Sufficient condition for $L^infty$-convergence in martingale convergence theorem.












1














Let $I$ denote the unit interval and $f:Ito I$ be a Borel measurable function.
For each $n$ let $P_n$ denote the partition of $I$ defined as
$$P_n= left{left[0, frac{1}{2^n}right), left[frac{1}{2^n}, frac{2}{2^n}right), left[frac{2}{2^n}, frac{3}{2^n}right), ldots, left[frac{2^n-2}{2^n}, frac{2^n-1}{2^n}right],left[frac{2^n-1}{2^n}, 1right]right}$$
and let $mathcal A_n$ be the $sigma$-algebra on $I$ generated by $P_n$.
Define $f_n=E[f|mathcal A_n]$, where the conditional expectation is with respect to the Lebesgue measure.



It is known by the martingale convergence theorem that $f_nto f$ pointwise a.e. and also in $L^1$.




Question. Are there some (nice) sufficient conditions on the nature of $f$ which ensure that the convergence above is in $L^infty$ also?











share|cite|improve this question





























    1














    Let $I$ denote the unit interval and $f:Ito I$ be a Borel measurable function.
    For each $n$ let $P_n$ denote the partition of $I$ defined as
    $$P_n= left{left[0, frac{1}{2^n}right), left[frac{1}{2^n}, frac{2}{2^n}right), left[frac{2}{2^n}, frac{3}{2^n}right), ldots, left[frac{2^n-2}{2^n}, frac{2^n-1}{2^n}right],left[frac{2^n-1}{2^n}, 1right]right}$$
    and let $mathcal A_n$ be the $sigma$-algebra on $I$ generated by $P_n$.
    Define $f_n=E[f|mathcal A_n]$, where the conditional expectation is with respect to the Lebesgue measure.



    It is known by the martingale convergence theorem that $f_nto f$ pointwise a.e. and also in $L^1$.




    Question. Are there some (nice) sufficient conditions on the nature of $f$ which ensure that the convergence above is in $L^infty$ also?











    share|cite|improve this question



























      1












      1








      1


      1





      Let $I$ denote the unit interval and $f:Ito I$ be a Borel measurable function.
      For each $n$ let $P_n$ denote the partition of $I$ defined as
      $$P_n= left{left[0, frac{1}{2^n}right), left[frac{1}{2^n}, frac{2}{2^n}right), left[frac{2}{2^n}, frac{3}{2^n}right), ldots, left[frac{2^n-2}{2^n}, frac{2^n-1}{2^n}right],left[frac{2^n-1}{2^n}, 1right]right}$$
      and let $mathcal A_n$ be the $sigma$-algebra on $I$ generated by $P_n$.
      Define $f_n=E[f|mathcal A_n]$, where the conditional expectation is with respect to the Lebesgue measure.



      It is known by the martingale convergence theorem that $f_nto f$ pointwise a.e. and also in $L^1$.




      Question. Are there some (nice) sufficient conditions on the nature of $f$ which ensure that the convergence above is in $L^infty$ also?











      share|cite|improve this question















      Let $I$ denote the unit interval and $f:Ito I$ be a Borel measurable function.
      For each $n$ let $P_n$ denote the partition of $I$ defined as
      $$P_n= left{left[0, frac{1}{2^n}right), left[frac{1}{2^n}, frac{2}{2^n}right), left[frac{2}{2^n}, frac{3}{2^n}right), ldots, left[frac{2^n-2}{2^n}, frac{2^n-1}{2^n}right],left[frac{2^n-1}{2^n}, 1right]right}$$
      and let $mathcal A_n$ be the $sigma$-algebra on $I$ generated by $P_n$.
      Define $f_n=E[f|mathcal A_n]$, where the conditional expectation is with respect to the Lebesgue measure.



      It is known by the martingale convergence theorem that $f_nto f$ pointwise a.e. and also in $L^1$.




      Question. Are there some (nice) sufficient conditions on the nature of $f$ which ensure that the convergence above is in $L^infty$ also?








      probability-theory measure-theory convergence lp-spaces martingales






      share|cite|improve this question















      share|cite|improve this question













      share|cite|improve this question




      share|cite|improve this question








      edited Nov 26 at 14:51









      Davide Giraudo

      125k16150259




      125k16150259










      asked Nov 23 at 19:38









      caffeinemachine

      6,49121250




      6,49121250






















          1 Answer
          1






          active

          oldest

          votes


















          1














          Denote by $I_{n,j}$ the interval $left[j2^{-n},(j+1)2^{-n}right)$. Then
          $$
          f_n(x)=2^nsum_{j=0}^{2^n-1}mathbb Eleft[fmathbf 1_{I_{n,j}}right]mathbf 1_{I_{n,j}}(x), xin I.
          $$

          If $f$ is continuous on $[0,1]$, then for all $xin I_{n,j}$, $$
          2^nmathbb Eleft[fmathbf 1_{I_{n,j}}right]-f(x)=frac1{2^{-n}}int_{j2^{-n}}^{(j+1)2^{-n}}left(f(t)-f(x)right)dt$$

          hence
          $$
          leftlvert f(x)-f_n(x)rightrvertleqslant sup_{substack{s,tin [0,1]\ leftlvert t-srightrvertleqslant 2^{-n}}}leftlvert f(t)-f(s)rightrvert
          $$

          and uniform continuity allows to conclude.



          However, in general, we cannot expect a convergence in $mathbb L^infty$ in the general case. Let $t$ be an element of $I$ such that for all $ngeqslant 1$ and $0leqslant jleqslant 2^n-1$, $xneq j2^{-n} $ (for example $t$ irrational).
          Let $n$ be fixed; then for some $j_0inleft{0,dots,2^n-1right}$, $tin I_{n.j_0}$. Therefore, letting $f=mathbf 1_{[0,t]}$, we have
          $$
          f_n(x)=mathbf 1_{left[0,j_02^{-n}right)}(x)+2^nleft(t-j_02^{-n}right)mathbf 1_{I_{n,j_0}}(x), xin I.$$

          Let $J$ and $J'$ be the intervals defined respectively by $J:=left[j_02^{-n},tright)$ and $J':=left[t, (j_0+1)2^{-n}right)$. By the assumption on $t$, these intervals have positive measure and letting $u:= 2^n left(t-j_02^{-n}right)$, it follows that
          $$
          leftlVert f-f_nrightrVert_inftygeqslant maxleft{sup_{xin J}leftlvert f(x)-f_n(x)rightrvert,sup_{xin J'}leftlvert f(x)-f_n(x)rightrvert
          right}geqslant max{u,1-u}geqslant 1/2.
          $$






          share|cite|improve this answer























          • In the display equation after "If $f$ is continuous on $[0, 1]$, then" I think the RHS is supposed to be $2^nsup_{s, tin [0, 1], |t-s|leq 1/2^n} |f(t)-f(s)|$. If this is indeed the case, then one cannot appeal to the uniform continuity of $f$ to argue that the $L^infty$ convergence occurs.
            – caffeinemachine
            Dec 10 at 13:03










          • @caffeinemachine There was a typo, but now I think it is correct.
            – Davide Giraudo
            Dec 10 at 17:04










          • I see. Thanks. It looks correct now.
            – caffeinemachine
            Dec 11 at 4:46










          • Actually I just realized that it was also correct before since $2^{-n}leq 1/n$, but not accurate.
            – Davide Giraudo
            Dec 11 at 9:22











          Your Answer





          StackExchange.ifUsing("editor", function () {
          return StackExchange.using("mathjaxEditing", function () {
          StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
          StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
          });
          });
          }, "mathjax-editing");

          StackExchange.ready(function() {
          var channelOptions = {
          tags: "".split(" "),
          id: "69"
          };
          initTagRenderer("".split(" "), "".split(" "), channelOptions);

          StackExchange.using("externalEditor", function() {
          // Have to fire editor after snippets, if snippets enabled
          if (StackExchange.settings.snippets.snippetsEnabled) {
          StackExchange.using("snippets", function() {
          createEditor();
          });
          }
          else {
          createEditor();
          }
          });

          function createEditor() {
          StackExchange.prepareEditor({
          heartbeatType: 'answer',
          autoActivateHeartbeat: false,
          convertImagesToLinks: true,
          noModals: true,
          showLowRepImageUploadWarning: true,
          reputationToPostImages: 10,
          bindNavPrevention: true,
          postfix: "",
          imageUploader: {
          brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
          contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
          allowUrls: true
          },
          noCode: true, onDemand: true,
          discardSelector: ".discard-answer"
          ,immediatelyShowMarkdownHelp:true
          });


          }
          });














          draft saved

          draft discarded


















          StackExchange.ready(
          function () {
          StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3010750%2fsufficient-condition-for-l-infty-convergence-in-martingale-convergence-theore%23new-answer', 'question_page');
          }
          );

          Post as a guest















          Required, but never shown

























          1 Answer
          1






          active

          oldest

          votes








          1 Answer
          1






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          1














          Denote by $I_{n,j}$ the interval $left[j2^{-n},(j+1)2^{-n}right)$. Then
          $$
          f_n(x)=2^nsum_{j=0}^{2^n-1}mathbb Eleft[fmathbf 1_{I_{n,j}}right]mathbf 1_{I_{n,j}}(x), xin I.
          $$

          If $f$ is continuous on $[0,1]$, then for all $xin I_{n,j}$, $$
          2^nmathbb Eleft[fmathbf 1_{I_{n,j}}right]-f(x)=frac1{2^{-n}}int_{j2^{-n}}^{(j+1)2^{-n}}left(f(t)-f(x)right)dt$$

          hence
          $$
          leftlvert f(x)-f_n(x)rightrvertleqslant sup_{substack{s,tin [0,1]\ leftlvert t-srightrvertleqslant 2^{-n}}}leftlvert f(t)-f(s)rightrvert
          $$

          and uniform continuity allows to conclude.



          However, in general, we cannot expect a convergence in $mathbb L^infty$ in the general case. Let $t$ be an element of $I$ such that for all $ngeqslant 1$ and $0leqslant jleqslant 2^n-1$, $xneq j2^{-n} $ (for example $t$ irrational).
          Let $n$ be fixed; then for some $j_0inleft{0,dots,2^n-1right}$, $tin I_{n.j_0}$. Therefore, letting $f=mathbf 1_{[0,t]}$, we have
          $$
          f_n(x)=mathbf 1_{left[0,j_02^{-n}right)}(x)+2^nleft(t-j_02^{-n}right)mathbf 1_{I_{n,j_0}}(x), xin I.$$

          Let $J$ and $J'$ be the intervals defined respectively by $J:=left[j_02^{-n},tright)$ and $J':=left[t, (j_0+1)2^{-n}right)$. By the assumption on $t$, these intervals have positive measure and letting $u:= 2^n left(t-j_02^{-n}right)$, it follows that
          $$
          leftlVert f-f_nrightrVert_inftygeqslant maxleft{sup_{xin J}leftlvert f(x)-f_n(x)rightrvert,sup_{xin J'}leftlvert f(x)-f_n(x)rightrvert
          right}geqslant max{u,1-u}geqslant 1/2.
          $$






          share|cite|improve this answer























          • In the display equation after "If $f$ is continuous on $[0, 1]$, then" I think the RHS is supposed to be $2^nsup_{s, tin [0, 1], |t-s|leq 1/2^n} |f(t)-f(s)|$. If this is indeed the case, then one cannot appeal to the uniform continuity of $f$ to argue that the $L^infty$ convergence occurs.
            – caffeinemachine
            Dec 10 at 13:03










          • @caffeinemachine There was a typo, but now I think it is correct.
            – Davide Giraudo
            Dec 10 at 17:04










          • I see. Thanks. It looks correct now.
            – caffeinemachine
            Dec 11 at 4:46










          • Actually I just realized that it was also correct before since $2^{-n}leq 1/n$, but not accurate.
            – Davide Giraudo
            Dec 11 at 9:22
















          1














          Denote by $I_{n,j}$ the interval $left[j2^{-n},(j+1)2^{-n}right)$. Then
          $$
          f_n(x)=2^nsum_{j=0}^{2^n-1}mathbb Eleft[fmathbf 1_{I_{n,j}}right]mathbf 1_{I_{n,j}}(x), xin I.
          $$

          If $f$ is continuous on $[0,1]$, then for all $xin I_{n,j}$, $$
          2^nmathbb Eleft[fmathbf 1_{I_{n,j}}right]-f(x)=frac1{2^{-n}}int_{j2^{-n}}^{(j+1)2^{-n}}left(f(t)-f(x)right)dt$$

          hence
          $$
          leftlvert f(x)-f_n(x)rightrvertleqslant sup_{substack{s,tin [0,1]\ leftlvert t-srightrvertleqslant 2^{-n}}}leftlvert f(t)-f(s)rightrvert
          $$

          and uniform continuity allows to conclude.



          However, in general, we cannot expect a convergence in $mathbb L^infty$ in the general case. Let $t$ be an element of $I$ such that for all $ngeqslant 1$ and $0leqslant jleqslant 2^n-1$, $xneq j2^{-n} $ (for example $t$ irrational).
          Let $n$ be fixed; then for some $j_0inleft{0,dots,2^n-1right}$, $tin I_{n.j_0}$. Therefore, letting $f=mathbf 1_{[0,t]}$, we have
          $$
          f_n(x)=mathbf 1_{left[0,j_02^{-n}right)}(x)+2^nleft(t-j_02^{-n}right)mathbf 1_{I_{n,j_0}}(x), xin I.$$

          Let $J$ and $J'$ be the intervals defined respectively by $J:=left[j_02^{-n},tright)$ and $J':=left[t, (j_0+1)2^{-n}right)$. By the assumption on $t$, these intervals have positive measure and letting $u:= 2^n left(t-j_02^{-n}right)$, it follows that
          $$
          leftlVert f-f_nrightrVert_inftygeqslant maxleft{sup_{xin J}leftlvert f(x)-f_n(x)rightrvert,sup_{xin J'}leftlvert f(x)-f_n(x)rightrvert
          right}geqslant max{u,1-u}geqslant 1/2.
          $$






          share|cite|improve this answer























          • In the display equation after "If $f$ is continuous on $[0, 1]$, then" I think the RHS is supposed to be $2^nsup_{s, tin [0, 1], |t-s|leq 1/2^n} |f(t)-f(s)|$. If this is indeed the case, then one cannot appeal to the uniform continuity of $f$ to argue that the $L^infty$ convergence occurs.
            – caffeinemachine
            Dec 10 at 13:03










          • @caffeinemachine There was a typo, but now I think it is correct.
            – Davide Giraudo
            Dec 10 at 17:04










          • I see. Thanks. It looks correct now.
            – caffeinemachine
            Dec 11 at 4:46










          • Actually I just realized that it was also correct before since $2^{-n}leq 1/n$, but not accurate.
            – Davide Giraudo
            Dec 11 at 9:22














          1












          1








          1






          Denote by $I_{n,j}$ the interval $left[j2^{-n},(j+1)2^{-n}right)$. Then
          $$
          f_n(x)=2^nsum_{j=0}^{2^n-1}mathbb Eleft[fmathbf 1_{I_{n,j}}right]mathbf 1_{I_{n,j}}(x), xin I.
          $$

          If $f$ is continuous on $[0,1]$, then for all $xin I_{n,j}$, $$
          2^nmathbb Eleft[fmathbf 1_{I_{n,j}}right]-f(x)=frac1{2^{-n}}int_{j2^{-n}}^{(j+1)2^{-n}}left(f(t)-f(x)right)dt$$

          hence
          $$
          leftlvert f(x)-f_n(x)rightrvertleqslant sup_{substack{s,tin [0,1]\ leftlvert t-srightrvertleqslant 2^{-n}}}leftlvert f(t)-f(s)rightrvert
          $$

          and uniform continuity allows to conclude.



          However, in general, we cannot expect a convergence in $mathbb L^infty$ in the general case. Let $t$ be an element of $I$ such that for all $ngeqslant 1$ and $0leqslant jleqslant 2^n-1$, $xneq j2^{-n} $ (for example $t$ irrational).
          Let $n$ be fixed; then for some $j_0inleft{0,dots,2^n-1right}$, $tin I_{n.j_0}$. Therefore, letting $f=mathbf 1_{[0,t]}$, we have
          $$
          f_n(x)=mathbf 1_{left[0,j_02^{-n}right)}(x)+2^nleft(t-j_02^{-n}right)mathbf 1_{I_{n,j_0}}(x), xin I.$$

          Let $J$ and $J'$ be the intervals defined respectively by $J:=left[j_02^{-n},tright)$ and $J':=left[t, (j_0+1)2^{-n}right)$. By the assumption on $t$, these intervals have positive measure and letting $u:= 2^n left(t-j_02^{-n}right)$, it follows that
          $$
          leftlVert f-f_nrightrVert_inftygeqslant maxleft{sup_{xin J}leftlvert f(x)-f_n(x)rightrvert,sup_{xin J'}leftlvert f(x)-f_n(x)rightrvert
          right}geqslant max{u,1-u}geqslant 1/2.
          $$






          share|cite|improve this answer














          Denote by $I_{n,j}$ the interval $left[j2^{-n},(j+1)2^{-n}right)$. Then
          $$
          f_n(x)=2^nsum_{j=0}^{2^n-1}mathbb Eleft[fmathbf 1_{I_{n,j}}right]mathbf 1_{I_{n,j}}(x), xin I.
          $$

          If $f$ is continuous on $[0,1]$, then for all $xin I_{n,j}$, $$
          2^nmathbb Eleft[fmathbf 1_{I_{n,j}}right]-f(x)=frac1{2^{-n}}int_{j2^{-n}}^{(j+1)2^{-n}}left(f(t)-f(x)right)dt$$

          hence
          $$
          leftlvert f(x)-f_n(x)rightrvertleqslant sup_{substack{s,tin [0,1]\ leftlvert t-srightrvertleqslant 2^{-n}}}leftlvert f(t)-f(s)rightrvert
          $$

          and uniform continuity allows to conclude.



          However, in general, we cannot expect a convergence in $mathbb L^infty$ in the general case. Let $t$ be an element of $I$ such that for all $ngeqslant 1$ and $0leqslant jleqslant 2^n-1$, $xneq j2^{-n} $ (for example $t$ irrational).
          Let $n$ be fixed; then for some $j_0inleft{0,dots,2^n-1right}$, $tin I_{n.j_0}$. Therefore, letting $f=mathbf 1_{[0,t]}$, we have
          $$
          f_n(x)=mathbf 1_{left[0,j_02^{-n}right)}(x)+2^nleft(t-j_02^{-n}right)mathbf 1_{I_{n,j_0}}(x), xin I.$$

          Let $J$ and $J'$ be the intervals defined respectively by $J:=left[j_02^{-n},tright)$ and $J':=left[t, (j_0+1)2^{-n}right)$. By the assumption on $t$, these intervals have positive measure and letting $u:= 2^n left(t-j_02^{-n}right)$, it follows that
          $$
          leftlVert f-f_nrightrVert_inftygeqslant maxleft{sup_{xin J}leftlvert f(x)-f_n(x)rightrvert,sup_{xin J'}leftlvert f(x)-f_n(x)rightrvert
          right}geqslant max{u,1-u}geqslant 1/2.
          $$







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited Dec 10 at 17:03

























          answered Nov 26 at 10:04









          Davide Giraudo

          125k16150259




          125k16150259












          • In the display equation after "If $f$ is continuous on $[0, 1]$, then" I think the RHS is supposed to be $2^nsup_{s, tin [0, 1], |t-s|leq 1/2^n} |f(t)-f(s)|$. If this is indeed the case, then one cannot appeal to the uniform continuity of $f$ to argue that the $L^infty$ convergence occurs.
            – caffeinemachine
            Dec 10 at 13:03










          • @caffeinemachine There was a typo, but now I think it is correct.
            – Davide Giraudo
            Dec 10 at 17:04










          • I see. Thanks. It looks correct now.
            – caffeinemachine
            Dec 11 at 4:46










          • Actually I just realized that it was also correct before since $2^{-n}leq 1/n$, but not accurate.
            – Davide Giraudo
            Dec 11 at 9:22


















          • In the display equation after "If $f$ is continuous on $[0, 1]$, then" I think the RHS is supposed to be $2^nsup_{s, tin [0, 1], |t-s|leq 1/2^n} |f(t)-f(s)|$. If this is indeed the case, then one cannot appeal to the uniform continuity of $f$ to argue that the $L^infty$ convergence occurs.
            – caffeinemachine
            Dec 10 at 13:03










          • @caffeinemachine There was a typo, but now I think it is correct.
            – Davide Giraudo
            Dec 10 at 17:04










          • I see. Thanks. It looks correct now.
            – caffeinemachine
            Dec 11 at 4:46










          • Actually I just realized that it was also correct before since $2^{-n}leq 1/n$, but not accurate.
            – Davide Giraudo
            Dec 11 at 9:22
















          In the display equation after "If $f$ is continuous on $[0, 1]$, then" I think the RHS is supposed to be $2^nsup_{s, tin [0, 1], |t-s|leq 1/2^n} |f(t)-f(s)|$. If this is indeed the case, then one cannot appeal to the uniform continuity of $f$ to argue that the $L^infty$ convergence occurs.
          – caffeinemachine
          Dec 10 at 13:03




          In the display equation after "If $f$ is continuous on $[0, 1]$, then" I think the RHS is supposed to be $2^nsup_{s, tin [0, 1], |t-s|leq 1/2^n} |f(t)-f(s)|$. If this is indeed the case, then one cannot appeal to the uniform continuity of $f$ to argue that the $L^infty$ convergence occurs.
          – caffeinemachine
          Dec 10 at 13:03












          @caffeinemachine There was a typo, but now I think it is correct.
          – Davide Giraudo
          Dec 10 at 17:04




          @caffeinemachine There was a typo, but now I think it is correct.
          – Davide Giraudo
          Dec 10 at 17:04












          I see. Thanks. It looks correct now.
          – caffeinemachine
          Dec 11 at 4:46




          I see. Thanks. It looks correct now.
          – caffeinemachine
          Dec 11 at 4:46












          Actually I just realized that it was also correct before since $2^{-n}leq 1/n$, but not accurate.
          – Davide Giraudo
          Dec 11 at 9:22




          Actually I just realized that it was also correct before since $2^{-n}leq 1/n$, but not accurate.
          – Davide Giraudo
          Dec 11 at 9:22


















          draft saved

          draft discarded




















































          Thanks for contributing an answer to Mathematics Stack Exchange!


          • Please be sure to answer the question. Provide details and share your research!

          But avoid



          • Asking for help, clarification, or responding to other answers.

          • Making statements based on opinion; back them up with references or personal experience.


          Use MathJax to format equations. MathJax reference.


          To learn more, see our tips on writing great answers.





          Some of your past answers have not been well-received, and you're in danger of being blocked from answering.


          Please pay close attention to the following guidance:


          • Please be sure to answer the question. Provide details and share your research!

          But avoid



          • Asking for help, clarification, or responding to other answers.

          • Making statements based on opinion; back them up with references or personal experience.


          To learn more, see our tips on writing great answers.




          draft saved


          draft discarded














          StackExchange.ready(
          function () {
          StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3010750%2fsufficient-condition-for-l-infty-convergence-in-martingale-convergence-theore%23new-answer', 'question_page');
          }
          );

          Post as a guest















          Required, but never shown





















































          Required, but never shown














          Required, but never shown












          Required, but never shown







          Required, but never shown

































          Required, but never shown














          Required, but never shown












          Required, but never shown







          Required, but never shown







          Popular posts from this blog

          Plaza Victoria

          Puebla de Zaragoza

          Musa