)undersetB_itextOSCf<varepsilon$




Definition for the integral of integrable function:




If $fcolon BtomathbbR$ is integrable then $int_Bf=sup_PiunderlineSleft(f,Piright)$
where $underlineS$ is the lower darboux sum



OR



If $fcolon BtomathbbR$ is integrable then $int_Bf=lim_textdiamPito0Sleft(f,Pi,Xright)$
where $Sleft(f,Pi,Xright)$ is a riemann sum








share|cite|improve this question





















  • What definition of integration are you using? Riemann integration?
    – Eric Wofsey
    Aug 2 at 15:45










  • @EricWofsey Edited in the question
    – Jon
    Aug 2 at 16:00












up vote
1
down vote

favorite
1









up vote
1
down vote

favorite
1






1





Let $BsubseteqmathbbR^n$ be a closed generalized rectangle
and let $f,gcolon BtomathbbR$ be two integrable functions. Prove
that if $B'=left xin Bmid fleft(xright)neq gleft(xright)right $
has measure zero then $int_Bf=int_Bg$.



My attempt:



$f,g$ are integrable then so is $f-g$ and $f-g=0$ for almost all
$xin B$. Moreover there are generalized rectangles $R_1,R_2,ldotssubseteq B$
where $R_i^intcap R_j^int=varnothing$ such that $B'subseteqbigcup R_i$
and $sum_iVleft(R_iright)<varepsilon$ for any $varepsilon$.



Now I thought if I could cover $B'$ by a finite number of rectangles
in $B$ I could then complete them also by a finite number of rectangles
to a partition $Pi$ of $B$. Then by splitting a Riemann
Sum over that partition I think I can finish.



So is there a way to cover $B'$ by a finite number of rectangles?



and/or Is there other way proving this?



EDIT:



Definitions for integrable function:




$fcolon BtomathbbR$ is integrable if for any $varepsilon>0$
there is a partition $Pi=left B_1,ldots,B_Nright $ of
$B$ such that $sum_i=1^NVleft(B_iright)undersetB_itextOSCf<varepsilon$.



OR



$fcolon BtomathbbR$ is integrable if for any $varepsilon>0$
there is $delta>0$ such that for any partition $Pi=left B_1,ldots,B_Nright $
of $B$ with $textdiamPi<delta$: $sum_i=1^NVleft(B_iright)undersetB_itextOSCf<varepsilon$




Definition for the integral of integrable function:




If $fcolon BtomathbbR$ is integrable then $int_Bf=sup_PiunderlineSleft(f,Piright)$
where $underlineS$ is the lower darboux sum



OR



If $fcolon BtomathbbR$ is integrable then $int_Bf=lim_textdiamPito0Sleft(f,Pi,Xright)$
where $Sleft(f,Pi,Xright)$ is a riemann sum








share|cite|improve this question













Let $BsubseteqmathbbR^n$ be a closed generalized rectangle
and let $f,gcolon BtomathbbR$ be two integrable functions. Prove
that if $B'=left xin Bmid fleft(xright)neq gleft(xright)right $
has measure zero then $int_Bf=int_Bg$.



My attempt:



$f,g$ are integrable then so is $f-g$ and $f-g=0$ for almost all
$xin B$. Moreover there are generalized rectangles $R_1,R_2,ldotssubseteq B$
where $R_i^intcap R_j^int=varnothing$ such that $B'subseteqbigcup R_i$
and $sum_iVleft(R_iright)<varepsilon$ for any $varepsilon$.



Now I thought if I could cover $B'$ by a finite number of rectangles
in $B$ I could then complete them also by a finite number of rectangles
to a partition $Pi$ of $B$. Then by splitting a Riemann
Sum over that partition I think I can finish.



So is there a way to cover $B'$ by a finite number of rectangles?



and/or Is there other way proving this?



EDIT:



Definitions for integrable function:




$fcolon BtomathbbR$ is integrable if for any $varepsilon>0$
there is a partition $Pi=left B_1,ldots,B_Nright $ of
$B$ such that $sum_i=1^NVleft(B_iright)undersetB_itextOSCf<varepsilon$.



OR



$fcolon BtomathbbR$ is integrable if for any $varepsilon>0$
there is $delta>0$ such that for any partition $Pi=left B_1,ldots,B_Nright $
of $B$ with $textdiamPi<delta$: $sum_i=1^NVleft(B_iright)undersetB_itextOSCf<varepsilon$




Definition for the integral of integrable function:




If $fcolon BtomathbbR$ is integrable then $int_Bf=sup_PiunderlineSleft(f,Piright)$
where $underlineS$ is the lower darboux sum



OR



If $fcolon BtomathbbR$ is integrable then $int_Bf=lim_textdiamPito0Sleft(f,Pi,Xright)$
where $Sleft(f,Pi,Xright)$ is a riemann sum










share|cite|improve this question












share|cite|improve this question




share|cite|improve this question








edited Aug 2 at 15:59
























asked Aug 2 at 15:42









Jon

480412




480412











  • What definition of integration are you using? Riemann integration?
    – Eric Wofsey
    Aug 2 at 15:45










  • @EricWofsey Edited in the question
    – Jon
    Aug 2 at 16:00
















  • What definition of integration are you using? Riemann integration?
    – Eric Wofsey
    Aug 2 at 15:45










  • @EricWofsey Edited in the question
    – Jon
    Aug 2 at 16:00















What definition of integration are you using? Riemann integration?
– Eric Wofsey
Aug 2 at 15:45




What definition of integration are you using? Riemann integration?
– Eric Wofsey
Aug 2 at 15:45












@EricWofsey Edited in the question
– Jon
Aug 2 at 16:00




@EricWofsey Edited in the question
– Jon
Aug 2 at 16:00










2 Answers
2






active

oldest

votes

















up vote
2
down vote



accepted










For the Lebesgue integral this follows almost directly from the definition. For the Riemann integral over a rectangle in $mathbbR^n$, the proof is more subtle in order to avoid measure theory -- other than the basic notion of zero measure.



Note that $h = f-g$ is Riemann integrable on $B$ (according to your definitions) and the set

$B' = x in B: h(x) = f(x)-g(x) neq 0$ has measure zero.



Take any partition $P$ of the rectangle $B$. Any subrectangle $R$ of $P$ has non-zero content and measure by definition of a partition. Hence $R$ is not a subset of $B'$ and must contain at least one point where $h(x) = 0$. This implies that $inf_R h(x) leqslant 0 leqslant sup_R h(x)$.



Forming upper and lower Riemann sums, we have for any partition $P$



$$underlineS(h ,P) leqslant 0 leqslant overlineS(h ,P)$$



Hence,



$$underlineint_B h = sup_P underlineS(h,P) leqslant 0 leqslant inf_P overlineS(h, P) = overlineint_B h $$



Since $h$ is Riemann integrable on $B$, the upper and lower integrals must be equal and



$$int_Bf - int_Bg =int_Bh = underlineint_B h = overlineint_B h = 0$$






share|cite|improve this answer






























    up vote
    0
    down vote













    $f-g=0$, except on a set of measure $0$. Therefore $int_B(f-g)=int_B0=0$, or $int_Bf=int_Bg$. Why isn't it obvious?



    If you need to be pedantic let $B=B_1cup B_2$, where $B_1$ is the set where $f=g$ and $B_2$ is the set of measure $0$ where $fne g$. Then $int_B(f-g)=int_B_1(f-g)+int_B_2(f-g)$. The first term=$0$, since the integrand is $0$, while the second term=$0$, since the domain of integration has measure $0$.






    share|cite|improve this answer























    • Can you sketch a proof of the fact that the Riemann integral on a set of measure zero (not content zero) is equal to zero ?
      – Tony Piccolo
      Aug 2 at 23:20











    • I was using measure theory (Lebesgue integral). It is a standard theorem.
      – herb steinberg
      Aug 3 at 0:05










    • My aim was to know if there exists a proof that doesn't use Lebesgue theory.
      – Tony Piccolo
      Aug 3 at 6:06










    • Once you use the term "measure zero", what else could you use?
      – herb steinberg
      Aug 3 at 15:16










    • See the accepted answer ..... I mean that.
      – Tony Piccolo
      Aug 3 at 19:02











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    2 Answers
    2






    active

    oldest

    votes








    2 Answers
    2






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes








    up vote
    2
    down vote



    accepted










    For the Lebesgue integral this follows almost directly from the definition. For the Riemann integral over a rectangle in $mathbbR^n$, the proof is more subtle in order to avoid measure theory -- other than the basic notion of zero measure.



    Note that $h = f-g$ is Riemann integrable on $B$ (according to your definitions) and the set

    $B' = x in B: h(x) = f(x)-g(x) neq 0$ has measure zero.



    Take any partition $P$ of the rectangle $B$. Any subrectangle $R$ of $P$ has non-zero content and measure by definition of a partition. Hence $R$ is not a subset of $B'$ and must contain at least one point where $h(x) = 0$. This implies that $inf_R h(x) leqslant 0 leqslant sup_R h(x)$.



    Forming upper and lower Riemann sums, we have for any partition $P$



    $$underlineS(h ,P) leqslant 0 leqslant overlineS(h ,P)$$



    Hence,



    $$underlineint_B h = sup_P underlineS(h,P) leqslant 0 leqslant inf_P overlineS(h, P) = overlineint_B h $$



    Since $h$ is Riemann integrable on $B$, the upper and lower integrals must be equal and



    $$int_Bf - int_Bg =int_Bh = underlineint_B h = overlineint_B h = 0$$






    share|cite|improve this answer



























      up vote
      2
      down vote



      accepted










      For the Lebesgue integral this follows almost directly from the definition. For the Riemann integral over a rectangle in $mathbbR^n$, the proof is more subtle in order to avoid measure theory -- other than the basic notion of zero measure.



      Note that $h = f-g$ is Riemann integrable on $B$ (according to your definitions) and the set

      $B' = x in B: h(x) = f(x)-g(x) neq 0$ has measure zero.



      Take any partition $P$ of the rectangle $B$. Any subrectangle $R$ of $P$ has non-zero content and measure by definition of a partition. Hence $R$ is not a subset of $B'$ and must contain at least one point where $h(x) = 0$. This implies that $inf_R h(x) leqslant 0 leqslant sup_R h(x)$.



      Forming upper and lower Riemann sums, we have for any partition $P$



      $$underlineS(h ,P) leqslant 0 leqslant overlineS(h ,P)$$



      Hence,



      $$underlineint_B h = sup_P underlineS(h,P) leqslant 0 leqslant inf_P overlineS(h, P) = overlineint_B h $$



      Since $h$ is Riemann integrable on $B$, the upper and lower integrals must be equal and



      $$int_Bf - int_Bg =int_Bh = underlineint_B h = overlineint_B h = 0$$






      share|cite|improve this answer

























        up vote
        2
        down vote



        accepted







        up vote
        2
        down vote



        accepted






        For the Lebesgue integral this follows almost directly from the definition. For the Riemann integral over a rectangle in $mathbbR^n$, the proof is more subtle in order to avoid measure theory -- other than the basic notion of zero measure.



        Note that $h = f-g$ is Riemann integrable on $B$ (according to your definitions) and the set

        $B' = x in B: h(x) = f(x)-g(x) neq 0$ has measure zero.



        Take any partition $P$ of the rectangle $B$. Any subrectangle $R$ of $P$ has non-zero content and measure by definition of a partition. Hence $R$ is not a subset of $B'$ and must contain at least one point where $h(x) = 0$. This implies that $inf_R h(x) leqslant 0 leqslant sup_R h(x)$.



        Forming upper and lower Riemann sums, we have for any partition $P$



        $$underlineS(h ,P) leqslant 0 leqslant overlineS(h ,P)$$



        Hence,



        $$underlineint_B h = sup_P underlineS(h,P) leqslant 0 leqslant inf_P overlineS(h, P) = overlineint_B h $$



        Since $h$ is Riemann integrable on $B$, the upper and lower integrals must be equal and



        $$int_Bf - int_Bg =int_Bh = underlineint_B h = overlineint_B h = 0$$






        share|cite|improve this answer















        For the Lebesgue integral this follows almost directly from the definition. For the Riemann integral over a rectangle in $mathbbR^n$, the proof is more subtle in order to avoid measure theory -- other than the basic notion of zero measure.



        Note that $h = f-g$ is Riemann integrable on $B$ (according to your definitions) and the set

        $B' = x in B: h(x) = f(x)-g(x) neq 0$ has measure zero.



        Take any partition $P$ of the rectangle $B$. Any subrectangle $R$ of $P$ has non-zero content and measure by definition of a partition. Hence $R$ is not a subset of $B'$ and must contain at least one point where $h(x) = 0$. This implies that $inf_R h(x) leqslant 0 leqslant sup_R h(x)$.



        Forming upper and lower Riemann sums, we have for any partition $P$



        $$underlineS(h ,P) leqslant 0 leqslant overlineS(h ,P)$$



        Hence,



        $$underlineint_B h = sup_P underlineS(h,P) leqslant 0 leqslant inf_P overlineS(h, P) = overlineint_B h $$



        Since $h$ is Riemann integrable on $B$, the upper and lower integrals must be equal and



        $$int_Bf - int_Bg =int_Bh = underlineint_B h = overlineint_B h = 0$$







        share|cite|improve this answer















        share|cite|improve this answer



        share|cite|improve this answer








        edited Aug 3 at 6:11


























        answered Aug 3 at 5:20









        RRL

        43.4k42260




        43.4k42260




















            up vote
            0
            down vote













            $f-g=0$, except on a set of measure $0$. Therefore $int_B(f-g)=int_B0=0$, or $int_Bf=int_Bg$. Why isn't it obvious?



            If you need to be pedantic let $B=B_1cup B_2$, where $B_1$ is the set where $f=g$ and $B_2$ is the set of measure $0$ where $fne g$. Then $int_B(f-g)=int_B_1(f-g)+int_B_2(f-g)$. The first term=$0$, since the integrand is $0$, while the second term=$0$, since the domain of integration has measure $0$.






            share|cite|improve this answer























            • Can you sketch a proof of the fact that the Riemann integral on a set of measure zero (not content zero) is equal to zero ?
              – Tony Piccolo
              Aug 2 at 23:20











            • I was using measure theory (Lebesgue integral). It is a standard theorem.
              – herb steinberg
              Aug 3 at 0:05










            • My aim was to know if there exists a proof that doesn't use Lebesgue theory.
              – Tony Piccolo
              Aug 3 at 6:06










            • Once you use the term "measure zero", what else could you use?
              – herb steinberg
              Aug 3 at 15:16










            • See the accepted answer ..... I mean that.
              – Tony Piccolo
              Aug 3 at 19:02















            up vote
            0
            down vote













            $f-g=0$, except on a set of measure $0$. Therefore $int_B(f-g)=int_B0=0$, or $int_Bf=int_Bg$. Why isn't it obvious?



            If you need to be pedantic let $B=B_1cup B_2$, where $B_1$ is the set where $f=g$ and $B_2$ is the set of measure $0$ where $fne g$. Then $int_B(f-g)=int_B_1(f-g)+int_B_2(f-g)$. The first term=$0$, since the integrand is $0$, while the second term=$0$, since the domain of integration has measure $0$.






            share|cite|improve this answer























            • Can you sketch a proof of the fact that the Riemann integral on a set of measure zero (not content zero) is equal to zero ?
              – Tony Piccolo
              Aug 2 at 23:20











            • I was using measure theory (Lebesgue integral). It is a standard theorem.
              – herb steinberg
              Aug 3 at 0:05










            • My aim was to know if there exists a proof that doesn't use Lebesgue theory.
              – Tony Piccolo
              Aug 3 at 6:06










            • Once you use the term "measure zero", what else could you use?
              – herb steinberg
              Aug 3 at 15:16










            • See the accepted answer ..... I mean that.
              – Tony Piccolo
              Aug 3 at 19:02













            up vote
            0
            down vote










            up vote
            0
            down vote









            $f-g=0$, except on a set of measure $0$. Therefore $int_B(f-g)=int_B0=0$, or $int_Bf=int_Bg$. Why isn't it obvious?



            If you need to be pedantic let $B=B_1cup B_2$, where $B_1$ is the set where $f=g$ and $B_2$ is the set of measure $0$ where $fne g$. Then $int_B(f-g)=int_B_1(f-g)+int_B_2(f-g)$. The first term=$0$, since the integrand is $0$, while the second term=$0$, since the domain of integration has measure $0$.






            share|cite|improve this answer















            $f-g=0$, except on a set of measure $0$. Therefore $int_B(f-g)=int_B0=0$, or $int_Bf=int_Bg$. Why isn't it obvious?



            If you need to be pedantic let $B=B_1cup B_2$, where $B_1$ is the set where $f=g$ and $B_2$ is the set of measure $0$ where $fne g$. Then $int_B(f-g)=int_B_1(f-g)+int_B_2(f-g)$. The first term=$0$, since the integrand is $0$, while the second term=$0$, since the domain of integration has measure $0$.







            share|cite|improve this answer















            share|cite|improve this answer



            share|cite|improve this answer








            edited Aug 2 at 19:19


























            answered Aug 2 at 19:05









            herb steinberg

            93529




            93529











            • Can you sketch a proof of the fact that the Riemann integral on a set of measure zero (not content zero) is equal to zero ?
              – Tony Piccolo
              Aug 2 at 23:20











            • I was using measure theory (Lebesgue integral). It is a standard theorem.
              – herb steinberg
              Aug 3 at 0:05










            • My aim was to know if there exists a proof that doesn't use Lebesgue theory.
              – Tony Piccolo
              Aug 3 at 6:06










            • Once you use the term "measure zero", what else could you use?
              – herb steinberg
              Aug 3 at 15:16










            • See the accepted answer ..... I mean that.
              – Tony Piccolo
              Aug 3 at 19:02

















            • Can you sketch a proof of the fact that the Riemann integral on a set of measure zero (not content zero) is equal to zero ?
              – Tony Piccolo
              Aug 2 at 23:20











            • I was using measure theory (Lebesgue integral). It is a standard theorem.
              – herb steinberg
              Aug 3 at 0:05










            • My aim was to know if there exists a proof that doesn't use Lebesgue theory.
              – Tony Piccolo
              Aug 3 at 6:06










            • Once you use the term "measure zero", what else could you use?
              – herb steinberg
              Aug 3 at 15:16










            • See the accepted answer ..... I mean that.
              – Tony Piccolo
              Aug 3 at 19:02
















            Can you sketch a proof of the fact that the Riemann integral on a set of measure zero (not content zero) is equal to zero ?
            – Tony Piccolo
            Aug 2 at 23:20





            Can you sketch a proof of the fact that the Riemann integral on a set of measure zero (not content zero) is equal to zero ?
            – Tony Piccolo
            Aug 2 at 23:20













            I was using measure theory (Lebesgue integral). It is a standard theorem.
            – herb steinberg
            Aug 3 at 0:05




            I was using measure theory (Lebesgue integral). It is a standard theorem.
            – herb steinberg
            Aug 3 at 0:05












            My aim was to know if there exists a proof that doesn't use Lebesgue theory.
            – Tony Piccolo
            Aug 3 at 6:06




            My aim was to know if there exists a proof that doesn't use Lebesgue theory.
            – Tony Piccolo
            Aug 3 at 6:06












            Once you use the term "measure zero", what else could you use?
            – herb steinberg
            Aug 3 at 15:16




            Once you use the term "measure zero", what else could you use?
            – herb steinberg
            Aug 3 at 15:16












            See the accepted answer ..... I mean that.
            – Tony Piccolo
            Aug 3 at 19:02





            See the accepted answer ..... I mean that.
            – Tony Piccolo
            Aug 3 at 19:02













             

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