Rewrite $ int_{mathcal{S}}dP_X=1 $ as conditions on boxes in $mathbb{R}^d$












5












$begingroup$


Take $rin mathbb{N}$ and let $dequiv r+binom{r}{2}$.



Consider a d-dimensional random vector $Xequiv (X_1,...,X_d)$. Let $P_X$ be the probability distribution of $X$. Assume that
$$
int_{mathcal{S}}dP_X=1
$$

where
$$
begin{aligned}
mathcal{S}equiv {(b_1,b_2,..., b_d)in mathbb{R}^{d}: text{ } & b_{r+1}=b_1-b_2, b_{r+2}=b_1-b_3, ...,b_{2r-1}=b_1-b_r, \
&b_{2r}=b_2-b_3, ..., b_{3r-3}=b_2-b_r,\
&...,\
& b_d=b_{r-1}-b_r}
end{aligned}
$$

For example, when $r=2$ ($d=3$) we have the surface
$$
begin{aligned}
mathcal{S}equiv {(b_1,b_2,b_3)in mathbb{R}^{3}: text{ } & b_3=b_1-b_2}={(b_1,b_2,b_3)in mathbb{R}^{3}: text{ } & b_1=b_2+b_3}
end{aligned}
$$

When $r=3$ ($d=6$) we have
$$
begin{aligned}
mathcal{S}equiv {(b_1,..., b_6)in mathbb{R}^{6}: text{ } & b_4=b_1-b_2, b_5=b_1-b_3, b_6=b_2-b_3}
end{aligned}
$$



My final goal: I'm interested in rewriting the condition $int_{mathcal{S}}dP_X=1$ as a collection of zero probability measure conditions on d-dimensional "boxes" in $mathbb{R}^d$. The idea is that any box in $mathbb{R}^d$ not intersecting $mathcal{S}$ should have probability measure equal to zero. Therefore, if we consider enough of these boxes, we should be able to equivalently rewrite $int_{mathcal{S}}dP_X=1$.





When $r=2$ ($d=3$), my goal is achieved by the following claim



Claim: For any two real numbers $(b,c)in mathbb{R}^2$, define the boxes $$B(b,c)equiv {(x,y,z)text{ s.t. } x> b+c, yleq b, zleq c}$$ and $$Q(b,c)equiv {(x,y,z)text{ s.t. } xleq b+c, y>b, z>c}$$



If $P_{X}(B(b,c))=0$ and $P_{X}(Q(b,c))=0$ $forall(b,c)in mathbb{Q}^2$, then $int_{mathcal{S}}dP_{X}=1$.



The proof of the claim is provided here





I would like your help to generalise the claim (and possibly the proof) to any $r$. What I find challenging is defining the relevant boxes for any $r>2$. I really can't see how to generalise the box definitions from $r=2$ to any $r$.










share|cite|improve this question











$endgroup$

















    5












    $begingroup$


    Take $rin mathbb{N}$ and let $dequiv r+binom{r}{2}$.



    Consider a d-dimensional random vector $Xequiv (X_1,...,X_d)$. Let $P_X$ be the probability distribution of $X$. Assume that
    $$
    int_{mathcal{S}}dP_X=1
    $$

    where
    $$
    begin{aligned}
    mathcal{S}equiv {(b_1,b_2,..., b_d)in mathbb{R}^{d}: text{ } & b_{r+1}=b_1-b_2, b_{r+2}=b_1-b_3, ...,b_{2r-1}=b_1-b_r, \
    &b_{2r}=b_2-b_3, ..., b_{3r-3}=b_2-b_r,\
    &...,\
    & b_d=b_{r-1}-b_r}
    end{aligned}
    $$

    For example, when $r=2$ ($d=3$) we have the surface
    $$
    begin{aligned}
    mathcal{S}equiv {(b_1,b_2,b_3)in mathbb{R}^{3}: text{ } & b_3=b_1-b_2}={(b_1,b_2,b_3)in mathbb{R}^{3}: text{ } & b_1=b_2+b_3}
    end{aligned}
    $$

    When $r=3$ ($d=6$) we have
    $$
    begin{aligned}
    mathcal{S}equiv {(b_1,..., b_6)in mathbb{R}^{6}: text{ } & b_4=b_1-b_2, b_5=b_1-b_3, b_6=b_2-b_3}
    end{aligned}
    $$



    My final goal: I'm interested in rewriting the condition $int_{mathcal{S}}dP_X=1$ as a collection of zero probability measure conditions on d-dimensional "boxes" in $mathbb{R}^d$. The idea is that any box in $mathbb{R}^d$ not intersecting $mathcal{S}$ should have probability measure equal to zero. Therefore, if we consider enough of these boxes, we should be able to equivalently rewrite $int_{mathcal{S}}dP_X=1$.





    When $r=2$ ($d=3$), my goal is achieved by the following claim



    Claim: For any two real numbers $(b,c)in mathbb{R}^2$, define the boxes $$B(b,c)equiv {(x,y,z)text{ s.t. } x> b+c, yleq b, zleq c}$$ and $$Q(b,c)equiv {(x,y,z)text{ s.t. } xleq b+c, y>b, z>c}$$



    If $P_{X}(B(b,c))=0$ and $P_{X}(Q(b,c))=0$ $forall(b,c)in mathbb{Q}^2$, then $int_{mathcal{S}}dP_{X}=1$.



    The proof of the claim is provided here





    I would like your help to generalise the claim (and possibly the proof) to any $r$. What I find challenging is defining the relevant boxes for any $r>2$. I really can't see how to generalise the box definitions from $r=2$ to any $r$.










    share|cite|improve this question











    $endgroup$















      5












      5








      5


      1



      $begingroup$


      Take $rin mathbb{N}$ and let $dequiv r+binom{r}{2}$.



      Consider a d-dimensional random vector $Xequiv (X_1,...,X_d)$. Let $P_X$ be the probability distribution of $X$. Assume that
      $$
      int_{mathcal{S}}dP_X=1
      $$

      where
      $$
      begin{aligned}
      mathcal{S}equiv {(b_1,b_2,..., b_d)in mathbb{R}^{d}: text{ } & b_{r+1}=b_1-b_2, b_{r+2}=b_1-b_3, ...,b_{2r-1}=b_1-b_r, \
      &b_{2r}=b_2-b_3, ..., b_{3r-3}=b_2-b_r,\
      &...,\
      & b_d=b_{r-1}-b_r}
      end{aligned}
      $$

      For example, when $r=2$ ($d=3$) we have the surface
      $$
      begin{aligned}
      mathcal{S}equiv {(b_1,b_2,b_3)in mathbb{R}^{3}: text{ } & b_3=b_1-b_2}={(b_1,b_2,b_3)in mathbb{R}^{3}: text{ } & b_1=b_2+b_3}
      end{aligned}
      $$

      When $r=3$ ($d=6$) we have
      $$
      begin{aligned}
      mathcal{S}equiv {(b_1,..., b_6)in mathbb{R}^{6}: text{ } & b_4=b_1-b_2, b_5=b_1-b_3, b_6=b_2-b_3}
      end{aligned}
      $$



      My final goal: I'm interested in rewriting the condition $int_{mathcal{S}}dP_X=1$ as a collection of zero probability measure conditions on d-dimensional "boxes" in $mathbb{R}^d$. The idea is that any box in $mathbb{R}^d$ not intersecting $mathcal{S}$ should have probability measure equal to zero. Therefore, if we consider enough of these boxes, we should be able to equivalently rewrite $int_{mathcal{S}}dP_X=1$.





      When $r=2$ ($d=3$), my goal is achieved by the following claim



      Claim: For any two real numbers $(b,c)in mathbb{R}^2$, define the boxes $$B(b,c)equiv {(x,y,z)text{ s.t. } x> b+c, yleq b, zleq c}$$ and $$Q(b,c)equiv {(x,y,z)text{ s.t. } xleq b+c, y>b, z>c}$$



      If $P_{X}(B(b,c))=0$ and $P_{X}(Q(b,c))=0$ $forall(b,c)in mathbb{Q}^2$, then $int_{mathcal{S}}dP_{X}=1$.



      The proof of the claim is provided here





      I would like your help to generalise the claim (and possibly the proof) to any $r$. What I find challenging is defining the relevant boxes for any $r>2$. I really can't see how to generalise the box definitions from $r=2$ to any $r$.










      share|cite|improve this question











      $endgroup$




      Take $rin mathbb{N}$ and let $dequiv r+binom{r}{2}$.



      Consider a d-dimensional random vector $Xequiv (X_1,...,X_d)$. Let $P_X$ be the probability distribution of $X$. Assume that
      $$
      int_{mathcal{S}}dP_X=1
      $$

      where
      $$
      begin{aligned}
      mathcal{S}equiv {(b_1,b_2,..., b_d)in mathbb{R}^{d}: text{ } & b_{r+1}=b_1-b_2, b_{r+2}=b_1-b_3, ...,b_{2r-1}=b_1-b_r, \
      &b_{2r}=b_2-b_3, ..., b_{3r-3}=b_2-b_r,\
      &...,\
      & b_d=b_{r-1}-b_r}
      end{aligned}
      $$

      For example, when $r=2$ ($d=3$) we have the surface
      $$
      begin{aligned}
      mathcal{S}equiv {(b_1,b_2,b_3)in mathbb{R}^{3}: text{ } & b_3=b_1-b_2}={(b_1,b_2,b_3)in mathbb{R}^{3}: text{ } & b_1=b_2+b_3}
      end{aligned}
      $$

      When $r=3$ ($d=6$) we have
      $$
      begin{aligned}
      mathcal{S}equiv {(b_1,..., b_6)in mathbb{R}^{6}: text{ } & b_4=b_1-b_2, b_5=b_1-b_3, b_6=b_2-b_3}
      end{aligned}
      $$



      My final goal: I'm interested in rewriting the condition $int_{mathcal{S}}dP_X=1$ as a collection of zero probability measure conditions on d-dimensional "boxes" in $mathbb{R}^d$. The idea is that any box in $mathbb{R}^d$ not intersecting $mathcal{S}$ should have probability measure equal to zero. Therefore, if we consider enough of these boxes, we should be able to equivalently rewrite $int_{mathcal{S}}dP_X=1$.





      When $r=2$ ($d=3$), my goal is achieved by the following claim



      Claim: For any two real numbers $(b,c)in mathbb{R}^2$, define the boxes $$B(b,c)equiv {(x,y,z)text{ s.t. } x> b+c, yleq b, zleq c}$$ and $$Q(b,c)equiv {(x,y,z)text{ s.t. } xleq b+c, y>b, z>c}$$



      If $P_{X}(B(b,c))=0$ and $P_{X}(Q(b,c))=0$ $forall(b,c)in mathbb{Q}^2$, then $int_{mathcal{S}}dP_{X}=1$.



      The proof of the claim is provided here





      I would like your help to generalise the claim (and possibly the proof) to any $r$. What I find challenging is defining the relevant boxes for any $r>2$. I really can't see how to generalise the box definitions from $r=2$ to any $r$.







      probability geometry probability-theory measure-theory lebesgue-measure






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      share|cite|improve this question













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      edited Jan 28 at 14:11







      STF

















      asked Jan 20 at 21:20









      STFSTF

      431422




      431422






















          1 Answer
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          active

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          1












          $begingroup$

          I try to post an answer. It is just an attempt, aiming to encourage comments from your side. The attempt mimics for any $r$ the claim and the proof provided here for the case $r=2$ ($d=3$).





          Claim



          For any $(bar{b}, tilde{b})in mathbb{R}^2$, consider the $d$-dimensional boxes in $mathbb{R}^d$
          $$
          B_{t,p,q}(bar{b}, tilde{b})equiv {(z_1,...,z_d)in mathbb{R}^d text{: } z_p leq bar{b}, text{ } z_qleq tilde{b}, text{ } z_t>bar{b}+tilde{b} }
          $$

          and
          $$
          Q_{t,p,q}(bar{b}, tilde{b})equiv {(z_1,...,z_d)in mathbb{R}^d text{: } z_p >bar{b}, text{ } z_q> tilde{b}, text{ } z_tleq bar{b}+tilde{b} }
          $$

          $forall t in {1,...,r-1}$ and $forall (p,q)in {(t+1,r), (t+2, r+1),...,(r, d)}$.
          Let $mathbb{Q}$ denote the set of rational numbers.
          If
          begin{equation}
          label{integral}
          begin{aligned}
          P_{X}(B_{t,p,q}(bar{b}, tilde{b}))=& P_{X}(Q_{t,p,q}(bar{b}, tilde{b}))=0 \
          & text{$forall t in {1,...,r-1}$, $forall (p,q)in {(t+1,r), (t+2, r+1),...,(r, d)}$}
          end{aligned}
          end{equation}

          $forall (b, tilde{b})in mathbb{Q}^{2}$, then $P_{X}(mathcal{S})=1$.





          Proof



          Step 1: Using the fact that $mathbb{Q}$ is dense in $mathbb{R}$, we can show that



          $$
          mathcal{S}^c= overbrace{bigcup_{(bar{b}, tilde{b})in mathbb{Q}^2} Big{bigcup_{substack{text{$tin{1,...,r_1}$} \ text{$(p,q)in {(t+1,r),...,(r,d)}$}}}{B_{t,q,p}(bar{b}, tilde{b}) cup Q_{t,q,p}(bar{b}, tilde{b})}Big}}^{equiv A}
          $$

          where $mathcal{S}^c$ denotes the complement of $mathcal{S}$.



          Step 2: Therefore
          $$
          mathbb{P}(A)=0 Leftrightarrow mathbb{P}(mathcal{S})=1
          $$

          from which the conclusion of the claim follows.






          share|cite|improve this answer











          $endgroup$













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            active

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            1












            $begingroup$

            I try to post an answer. It is just an attempt, aiming to encourage comments from your side. The attempt mimics for any $r$ the claim and the proof provided here for the case $r=2$ ($d=3$).





            Claim



            For any $(bar{b}, tilde{b})in mathbb{R}^2$, consider the $d$-dimensional boxes in $mathbb{R}^d$
            $$
            B_{t,p,q}(bar{b}, tilde{b})equiv {(z_1,...,z_d)in mathbb{R}^d text{: } z_p leq bar{b}, text{ } z_qleq tilde{b}, text{ } z_t>bar{b}+tilde{b} }
            $$

            and
            $$
            Q_{t,p,q}(bar{b}, tilde{b})equiv {(z_1,...,z_d)in mathbb{R}^d text{: } z_p >bar{b}, text{ } z_q> tilde{b}, text{ } z_tleq bar{b}+tilde{b} }
            $$

            $forall t in {1,...,r-1}$ and $forall (p,q)in {(t+1,r), (t+2, r+1),...,(r, d)}$.
            Let $mathbb{Q}$ denote the set of rational numbers.
            If
            begin{equation}
            label{integral}
            begin{aligned}
            P_{X}(B_{t,p,q}(bar{b}, tilde{b}))=& P_{X}(Q_{t,p,q}(bar{b}, tilde{b}))=0 \
            & text{$forall t in {1,...,r-1}$, $forall (p,q)in {(t+1,r), (t+2, r+1),...,(r, d)}$}
            end{aligned}
            end{equation}

            $forall (b, tilde{b})in mathbb{Q}^{2}$, then $P_{X}(mathcal{S})=1$.





            Proof



            Step 1: Using the fact that $mathbb{Q}$ is dense in $mathbb{R}$, we can show that



            $$
            mathcal{S}^c= overbrace{bigcup_{(bar{b}, tilde{b})in mathbb{Q}^2} Big{bigcup_{substack{text{$tin{1,...,r_1}$} \ text{$(p,q)in {(t+1,r),...,(r,d)}$}}}{B_{t,q,p}(bar{b}, tilde{b}) cup Q_{t,q,p}(bar{b}, tilde{b})}Big}}^{equiv A}
            $$

            where $mathcal{S}^c$ denotes the complement of $mathcal{S}$.



            Step 2: Therefore
            $$
            mathbb{P}(A)=0 Leftrightarrow mathbb{P}(mathcal{S})=1
            $$

            from which the conclusion of the claim follows.






            share|cite|improve this answer











            $endgroup$


















              1












              $begingroup$

              I try to post an answer. It is just an attempt, aiming to encourage comments from your side. The attempt mimics for any $r$ the claim and the proof provided here for the case $r=2$ ($d=3$).





              Claim



              For any $(bar{b}, tilde{b})in mathbb{R}^2$, consider the $d$-dimensional boxes in $mathbb{R}^d$
              $$
              B_{t,p,q}(bar{b}, tilde{b})equiv {(z_1,...,z_d)in mathbb{R}^d text{: } z_p leq bar{b}, text{ } z_qleq tilde{b}, text{ } z_t>bar{b}+tilde{b} }
              $$

              and
              $$
              Q_{t,p,q}(bar{b}, tilde{b})equiv {(z_1,...,z_d)in mathbb{R}^d text{: } z_p >bar{b}, text{ } z_q> tilde{b}, text{ } z_tleq bar{b}+tilde{b} }
              $$

              $forall t in {1,...,r-1}$ and $forall (p,q)in {(t+1,r), (t+2, r+1),...,(r, d)}$.
              Let $mathbb{Q}$ denote the set of rational numbers.
              If
              begin{equation}
              label{integral}
              begin{aligned}
              P_{X}(B_{t,p,q}(bar{b}, tilde{b}))=& P_{X}(Q_{t,p,q}(bar{b}, tilde{b}))=0 \
              & text{$forall t in {1,...,r-1}$, $forall (p,q)in {(t+1,r), (t+2, r+1),...,(r, d)}$}
              end{aligned}
              end{equation}

              $forall (b, tilde{b})in mathbb{Q}^{2}$, then $P_{X}(mathcal{S})=1$.





              Proof



              Step 1: Using the fact that $mathbb{Q}$ is dense in $mathbb{R}$, we can show that



              $$
              mathcal{S}^c= overbrace{bigcup_{(bar{b}, tilde{b})in mathbb{Q}^2} Big{bigcup_{substack{text{$tin{1,...,r_1}$} \ text{$(p,q)in {(t+1,r),...,(r,d)}$}}}{B_{t,q,p}(bar{b}, tilde{b}) cup Q_{t,q,p}(bar{b}, tilde{b})}Big}}^{equiv A}
              $$

              where $mathcal{S}^c$ denotes the complement of $mathcal{S}$.



              Step 2: Therefore
              $$
              mathbb{P}(A)=0 Leftrightarrow mathbb{P}(mathcal{S})=1
              $$

              from which the conclusion of the claim follows.






              share|cite|improve this answer











              $endgroup$
















                1












                1








                1





                $begingroup$

                I try to post an answer. It is just an attempt, aiming to encourage comments from your side. The attempt mimics for any $r$ the claim and the proof provided here for the case $r=2$ ($d=3$).





                Claim



                For any $(bar{b}, tilde{b})in mathbb{R}^2$, consider the $d$-dimensional boxes in $mathbb{R}^d$
                $$
                B_{t,p,q}(bar{b}, tilde{b})equiv {(z_1,...,z_d)in mathbb{R}^d text{: } z_p leq bar{b}, text{ } z_qleq tilde{b}, text{ } z_t>bar{b}+tilde{b} }
                $$

                and
                $$
                Q_{t,p,q}(bar{b}, tilde{b})equiv {(z_1,...,z_d)in mathbb{R}^d text{: } z_p >bar{b}, text{ } z_q> tilde{b}, text{ } z_tleq bar{b}+tilde{b} }
                $$

                $forall t in {1,...,r-1}$ and $forall (p,q)in {(t+1,r), (t+2, r+1),...,(r, d)}$.
                Let $mathbb{Q}$ denote the set of rational numbers.
                If
                begin{equation}
                label{integral}
                begin{aligned}
                P_{X}(B_{t,p,q}(bar{b}, tilde{b}))=& P_{X}(Q_{t,p,q}(bar{b}, tilde{b}))=0 \
                & text{$forall t in {1,...,r-1}$, $forall (p,q)in {(t+1,r), (t+2, r+1),...,(r, d)}$}
                end{aligned}
                end{equation}

                $forall (b, tilde{b})in mathbb{Q}^{2}$, then $P_{X}(mathcal{S})=1$.





                Proof



                Step 1: Using the fact that $mathbb{Q}$ is dense in $mathbb{R}$, we can show that



                $$
                mathcal{S}^c= overbrace{bigcup_{(bar{b}, tilde{b})in mathbb{Q}^2} Big{bigcup_{substack{text{$tin{1,...,r_1}$} \ text{$(p,q)in {(t+1,r),...,(r,d)}$}}}{B_{t,q,p}(bar{b}, tilde{b}) cup Q_{t,q,p}(bar{b}, tilde{b})}Big}}^{equiv A}
                $$

                where $mathcal{S}^c$ denotes the complement of $mathcal{S}$.



                Step 2: Therefore
                $$
                mathbb{P}(A)=0 Leftrightarrow mathbb{P}(mathcal{S})=1
                $$

                from which the conclusion of the claim follows.






                share|cite|improve this answer











                $endgroup$



                I try to post an answer. It is just an attempt, aiming to encourage comments from your side. The attempt mimics for any $r$ the claim and the proof provided here for the case $r=2$ ($d=3$).





                Claim



                For any $(bar{b}, tilde{b})in mathbb{R}^2$, consider the $d$-dimensional boxes in $mathbb{R}^d$
                $$
                B_{t,p,q}(bar{b}, tilde{b})equiv {(z_1,...,z_d)in mathbb{R}^d text{: } z_p leq bar{b}, text{ } z_qleq tilde{b}, text{ } z_t>bar{b}+tilde{b} }
                $$

                and
                $$
                Q_{t,p,q}(bar{b}, tilde{b})equiv {(z_1,...,z_d)in mathbb{R}^d text{: } z_p >bar{b}, text{ } z_q> tilde{b}, text{ } z_tleq bar{b}+tilde{b} }
                $$

                $forall t in {1,...,r-1}$ and $forall (p,q)in {(t+1,r), (t+2, r+1),...,(r, d)}$.
                Let $mathbb{Q}$ denote the set of rational numbers.
                If
                begin{equation}
                label{integral}
                begin{aligned}
                P_{X}(B_{t,p,q}(bar{b}, tilde{b}))=& P_{X}(Q_{t,p,q}(bar{b}, tilde{b}))=0 \
                & text{$forall t in {1,...,r-1}$, $forall (p,q)in {(t+1,r), (t+2, r+1),...,(r, d)}$}
                end{aligned}
                end{equation}

                $forall (b, tilde{b})in mathbb{Q}^{2}$, then $P_{X}(mathcal{S})=1$.





                Proof



                Step 1: Using the fact that $mathbb{Q}$ is dense in $mathbb{R}$, we can show that



                $$
                mathcal{S}^c= overbrace{bigcup_{(bar{b}, tilde{b})in mathbb{Q}^2} Big{bigcup_{substack{text{$tin{1,...,r_1}$} \ text{$(p,q)in {(t+1,r),...,(r,d)}$}}}{B_{t,q,p}(bar{b}, tilde{b}) cup Q_{t,q,p}(bar{b}, tilde{b})}Big}}^{equiv A}
                $$

                where $mathcal{S}^c$ denotes the complement of $mathcal{S}$.



                Step 2: Therefore
                $$
                mathbb{P}(A)=0 Leftrightarrow mathbb{P}(mathcal{S})=1
                $$

                from which the conclusion of the claim follows.







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                edited Jan 28 at 15:44

























                answered Jan 28 at 14:35









                STFSTF

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