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Inequality of vector norms with projections



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1












$begingroup$


Let $W$ be a vector subspace of $V$, a space with a dot product; $vin V$. Let $p_W(v)$ be the orthogonal projection of $v$ onto $W$ and $win W, wneq p_W(v)$.



How can i prove that $||v-w|| > ||v-p_W(v)||$?










share|cite|improve this question











$endgroup$












  • $begingroup$
    Pray tell, what is $w'$?
    $endgroup$
    – Robert Lewis
    Mar 16 at 17:49






  • 1




    $begingroup$
    My bad, I meant $w$
    $endgroup$
    – Noé Duarte González
    Mar 16 at 17:55










  • $begingroup$
    Thanks for the correction!
    $endgroup$
    – Robert Lewis
    Mar 16 at 18:06
















1












$begingroup$


Let $W$ be a vector subspace of $V$, a space with a dot product; $vin V$. Let $p_W(v)$ be the orthogonal projection of $v$ onto $W$ and $win W, wneq p_W(v)$.



How can i prove that $||v-w|| > ||v-p_W(v)||$?










share|cite|improve this question











$endgroup$












  • $begingroup$
    Pray tell, what is $w'$?
    $endgroup$
    – Robert Lewis
    Mar 16 at 17:49






  • 1




    $begingroup$
    My bad, I meant $w$
    $endgroup$
    – Noé Duarte González
    Mar 16 at 17:55










  • $begingroup$
    Thanks for the correction!
    $endgroup$
    – Robert Lewis
    Mar 16 at 18:06














1












1








1


1



$begingroup$


Let $W$ be a vector subspace of $V$, a space with a dot product; $vin V$. Let $p_W(v)$ be the orthogonal projection of $v$ onto $W$ and $win W, wneq p_W(v)$.



How can i prove that $||v-w|| > ||v-p_W(v)||$?










share|cite|improve this question











$endgroup$




Let $W$ be a vector subspace of $V$, a space with a dot product; $vin V$. Let $p_W(v)$ be the orthogonal projection of $v$ onto $W$ and $win W, wneq p_W(v)$.



How can i prove that $||v-w|| > ||v-p_W(v)||$?







linear-algebra vector-spaces norm orthogonality






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Mar 16 at 17:54







Noé Duarte González

















asked Mar 16 at 17:42









Noé Duarte GonzálezNoé Duarte González

63




63












  • $begingroup$
    Pray tell, what is $w'$?
    $endgroup$
    – Robert Lewis
    Mar 16 at 17:49






  • 1




    $begingroup$
    My bad, I meant $w$
    $endgroup$
    – Noé Duarte González
    Mar 16 at 17:55










  • $begingroup$
    Thanks for the correction!
    $endgroup$
    – Robert Lewis
    Mar 16 at 18:06


















  • $begingroup$
    Pray tell, what is $w'$?
    $endgroup$
    – Robert Lewis
    Mar 16 at 17:49






  • 1




    $begingroup$
    My bad, I meant $w$
    $endgroup$
    – Noé Duarte González
    Mar 16 at 17:55










  • $begingroup$
    Thanks for the correction!
    $endgroup$
    – Robert Lewis
    Mar 16 at 18:06
















$begingroup$
Pray tell, what is $w'$?
$endgroup$
– Robert Lewis
Mar 16 at 17:49




$begingroup$
Pray tell, what is $w'$?
$endgroup$
– Robert Lewis
Mar 16 at 17:49




1




1




$begingroup$
My bad, I meant $w$
$endgroup$
– Noé Duarte González
Mar 16 at 17:55




$begingroup$
My bad, I meant $w$
$endgroup$
– Noé Duarte González
Mar 16 at 17:55












$begingroup$
Thanks for the correction!
$endgroup$
– Robert Lewis
Mar 16 at 18:06




$begingroup$
Thanks for the correction!
$endgroup$
– Robert Lewis
Mar 16 at 18:06










2 Answers
2






active

oldest

votes


















0












$begingroup$

Let $v = v_| + v_bot $, where $v_| = p_W(v)$, $v_bot in W^bot$.



$ |v-w|^2 = (v-w)^2 = v^2 + w^2 - 2vw = v_bot^2 + v_|^2 + w^2 - 2v_|w = v_bot^2 + (v_|-w)^2$



$ |v-v_||^2 = v_bot^2 < v_bot^2 + (v_|-w)^2 $, QED.






share|cite|improve this answer









$endgroup$





















    0












    $begingroup$

    Since $W subset V$ is a fixed subspace, I am going to drop the subscript "$W"$ from $P$; in this answer, $P = P_W$.



    Recall that an orthogonal projection $P$ satisfies



    $P^2 = P = P^T; tag 1$



    thus for any



    $x, y in V tag 2$



    we have



    $langle x, Py rangle = langle P^Tx, y rangle = langle Px, y rangle; tag 3$



    now consider the three vectors $v$, $Pv$, and $w$;



    $Pv, w in W Longrightarrow Pv - w in W, tag 4$



    $langle v - Pv, w rangle = langle v, w rangle - langle Pv, w rangle = langle v, w rangle - langle v, Pw rangle = langle v, w rangle - langle v, w rangle = 0, tag 5$



    where we have used (3) and the fact that $Pw = w$ for $w in W$; since this holds for every $w in W$, we have established that



    $v - Pv in W^bot; tag 6$



    we write



    $v - w = (v - Pv) + (Pv - w), tag 7$



    and by virtue of (4) and (6)



    $langle v - Pv, Pv - w rangle = 0, tag 8$



    whence



    $Vert v - w Vert^2 = langle v - w, v - w rangle = langle (v - Pv) + (Pv - w), (v - Pv) + (Pv - w) rangle$
    $= langle v - Pv, v - Pv rangle -2langle v - Pv, Pv - w rangle + langle Pv - w, Pv - w rangle$
    $= Vert v - Pv Vert^2 + Vert Pv - w Vert^2;tag 9$



    since each term on the right is non-negative, we find that



    $Vert v - w Vert^2 ge Vert v - Pv Vert^2, tag{10}$



    whence



    $Vert v - w Vert ge Vert v - Pv Vert, ; forall w in W, tag{11}$



    with equality holding precisely when



    $Vert w - Pv Vert = 0 Longleftrightarrow w = Pv; tag{12}$



    thus



    $w ne Pv Longrightarrow Vert v - w Vert > Vert v - Pv Vert. tag{13}$



    $OEDelta$.



    Inequality of vector norms with projections






    share|cite|improve this answer









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      2 Answers
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      2 Answers
      2






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      active

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      0












      $begingroup$

      Let $v = v_| + v_bot $, where $v_| = p_W(v)$, $v_bot in W^bot$.



      $ |v-w|^2 = (v-w)^2 = v^2 + w^2 - 2vw = v_bot^2 + v_|^2 + w^2 - 2v_|w = v_bot^2 + (v_|-w)^2$



      $ |v-v_||^2 = v_bot^2 < v_bot^2 + (v_|-w)^2 $, QED.






      share|cite|improve this answer









      $endgroup$


















        0












        $begingroup$

        Let $v = v_| + v_bot $, where $v_| = p_W(v)$, $v_bot in W^bot$.



        $ |v-w|^2 = (v-w)^2 = v^2 + w^2 - 2vw = v_bot^2 + v_|^2 + w^2 - 2v_|w = v_bot^2 + (v_|-w)^2$



        $ |v-v_||^2 = v_bot^2 < v_bot^2 + (v_|-w)^2 $, QED.






        share|cite|improve this answer









        $endgroup$
















          0












          0








          0





          $begingroup$

          Let $v = v_| + v_bot $, where $v_| = p_W(v)$, $v_bot in W^bot$.



          $ |v-w|^2 = (v-w)^2 = v^2 + w^2 - 2vw = v_bot^2 + v_|^2 + w^2 - 2v_|w = v_bot^2 + (v_|-w)^2$



          $ |v-v_||^2 = v_bot^2 < v_bot^2 + (v_|-w)^2 $, QED.






          share|cite|improve this answer









          $endgroup$



          Let $v = v_| + v_bot $, where $v_| = p_W(v)$, $v_bot in W^bot$.



          $ |v-w|^2 = (v-w)^2 = v^2 + w^2 - 2vw = v_bot^2 + v_|^2 + w^2 - 2v_|w = v_bot^2 + (v_|-w)^2$



          $ |v-v_||^2 = v_bot^2 < v_bot^2 + (v_|-w)^2 $, QED.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Mar 16 at 18:14









          colt_browningcolt_browning

          768110




          768110























              0












              $begingroup$

              Since $W subset V$ is a fixed subspace, I am going to drop the subscript "$W"$ from $P$; in this answer, $P = P_W$.



              Recall that an orthogonal projection $P$ satisfies



              $P^2 = P = P^T; tag 1$



              thus for any



              $x, y in V tag 2$



              we have



              $langle x, Py rangle = langle P^Tx, y rangle = langle Px, y rangle; tag 3$



              now consider the three vectors $v$, $Pv$, and $w$;



              $Pv, w in W Longrightarrow Pv - w in W, tag 4$



              $langle v - Pv, w rangle = langle v, w rangle - langle Pv, w rangle = langle v, w rangle - langle v, Pw rangle = langle v, w rangle - langle v, w rangle = 0, tag 5$



              where we have used (3) and the fact that $Pw = w$ for $w in W$; since this holds for every $w in W$, we have established that



              $v - Pv in W^bot; tag 6$



              we write



              $v - w = (v - Pv) + (Pv - w), tag 7$



              and by virtue of (4) and (6)



              $langle v - Pv, Pv - w rangle = 0, tag 8$



              whence



              $Vert v - w Vert^2 = langle v - w, v - w rangle = langle (v - Pv) + (Pv - w), (v - Pv) + (Pv - w) rangle$
              $= langle v - Pv, v - Pv rangle -2langle v - Pv, Pv - w rangle + langle Pv - w, Pv - w rangle$
              $= Vert v - Pv Vert^2 + Vert Pv - w Vert^2;tag 9$



              since each term on the right is non-negative, we find that



              $Vert v - w Vert^2 ge Vert v - Pv Vert^2, tag{10}$



              whence



              $Vert v - w Vert ge Vert v - Pv Vert, ; forall w in W, tag{11}$



              with equality holding precisely when



              $Vert w - Pv Vert = 0 Longleftrightarrow w = Pv; tag{12}$



              thus



              $w ne Pv Longrightarrow Vert v - w Vert > Vert v - Pv Vert. tag{13}$



              $OEDelta$.



              Inequality of vector norms with projections






              share|cite|improve this answer









              $endgroup$


















                0












                $begingroup$

                Since $W subset V$ is a fixed subspace, I am going to drop the subscript "$W"$ from $P$; in this answer, $P = P_W$.



                Recall that an orthogonal projection $P$ satisfies



                $P^2 = P = P^T; tag 1$



                thus for any



                $x, y in V tag 2$



                we have



                $langle x, Py rangle = langle P^Tx, y rangle = langle Px, y rangle; tag 3$



                now consider the three vectors $v$, $Pv$, and $w$;



                $Pv, w in W Longrightarrow Pv - w in W, tag 4$



                $langle v - Pv, w rangle = langle v, w rangle - langle Pv, w rangle = langle v, w rangle - langle v, Pw rangle = langle v, w rangle - langle v, w rangle = 0, tag 5$



                where we have used (3) and the fact that $Pw = w$ for $w in W$; since this holds for every $w in W$, we have established that



                $v - Pv in W^bot; tag 6$



                we write



                $v - w = (v - Pv) + (Pv - w), tag 7$



                and by virtue of (4) and (6)



                $langle v - Pv, Pv - w rangle = 0, tag 8$



                whence



                $Vert v - w Vert^2 = langle v - w, v - w rangle = langle (v - Pv) + (Pv - w), (v - Pv) + (Pv - w) rangle$
                $= langle v - Pv, v - Pv rangle -2langle v - Pv, Pv - w rangle + langle Pv - w, Pv - w rangle$
                $= Vert v - Pv Vert^2 + Vert Pv - w Vert^2;tag 9$



                since each term on the right is non-negative, we find that



                $Vert v - w Vert^2 ge Vert v - Pv Vert^2, tag{10}$



                whence



                $Vert v - w Vert ge Vert v - Pv Vert, ; forall w in W, tag{11}$



                with equality holding precisely when



                $Vert w - Pv Vert = 0 Longleftrightarrow w = Pv; tag{12}$



                thus



                $w ne Pv Longrightarrow Vert v - w Vert > Vert v - Pv Vert. tag{13}$



                $OEDelta$.



                Inequality of vector norms with projections






                share|cite|improve this answer









                $endgroup$
















                  0












                  0








                  0





                  $begingroup$

                  Since $W subset V$ is a fixed subspace, I am going to drop the subscript "$W"$ from $P$; in this answer, $P = P_W$.



                  Recall that an orthogonal projection $P$ satisfies



                  $P^2 = P = P^T; tag 1$



                  thus for any



                  $x, y in V tag 2$



                  we have



                  $langle x, Py rangle = langle P^Tx, y rangle = langle Px, y rangle; tag 3$



                  now consider the three vectors $v$, $Pv$, and $w$;



                  $Pv, w in W Longrightarrow Pv - w in W, tag 4$



                  $langle v - Pv, w rangle = langle v, w rangle - langle Pv, w rangle = langle v, w rangle - langle v, Pw rangle = langle v, w rangle - langle v, w rangle = 0, tag 5$



                  where we have used (3) and the fact that $Pw = w$ for $w in W$; since this holds for every $w in W$, we have established that



                  $v - Pv in W^bot; tag 6$



                  we write



                  $v - w = (v - Pv) + (Pv - w), tag 7$



                  and by virtue of (4) and (6)



                  $langle v - Pv, Pv - w rangle = 0, tag 8$



                  whence



                  $Vert v - w Vert^2 = langle v - w, v - w rangle = langle (v - Pv) + (Pv - w), (v - Pv) + (Pv - w) rangle$
                  $= langle v - Pv, v - Pv rangle -2langle v - Pv, Pv - w rangle + langle Pv - w, Pv - w rangle$
                  $= Vert v - Pv Vert^2 + Vert Pv - w Vert^2;tag 9$



                  since each term on the right is non-negative, we find that



                  $Vert v - w Vert^2 ge Vert v - Pv Vert^2, tag{10}$



                  whence



                  $Vert v - w Vert ge Vert v - Pv Vert, ; forall w in W, tag{11}$



                  with equality holding precisely when



                  $Vert w - Pv Vert = 0 Longleftrightarrow w = Pv; tag{12}$



                  thus



                  $w ne Pv Longrightarrow Vert v - w Vert > Vert v - Pv Vert. tag{13}$



                  $OEDelta$.



                  Inequality of vector norms with projections






                  share|cite|improve this answer









                  $endgroup$



                  Since $W subset V$ is a fixed subspace, I am going to drop the subscript "$W"$ from $P$; in this answer, $P = P_W$.



                  Recall that an orthogonal projection $P$ satisfies



                  $P^2 = P = P^T; tag 1$



                  thus for any



                  $x, y in V tag 2$



                  we have



                  $langle x, Py rangle = langle P^Tx, y rangle = langle Px, y rangle; tag 3$



                  now consider the three vectors $v$, $Pv$, and $w$;



                  $Pv, w in W Longrightarrow Pv - w in W, tag 4$



                  $langle v - Pv, w rangle = langle v, w rangle - langle Pv, w rangle = langle v, w rangle - langle v, Pw rangle = langle v, w rangle - langle v, w rangle = 0, tag 5$



                  where we have used (3) and the fact that $Pw = w$ for $w in W$; since this holds for every $w in W$, we have established that



                  $v - Pv in W^bot; tag 6$



                  we write



                  $v - w = (v - Pv) + (Pv - w), tag 7$



                  and by virtue of (4) and (6)



                  $langle v - Pv, Pv - w rangle = 0, tag 8$



                  whence



                  $Vert v - w Vert^2 = langle v - w, v - w rangle = langle (v - Pv) + (Pv - w), (v - Pv) + (Pv - w) rangle$
                  $= langle v - Pv, v - Pv rangle -2langle v - Pv, Pv - w rangle + langle Pv - w, Pv - w rangle$
                  $= Vert v - Pv Vert^2 + Vert Pv - w Vert^2;tag 9$



                  since each term on the right is non-negative, we find that



                  $Vert v - w Vert^2 ge Vert v - Pv Vert^2, tag{10}$



                  whence



                  $Vert v - w Vert ge Vert v - Pv Vert, ; forall w in W, tag{11}$



                  with equality holding precisely when



                  $Vert w - Pv Vert = 0 Longleftrightarrow w = Pv; tag{12}$



                  thus



                  $w ne Pv Longrightarrow Vert v - w Vert > Vert v - Pv Vert. tag{13}$



                  $OEDelta$.



                  Inequality of vector norms with projections







                  share|cite|improve this answer












                  share|cite|improve this answer



                  share|cite|improve this answer










                  answered Mar 17 at 19:46









                  Robert LewisRobert Lewis

                  48.5k23167




                  48.5k23167






























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