If $langle f'(x) cdot v , v rangle > 0$ then $f$ is injective












4












$begingroup$


Question:




Let $f: U to mathbb R^m$ differentiable at the convex set $U subseteq mathbb R^m$. If $$langle f'(x) cdot v , v rangle > 0 , ,,, forall,, x in U, v neq 0 in mathbb R^m $$
then $f$ is injective. If $f in C^1$ then $f$ is a diffeomorphism of $U$ over a subset of $mathbb R^m$. Give an example such that $U = mathbb R^m$, but $f$ is not surjective.




Attempt: The idea is to show $$|f(x+v) - f(x)| > 0$$



As $U$ is convex and $f$ id differentiable in $U$ then $[x,x+v] subseteq U$ for any $x, x + v in U$, by the Mean Value Theorem there exists $theta in (0,1)$ such that $$f(x + v) - f(x) = frac{partial f}{partial v}(x + theta v) = f'(x + theta v) cdot v$$



Then $$langle f(x + v) - f(x) , vrangle = langle f'(x + theta v) cdot v , vrangle > 0 $$



and I couldn't conclude anything.



The second part is o.k.



Any thoughts?



Edit: I can't use the Mean Value Theorem here.










share|cite|improve this question











$endgroup$

















    4












    $begingroup$


    Question:




    Let $f: U to mathbb R^m$ differentiable at the convex set $U subseteq mathbb R^m$. If $$langle f'(x) cdot v , v rangle > 0 , ,,, forall,, x in U, v neq 0 in mathbb R^m $$
    then $f$ is injective. If $f in C^1$ then $f$ is a diffeomorphism of $U$ over a subset of $mathbb R^m$. Give an example such that $U = mathbb R^m$, but $f$ is not surjective.




    Attempt: The idea is to show $$|f(x+v) - f(x)| > 0$$



    As $U$ is convex and $f$ id differentiable in $U$ then $[x,x+v] subseteq U$ for any $x, x + v in U$, by the Mean Value Theorem there exists $theta in (0,1)$ such that $$f(x + v) - f(x) = frac{partial f}{partial v}(x + theta v) = f'(x + theta v) cdot v$$



    Then $$langle f(x + v) - f(x) , vrangle = langle f'(x + theta v) cdot v , vrangle > 0 $$



    and I couldn't conclude anything.



    The second part is o.k.



    Any thoughts?



    Edit: I can't use the Mean Value Theorem here.










    share|cite|improve this question











    $endgroup$















      4












      4








      4





      $begingroup$


      Question:




      Let $f: U to mathbb R^m$ differentiable at the convex set $U subseteq mathbb R^m$. If $$langle f'(x) cdot v , v rangle > 0 , ,,, forall,, x in U, v neq 0 in mathbb R^m $$
      then $f$ is injective. If $f in C^1$ then $f$ is a diffeomorphism of $U$ over a subset of $mathbb R^m$. Give an example such that $U = mathbb R^m$, but $f$ is not surjective.




      Attempt: The idea is to show $$|f(x+v) - f(x)| > 0$$



      As $U$ is convex and $f$ id differentiable in $U$ then $[x,x+v] subseteq U$ for any $x, x + v in U$, by the Mean Value Theorem there exists $theta in (0,1)$ such that $$f(x + v) - f(x) = frac{partial f}{partial v}(x + theta v) = f'(x + theta v) cdot v$$



      Then $$langle f(x + v) - f(x) , vrangle = langle f'(x + theta v) cdot v , vrangle > 0 $$



      and I couldn't conclude anything.



      The second part is o.k.



      Any thoughts?



      Edit: I can't use the Mean Value Theorem here.










      share|cite|improve this question











      $endgroup$




      Question:




      Let $f: U to mathbb R^m$ differentiable at the convex set $U subseteq mathbb R^m$. If $$langle f'(x) cdot v , v rangle > 0 , ,,, forall,, x in U, v neq 0 in mathbb R^m $$
      then $f$ is injective. If $f in C^1$ then $f$ is a diffeomorphism of $U$ over a subset of $mathbb R^m$. Give an example such that $U = mathbb R^m$, but $f$ is not surjective.




      Attempt: The idea is to show $$|f(x+v) - f(x)| > 0$$



      As $U$ is convex and $f$ id differentiable in $U$ then $[x,x+v] subseteq U$ for any $x, x + v in U$, by the Mean Value Theorem there exists $theta in (0,1)$ such that $$f(x + v) - f(x) = frac{partial f}{partial v}(x + theta v) = f'(x + theta v) cdot v$$



      Then $$langle f(x + v) - f(x) , vrangle = langle f'(x + theta v) cdot v , vrangle > 0 $$



      and I couldn't conclude anything.



      The second part is o.k.



      Any thoughts?



      Edit: I can't use the Mean Value Theorem here.







      analysis multivariable-calculus






      share|cite|improve this question















      share|cite|improve this question













      share|cite|improve this question




      share|cite|improve this question








      edited Jul 2 '15 at 23:14







      Aaron Maroja

















      asked Jul 2 '15 at 2:39









      Aaron MarojaAaron Maroja

      15.5k51447




      15.5k51447






















          3 Answers
          3






          active

          oldest

          votes


















          1












          $begingroup$

          I think that you have already solved the problem. Pick $x, yin U$, define your $v=y-x$, and proceed. Note that $||f(y)-f(x)||cdot||y-x||>0$. As you picked $xneq y$, then $f$ must be injective. Sorry for my english.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Cauchy-Schwarz inequality
            $endgroup$
            – Micael
            Jul 2 '15 at 3:02












          • $begingroup$
            I just found a counter example where I can't use the Mean Value Theorem for $f: U to mathbb R^m$, is there any way to walk around this?
            $endgroup$
            – Aaron Maroja
            Jul 2 '15 at 15:09












          • $begingroup$
            Just for the record, this does not answer the question.
            $endgroup$
            – Aaron Maroja
            Jul 2 '15 at 23:29










          • $begingroup$
            The point here is that, when you parametrize the segment $[x, y]subset U$ you can use the M.V.T. for functions from $R$ into $R^m$.
            $endgroup$
            – Micael
            Jul 3 '15 at 6:11










          • $begingroup$
            Could you show me the proof somewhere, a paper or a reference?
            $endgroup$
            – Aaron Maroja
            Jul 3 '15 at 11:55





















          0












          $begingroup$

          After a while I figured it out.



          Consider the function $$begin{align}psi: [0,1] &to mathbb R\ t &mapsto langle f(a + tv), v rangleend{align}$$



          defined at $[0,1]$, differentiable with



          $$begin{align}psi' (t) &= langle f'(a + tv)cdot v ,vrangle + langle f(a+tv),0 rangle \ &= langle f'(a + tv)cdot v rangle > 0end{align} tag {*}$$



          for $v = b -a$ and for all $t in [0,1]$, then $psi$ is strictly increasing at this closed interval. Now suppose that for $a neq b$ we have $f(a) = f(b)$.



          $$begin{align}psi (1) - psi (0) &= langle f(b), v rangle - langle f(a),v rangle\&= langle f(b) - f(a), v rangle\ &= 0end{align}$$



          As $psi$ is continuous and $psi (1) = psi (0)$ by Rolle's Theorem there exists $c in (0,1)$ such that $psi ' (c) = 0$, which contradicts $(*)$ .Thus $f(a) neq f(b)$ and it follows that $f$ is injective.






          share|cite|improve this answer











          $endgroup$





















            0












            $begingroup$

            What about the second part? How did you prove that $f$ is a homeomorphism over $f(U)$?






            share|cite|improve this answer









            $endgroup$













              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%2f1346560%2fif-langle-fx-cdot-v-v-rangle-0-then-f-is-injective%23new-answer', 'question_page');
              }
              );

              Post as a guest















              Required, but never shown

























              3 Answers
              3






              active

              oldest

              votes








              3 Answers
              3






              active

              oldest

              votes









              active

              oldest

              votes






              active

              oldest

              votes









              1












              $begingroup$

              I think that you have already solved the problem. Pick $x, yin U$, define your $v=y-x$, and proceed. Note that $||f(y)-f(x)||cdot||y-x||>0$. As you picked $xneq y$, then $f$ must be injective. Sorry for my english.






              share|cite|improve this answer











              $endgroup$













              • $begingroup$
                Cauchy-Schwarz inequality
                $endgroup$
                – Micael
                Jul 2 '15 at 3:02












              • $begingroup$
                I just found a counter example where I can't use the Mean Value Theorem for $f: U to mathbb R^m$, is there any way to walk around this?
                $endgroup$
                – Aaron Maroja
                Jul 2 '15 at 15:09












              • $begingroup$
                Just for the record, this does not answer the question.
                $endgroup$
                – Aaron Maroja
                Jul 2 '15 at 23:29










              • $begingroup$
                The point here is that, when you parametrize the segment $[x, y]subset U$ you can use the M.V.T. for functions from $R$ into $R^m$.
                $endgroup$
                – Micael
                Jul 3 '15 at 6:11










              • $begingroup$
                Could you show me the proof somewhere, a paper or a reference?
                $endgroup$
                – Aaron Maroja
                Jul 3 '15 at 11:55


















              1












              $begingroup$

              I think that you have already solved the problem. Pick $x, yin U$, define your $v=y-x$, and proceed. Note that $||f(y)-f(x)||cdot||y-x||>0$. As you picked $xneq y$, then $f$ must be injective. Sorry for my english.






              share|cite|improve this answer











              $endgroup$













              • $begingroup$
                Cauchy-Schwarz inequality
                $endgroup$
                – Micael
                Jul 2 '15 at 3:02












              • $begingroup$
                I just found a counter example where I can't use the Mean Value Theorem for $f: U to mathbb R^m$, is there any way to walk around this?
                $endgroup$
                – Aaron Maroja
                Jul 2 '15 at 15:09












              • $begingroup$
                Just for the record, this does not answer the question.
                $endgroup$
                – Aaron Maroja
                Jul 2 '15 at 23:29










              • $begingroup$
                The point here is that, when you parametrize the segment $[x, y]subset U$ you can use the M.V.T. for functions from $R$ into $R^m$.
                $endgroup$
                – Micael
                Jul 3 '15 at 6:11










              • $begingroup$
                Could you show me the proof somewhere, a paper or a reference?
                $endgroup$
                – Aaron Maroja
                Jul 3 '15 at 11:55
















              1












              1








              1





              $begingroup$

              I think that you have already solved the problem. Pick $x, yin U$, define your $v=y-x$, and proceed. Note that $||f(y)-f(x)||cdot||y-x||>0$. As you picked $xneq y$, then $f$ must be injective. Sorry for my english.






              share|cite|improve this answer











              $endgroup$



              I think that you have already solved the problem. Pick $x, yin U$, define your $v=y-x$, and proceed. Note that $||f(y)-f(x)||cdot||y-x||>0$. As you picked $xneq y$, then $f$ must be injective. Sorry for my english.







              share|cite|improve this answer














              share|cite|improve this answer



              share|cite|improve this answer








              edited Jul 2 '15 at 15:41









              Aaron Maroja

              15.5k51447




              15.5k51447










              answered Jul 2 '15 at 2:54









              MicaelMicael

              18319




              18319












              • $begingroup$
                Cauchy-Schwarz inequality
                $endgroup$
                – Micael
                Jul 2 '15 at 3:02












              • $begingroup$
                I just found a counter example where I can't use the Mean Value Theorem for $f: U to mathbb R^m$, is there any way to walk around this?
                $endgroup$
                – Aaron Maroja
                Jul 2 '15 at 15:09












              • $begingroup$
                Just for the record, this does not answer the question.
                $endgroup$
                – Aaron Maroja
                Jul 2 '15 at 23:29










              • $begingroup$
                The point here is that, when you parametrize the segment $[x, y]subset U$ you can use the M.V.T. for functions from $R$ into $R^m$.
                $endgroup$
                – Micael
                Jul 3 '15 at 6:11










              • $begingroup$
                Could you show me the proof somewhere, a paper or a reference?
                $endgroup$
                – Aaron Maroja
                Jul 3 '15 at 11:55




















              • $begingroup$
                Cauchy-Schwarz inequality
                $endgroup$
                – Micael
                Jul 2 '15 at 3:02












              • $begingroup$
                I just found a counter example where I can't use the Mean Value Theorem for $f: U to mathbb R^m$, is there any way to walk around this?
                $endgroup$
                – Aaron Maroja
                Jul 2 '15 at 15:09












              • $begingroup$
                Just for the record, this does not answer the question.
                $endgroup$
                – Aaron Maroja
                Jul 2 '15 at 23:29










              • $begingroup$
                The point here is that, when you parametrize the segment $[x, y]subset U$ you can use the M.V.T. for functions from $R$ into $R^m$.
                $endgroup$
                – Micael
                Jul 3 '15 at 6:11










              • $begingroup$
                Could you show me the proof somewhere, a paper or a reference?
                $endgroup$
                – Aaron Maroja
                Jul 3 '15 at 11:55


















              $begingroup$
              Cauchy-Schwarz inequality
              $endgroup$
              – Micael
              Jul 2 '15 at 3:02






              $begingroup$
              Cauchy-Schwarz inequality
              $endgroup$
              – Micael
              Jul 2 '15 at 3:02














              $begingroup$
              I just found a counter example where I can't use the Mean Value Theorem for $f: U to mathbb R^m$, is there any way to walk around this?
              $endgroup$
              – Aaron Maroja
              Jul 2 '15 at 15:09






              $begingroup$
              I just found a counter example where I can't use the Mean Value Theorem for $f: U to mathbb R^m$, is there any way to walk around this?
              $endgroup$
              – Aaron Maroja
              Jul 2 '15 at 15:09














              $begingroup$
              Just for the record, this does not answer the question.
              $endgroup$
              – Aaron Maroja
              Jul 2 '15 at 23:29




              $begingroup$
              Just for the record, this does not answer the question.
              $endgroup$
              – Aaron Maroja
              Jul 2 '15 at 23:29












              $begingroup$
              The point here is that, when you parametrize the segment $[x, y]subset U$ you can use the M.V.T. for functions from $R$ into $R^m$.
              $endgroup$
              – Micael
              Jul 3 '15 at 6:11




              $begingroup$
              The point here is that, when you parametrize the segment $[x, y]subset U$ you can use the M.V.T. for functions from $R$ into $R^m$.
              $endgroup$
              – Micael
              Jul 3 '15 at 6:11












              $begingroup$
              Could you show me the proof somewhere, a paper or a reference?
              $endgroup$
              – Aaron Maroja
              Jul 3 '15 at 11:55






              $begingroup$
              Could you show me the proof somewhere, a paper or a reference?
              $endgroup$
              – Aaron Maroja
              Jul 3 '15 at 11:55













              0












              $begingroup$

              After a while I figured it out.



              Consider the function $$begin{align}psi: [0,1] &to mathbb R\ t &mapsto langle f(a + tv), v rangleend{align}$$



              defined at $[0,1]$, differentiable with



              $$begin{align}psi' (t) &= langle f'(a + tv)cdot v ,vrangle + langle f(a+tv),0 rangle \ &= langle f'(a + tv)cdot v rangle > 0end{align} tag {*}$$



              for $v = b -a$ and for all $t in [0,1]$, then $psi$ is strictly increasing at this closed interval. Now suppose that for $a neq b$ we have $f(a) = f(b)$.



              $$begin{align}psi (1) - psi (0) &= langle f(b), v rangle - langle f(a),v rangle\&= langle f(b) - f(a), v rangle\ &= 0end{align}$$



              As $psi$ is continuous and $psi (1) = psi (0)$ by Rolle's Theorem there exists $c in (0,1)$ such that $psi ' (c) = 0$, which contradicts $(*)$ .Thus $f(a) neq f(b)$ and it follows that $f$ is injective.






              share|cite|improve this answer











              $endgroup$


















                0












                $begingroup$

                After a while I figured it out.



                Consider the function $$begin{align}psi: [0,1] &to mathbb R\ t &mapsto langle f(a + tv), v rangleend{align}$$



                defined at $[0,1]$, differentiable with



                $$begin{align}psi' (t) &= langle f'(a + tv)cdot v ,vrangle + langle f(a+tv),0 rangle \ &= langle f'(a + tv)cdot v rangle > 0end{align} tag {*}$$



                for $v = b -a$ and for all $t in [0,1]$, then $psi$ is strictly increasing at this closed interval. Now suppose that for $a neq b$ we have $f(a) = f(b)$.



                $$begin{align}psi (1) - psi (0) &= langle f(b), v rangle - langle f(a),v rangle\&= langle f(b) - f(a), v rangle\ &= 0end{align}$$



                As $psi$ is continuous and $psi (1) = psi (0)$ by Rolle's Theorem there exists $c in (0,1)$ such that $psi ' (c) = 0$, which contradicts $(*)$ .Thus $f(a) neq f(b)$ and it follows that $f$ is injective.






                share|cite|improve this answer











                $endgroup$
















                  0












                  0








                  0





                  $begingroup$

                  After a while I figured it out.



                  Consider the function $$begin{align}psi: [0,1] &to mathbb R\ t &mapsto langle f(a + tv), v rangleend{align}$$



                  defined at $[0,1]$, differentiable with



                  $$begin{align}psi' (t) &= langle f'(a + tv)cdot v ,vrangle + langle f(a+tv),0 rangle \ &= langle f'(a + tv)cdot v rangle > 0end{align} tag {*}$$



                  for $v = b -a$ and for all $t in [0,1]$, then $psi$ is strictly increasing at this closed interval. Now suppose that for $a neq b$ we have $f(a) = f(b)$.



                  $$begin{align}psi (1) - psi (0) &= langle f(b), v rangle - langle f(a),v rangle\&= langle f(b) - f(a), v rangle\ &= 0end{align}$$



                  As $psi$ is continuous and $psi (1) = psi (0)$ by Rolle's Theorem there exists $c in (0,1)$ such that $psi ' (c) = 0$, which contradicts $(*)$ .Thus $f(a) neq f(b)$ and it follows that $f$ is injective.






                  share|cite|improve this answer











                  $endgroup$



                  After a while I figured it out.



                  Consider the function $$begin{align}psi: [0,1] &to mathbb R\ t &mapsto langle f(a + tv), v rangleend{align}$$



                  defined at $[0,1]$, differentiable with



                  $$begin{align}psi' (t) &= langle f'(a + tv)cdot v ,vrangle + langle f(a+tv),0 rangle \ &= langle f'(a + tv)cdot v rangle > 0end{align} tag {*}$$



                  for $v = b -a$ and for all $t in [0,1]$, then $psi$ is strictly increasing at this closed interval. Now suppose that for $a neq b$ we have $f(a) = f(b)$.



                  $$begin{align}psi (1) - psi (0) &= langle f(b), v rangle - langle f(a),v rangle\&= langle f(b) - f(a), v rangle\ &= 0end{align}$$



                  As $psi$ is continuous and $psi (1) = psi (0)$ by Rolle's Theorem there exists $c in (0,1)$ such that $psi ' (c) = 0$, which contradicts $(*)$ .Thus $f(a) neq f(b)$ and it follows that $f$ is injective.







                  share|cite|improve this answer














                  share|cite|improve this answer



                  share|cite|improve this answer








                  edited Feb 25 '16 at 13:32

























                  answered Jul 2 '15 at 23:11









                  Aaron MarojaAaron Maroja

                  15.5k51447




                  15.5k51447























                      0












                      $begingroup$

                      What about the second part? How did you prove that $f$ is a homeomorphism over $f(U)$?






                      share|cite|improve this answer









                      $endgroup$


















                        0












                        $begingroup$

                        What about the second part? How did you prove that $f$ is a homeomorphism over $f(U)$?






                        share|cite|improve this answer









                        $endgroup$
















                          0












                          0








                          0





                          $begingroup$

                          What about the second part? How did you prove that $f$ is a homeomorphism over $f(U)$?






                          share|cite|improve this answer









                          $endgroup$



                          What about the second part? How did you prove that $f$ is a homeomorphism over $f(U)$?







                          share|cite|improve this answer












                          share|cite|improve this answer



                          share|cite|improve this answer










                          answered Jan 22 at 22:39









                          João CostaJoão Costa

                          755




                          755






























                              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.




                              draft saved


                              draft discarded














                              StackExchange.ready(
                              function () {
                              StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f1346560%2fif-langle-fx-cdot-v-v-rangle-0-then-f-is-injective%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

                              Mario Kart Wii

                              The Binding of Isaac: Rebirth/Afterbirth

                              What does “Dominus providebit” mean?