Prove that function is bi-lipshitz given it'a differential is zero












1












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Let $f$ be a function $f:R^n -> R^n$ which is differentially continuous. Moreover, it is given that in the origin: $D_f(0)=Id$, which is the identity function.



I need to prove that around the origin, there exist $A,B>0$ that satisfy:
$$A||x-y|| le ||f(x)-f(y)|| le B||x-y||$$



I tried to maybe somehow use the Implicit Function Theorem, but no luck yet.



If someone could give me a hint or some direction, it will be very appreciated.










share|cite|improve this question









$endgroup$

















    1












    $begingroup$


    Let $f$ be a function $f:R^n -> R^n$ which is differentially continuous. Moreover, it is given that in the origin: $D_f(0)=Id$, which is the identity function.



    I need to prove that around the origin, there exist $A,B>0$ that satisfy:
    $$A||x-y|| le ||f(x)-f(y)|| le B||x-y||$$



    I tried to maybe somehow use the Implicit Function Theorem, but no luck yet.



    If someone could give me a hint or some direction, it will be very appreciated.










    share|cite|improve this question









    $endgroup$















      1












      1








      1





      $begingroup$


      Let $f$ be a function $f:R^n -> R^n$ which is differentially continuous. Moreover, it is given that in the origin: $D_f(0)=Id$, which is the identity function.



      I need to prove that around the origin, there exist $A,B>0$ that satisfy:
      $$A||x-y|| le ||f(x)-f(y)|| le B||x-y||$$



      I tried to maybe somehow use the Implicit Function Theorem, but no luck yet.



      If someone could give me a hint or some direction, it will be very appreciated.










      share|cite|improve this question









      $endgroup$




      Let $f$ be a function $f:R^n -> R^n$ which is differentially continuous. Moreover, it is given that in the origin: $D_f(0)=Id$, which is the identity function.



      I need to prove that around the origin, there exist $A,B>0$ that satisfy:
      $$A||x-y|| le ||f(x)-f(y)|| le B||x-y||$$



      I tried to maybe somehow use the Implicit Function Theorem, but no luck yet.



      If someone could give me a hint or some direction, it will be very appreciated.







      real-analysis calculus multivariable-calculus






      share|cite|improve this question













      share|cite|improve this question











      share|cite|improve this question




      share|cite|improve this question










      asked Dec 5 '18 at 2:16









      Gabi GGabi G

      39819




      39819






















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          $begingroup$

          By the Inverse Function Theorem, there exists two open sets $U$ and $V$ such that $0in U$, $f(U)=V$ and the restriction $f:U to V$ is a diffeomorphism. Let $varepsilon$ be a positive real number such that the closed ball $overline{B_varepsilon(0)}$ is included in $U$. Recall that the norm of $||cdot||$ of $mathbb{R}^n$ induces a norm on the vector space $mathbb{R}^{ntimes n}$ of real matrices of size $ntimes n$ defined by
          $$
          ||A||=sup{||Ax||:||x||=1},
          $$

          for every $A$ in $mathbb{R}^{ntimes n}$. The main property of this induced norm is the inequality $||Ax||leq ||A||.||x||$, which holds for every matrix $A$ and every vector $x$. Now, the functions $Phi:xiinoverline{B_varepsilon(0)}mapsto Df(xi)in mathbb{R}^{ntimes n}$ and $Psi:xiin overline{B_varepsilon(0)}mapsto Df^{-1}(f(xi))in mathbb{R}^{ntimes n}$ are continuous and they are defined on a compact space. So there exists positive real numbers $A$ and $C$ such that
          $$
          ||Phi(xi)||=||Df(xi)||leq A quadtext{ and }quad ||Psi(xi)||=||Df^{-1}(f(xi))||leq C,
          $$

          for every $xi$ in $overline{B_varepsilon(0)}$. Now, let $x$ and $y$ be different points in $B_varepsilon(0)$. By the Mean Value Theorem for Several Variables (if you do not know this version let me know and I will add the details), there exists $xi$ in the segment between $x$ and $y$ such that
          $$
          f(x)-f(y)=Df(xi).(x-y)
          $$

          And we know that
          $$
          ||Df(xi).(x-y)||leq ||Df(xi)||.||x-y||leq A||x-y||
          $$

          and
          $$
          ||x-y||=||Df^{-1}(f(xi)).Df(xi).(x-y)||leq ||Df^{-1}(f(xi))||.||Df(xi).(x-y)||leq C||Df(xi).(x-y)||,
          $$

          so $frac{1}{C}||x-y||leq ||Df(xi).(x-y)||$. If we set $B=frac{1}{C}$, it follows that
          $$
          B||x-y||leq ||f(x)-f(y)|| leq A||x-y||,
          $$

          for every $x$ and $y$ in $B_epsilon(0)$.



          Note that we only need $Df(0)$ to be invertible.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Nice answer! Makes me wonder why in the question it was given that the differential at zero is the identity function, and not merely invertible.
            $endgroup$
            – Gabi G
            Dec 5 '18 at 23:59










          • $begingroup$
            Thanks! Well, you can use the precise value of $||Df(0)||$ and $||Df^{−1}(f(0))||$ to control the constants $A$ and $B$ in the following way: For every $delta >0$ there exists an $varepsilon >0$ such that the constansts $A$ and $B$ in the answer above can be chosen $||Df(0)||leq A<||Df(0)||+δ$ and $1/||Df^{−1}(f(0))||geq C>1/(||Df^{−1}(f(0))||+δ)$.
            $endgroup$
            – Dante Grevino
            Dec 6 '18 at 1:04













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          $begingroup$

          By the Inverse Function Theorem, there exists two open sets $U$ and $V$ such that $0in U$, $f(U)=V$ and the restriction $f:U to V$ is a diffeomorphism. Let $varepsilon$ be a positive real number such that the closed ball $overline{B_varepsilon(0)}$ is included in $U$. Recall that the norm of $||cdot||$ of $mathbb{R}^n$ induces a norm on the vector space $mathbb{R}^{ntimes n}$ of real matrices of size $ntimes n$ defined by
          $$
          ||A||=sup{||Ax||:||x||=1},
          $$

          for every $A$ in $mathbb{R}^{ntimes n}$. The main property of this induced norm is the inequality $||Ax||leq ||A||.||x||$, which holds for every matrix $A$ and every vector $x$. Now, the functions $Phi:xiinoverline{B_varepsilon(0)}mapsto Df(xi)in mathbb{R}^{ntimes n}$ and $Psi:xiin overline{B_varepsilon(0)}mapsto Df^{-1}(f(xi))in mathbb{R}^{ntimes n}$ are continuous and they are defined on a compact space. So there exists positive real numbers $A$ and $C$ such that
          $$
          ||Phi(xi)||=||Df(xi)||leq A quadtext{ and }quad ||Psi(xi)||=||Df^{-1}(f(xi))||leq C,
          $$

          for every $xi$ in $overline{B_varepsilon(0)}$. Now, let $x$ and $y$ be different points in $B_varepsilon(0)$. By the Mean Value Theorem for Several Variables (if you do not know this version let me know and I will add the details), there exists $xi$ in the segment between $x$ and $y$ such that
          $$
          f(x)-f(y)=Df(xi).(x-y)
          $$

          And we know that
          $$
          ||Df(xi).(x-y)||leq ||Df(xi)||.||x-y||leq A||x-y||
          $$

          and
          $$
          ||x-y||=||Df^{-1}(f(xi)).Df(xi).(x-y)||leq ||Df^{-1}(f(xi))||.||Df(xi).(x-y)||leq C||Df(xi).(x-y)||,
          $$

          so $frac{1}{C}||x-y||leq ||Df(xi).(x-y)||$. If we set $B=frac{1}{C}$, it follows that
          $$
          B||x-y||leq ||f(x)-f(y)|| leq A||x-y||,
          $$

          for every $x$ and $y$ in $B_epsilon(0)$.



          Note that we only need $Df(0)$ to be invertible.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Nice answer! Makes me wonder why in the question it was given that the differential at zero is the identity function, and not merely invertible.
            $endgroup$
            – Gabi G
            Dec 5 '18 at 23:59










          • $begingroup$
            Thanks! Well, you can use the precise value of $||Df(0)||$ and $||Df^{−1}(f(0))||$ to control the constants $A$ and $B$ in the following way: For every $delta >0$ there exists an $varepsilon >0$ such that the constansts $A$ and $B$ in the answer above can be chosen $||Df(0)||leq A<||Df(0)||+δ$ and $1/||Df^{−1}(f(0))||geq C>1/(||Df^{−1}(f(0))||+δ)$.
            $endgroup$
            – Dante Grevino
            Dec 6 '18 at 1:04


















          1












          $begingroup$

          By the Inverse Function Theorem, there exists two open sets $U$ and $V$ such that $0in U$, $f(U)=V$ and the restriction $f:U to V$ is a diffeomorphism. Let $varepsilon$ be a positive real number such that the closed ball $overline{B_varepsilon(0)}$ is included in $U$. Recall that the norm of $||cdot||$ of $mathbb{R}^n$ induces a norm on the vector space $mathbb{R}^{ntimes n}$ of real matrices of size $ntimes n$ defined by
          $$
          ||A||=sup{||Ax||:||x||=1},
          $$

          for every $A$ in $mathbb{R}^{ntimes n}$. The main property of this induced norm is the inequality $||Ax||leq ||A||.||x||$, which holds for every matrix $A$ and every vector $x$. Now, the functions $Phi:xiinoverline{B_varepsilon(0)}mapsto Df(xi)in mathbb{R}^{ntimes n}$ and $Psi:xiin overline{B_varepsilon(0)}mapsto Df^{-1}(f(xi))in mathbb{R}^{ntimes n}$ are continuous and they are defined on a compact space. So there exists positive real numbers $A$ and $C$ such that
          $$
          ||Phi(xi)||=||Df(xi)||leq A quadtext{ and }quad ||Psi(xi)||=||Df^{-1}(f(xi))||leq C,
          $$

          for every $xi$ in $overline{B_varepsilon(0)}$. Now, let $x$ and $y$ be different points in $B_varepsilon(0)$. By the Mean Value Theorem for Several Variables (if you do not know this version let me know and I will add the details), there exists $xi$ in the segment between $x$ and $y$ such that
          $$
          f(x)-f(y)=Df(xi).(x-y)
          $$

          And we know that
          $$
          ||Df(xi).(x-y)||leq ||Df(xi)||.||x-y||leq A||x-y||
          $$

          and
          $$
          ||x-y||=||Df^{-1}(f(xi)).Df(xi).(x-y)||leq ||Df^{-1}(f(xi))||.||Df(xi).(x-y)||leq C||Df(xi).(x-y)||,
          $$

          so $frac{1}{C}||x-y||leq ||Df(xi).(x-y)||$. If we set $B=frac{1}{C}$, it follows that
          $$
          B||x-y||leq ||f(x)-f(y)|| leq A||x-y||,
          $$

          for every $x$ and $y$ in $B_epsilon(0)$.



          Note that we only need $Df(0)$ to be invertible.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Nice answer! Makes me wonder why in the question it was given that the differential at zero is the identity function, and not merely invertible.
            $endgroup$
            – Gabi G
            Dec 5 '18 at 23:59










          • $begingroup$
            Thanks! Well, you can use the precise value of $||Df(0)||$ and $||Df^{−1}(f(0))||$ to control the constants $A$ and $B$ in the following way: For every $delta >0$ there exists an $varepsilon >0$ such that the constansts $A$ and $B$ in the answer above can be chosen $||Df(0)||leq A<||Df(0)||+δ$ and $1/||Df^{−1}(f(0))||geq C>1/(||Df^{−1}(f(0))||+δ)$.
            $endgroup$
            – Dante Grevino
            Dec 6 '18 at 1:04
















          1












          1








          1





          $begingroup$

          By the Inverse Function Theorem, there exists two open sets $U$ and $V$ such that $0in U$, $f(U)=V$ and the restriction $f:U to V$ is a diffeomorphism. Let $varepsilon$ be a positive real number such that the closed ball $overline{B_varepsilon(0)}$ is included in $U$. Recall that the norm of $||cdot||$ of $mathbb{R}^n$ induces a norm on the vector space $mathbb{R}^{ntimes n}$ of real matrices of size $ntimes n$ defined by
          $$
          ||A||=sup{||Ax||:||x||=1},
          $$

          for every $A$ in $mathbb{R}^{ntimes n}$. The main property of this induced norm is the inequality $||Ax||leq ||A||.||x||$, which holds for every matrix $A$ and every vector $x$. Now, the functions $Phi:xiinoverline{B_varepsilon(0)}mapsto Df(xi)in mathbb{R}^{ntimes n}$ and $Psi:xiin overline{B_varepsilon(0)}mapsto Df^{-1}(f(xi))in mathbb{R}^{ntimes n}$ are continuous and they are defined on a compact space. So there exists positive real numbers $A$ and $C$ such that
          $$
          ||Phi(xi)||=||Df(xi)||leq A quadtext{ and }quad ||Psi(xi)||=||Df^{-1}(f(xi))||leq C,
          $$

          for every $xi$ in $overline{B_varepsilon(0)}$. Now, let $x$ and $y$ be different points in $B_varepsilon(0)$. By the Mean Value Theorem for Several Variables (if you do not know this version let me know and I will add the details), there exists $xi$ in the segment between $x$ and $y$ such that
          $$
          f(x)-f(y)=Df(xi).(x-y)
          $$

          And we know that
          $$
          ||Df(xi).(x-y)||leq ||Df(xi)||.||x-y||leq A||x-y||
          $$

          and
          $$
          ||x-y||=||Df^{-1}(f(xi)).Df(xi).(x-y)||leq ||Df^{-1}(f(xi))||.||Df(xi).(x-y)||leq C||Df(xi).(x-y)||,
          $$

          so $frac{1}{C}||x-y||leq ||Df(xi).(x-y)||$. If we set $B=frac{1}{C}$, it follows that
          $$
          B||x-y||leq ||f(x)-f(y)|| leq A||x-y||,
          $$

          for every $x$ and $y$ in $B_epsilon(0)$.



          Note that we only need $Df(0)$ to be invertible.






          share|cite|improve this answer











          $endgroup$



          By the Inverse Function Theorem, there exists two open sets $U$ and $V$ such that $0in U$, $f(U)=V$ and the restriction $f:U to V$ is a diffeomorphism. Let $varepsilon$ be a positive real number such that the closed ball $overline{B_varepsilon(0)}$ is included in $U$. Recall that the norm of $||cdot||$ of $mathbb{R}^n$ induces a norm on the vector space $mathbb{R}^{ntimes n}$ of real matrices of size $ntimes n$ defined by
          $$
          ||A||=sup{||Ax||:||x||=1},
          $$

          for every $A$ in $mathbb{R}^{ntimes n}$. The main property of this induced norm is the inequality $||Ax||leq ||A||.||x||$, which holds for every matrix $A$ and every vector $x$. Now, the functions $Phi:xiinoverline{B_varepsilon(0)}mapsto Df(xi)in mathbb{R}^{ntimes n}$ and $Psi:xiin overline{B_varepsilon(0)}mapsto Df^{-1}(f(xi))in mathbb{R}^{ntimes n}$ are continuous and they are defined on a compact space. So there exists positive real numbers $A$ and $C$ such that
          $$
          ||Phi(xi)||=||Df(xi)||leq A quadtext{ and }quad ||Psi(xi)||=||Df^{-1}(f(xi))||leq C,
          $$

          for every $xi$ in $overline{B_varepsilon(0)}$. Now, let $x$ and $y$ be different points in $B_varepsilon(0)$. By the Mean Value Theorem for Several Variables (if you do not know this version let me know and I will add the details), there exists $xi$ in the segment between $x$ and $y$ such that
          $$
          f(x)-f(y)=Df(xi).(x-y)
          $$

          And we know that
          $$
          ||Df(xi).(x-y)||leq ||Df(xi)||.||x-y||leq A||x-y||
          $$

          and
          $$
          ||x-y||=||Df^{-1}(f(xi)).Df(xi).(x-y)||leq ||Df^{-1}(f(xi))||.||Df(xi).(x-y)||leq C||Df(xi).(x-y)||,
          $$

          so $frac{1}{C}||x-y||leq ||Df(xi).(x-y)||$. If we set $B=frac{1}{C}$, it follows that
          $$
          B||x-y||leq ||f(x)-f(y)|| leq A||x-y||,
          $$

          for every $x$ and $y$ in $B_epsilon(0)$.



          Note that we only need $Df(0)$ to be invertible.







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited Dec 6 '18 at 0:31

























          answered Dec 5 '18 at 22:48









          Dante GrevinoDante Grevino

          96319




          96319












          • $begingroup$
            Nice answer! Makes me wonder why in the question it was given that the differential at zero is the identity function, and not merely invertible.
            $endgroup$
            – Gabi G
            Dec 5 '18 at 23:59










          • $begingroup$
            Thanks! Well, you can use the precise value of $||Df(0)||$ and $||Df^{−1}(f(0))||$ to control the constants $A$ and $B$ in the following way: For every $delta >0$ there exists an $varepsilon >0$ such that the constansts $A$ and $B$ in the answer above can be chosen $||Df(0)||leq A<||Df(0)||+δ$ and $1/||Df^{−1}(f(0))||geq C>1/(||Df^{−1}(f(0))||+δ)$.
            $endgroup$
            – Dante Grevino
            Dec 6 '18 at 1:04




















          • $begingroup$
            Nice answer! Makes me wonder why in the question it was given that the differential at zero is the identity function, and not merely invertible.
            $endgroup$
            – Gabi G
            Dec 5 '18 at 23:59










          • $begingroup$
            Thanks! Well, you can use the precise value of $||Df(0)||$ and $||Df^{−1}(f(0))||$ to control the constants $A$ and $B$ in the following way: For every $delta >0$ there exists an $varepsilon >0$ such that the constansts $A$ and $B$ in the answer above can be chosen $||Df(0)||leq A<||Df(0)||+δ$ and $1/||Df^{−1}(f(0))||geq C>1/(||Df^{−1}(f(0))||+δ)$.
            $endgroup$
            – Dante Grevino
            Dec 6 '18 at 1:04


















          $begingroup$
          Nice answer! Makes me wonder why in the question it was given that the differential at zero is the identity function, and not merely invertible.
          $endgroup$
          – Gabi G
          Dec 5 '18 at 23:59




          $begingroup$
          Nice answer! Makes me wonder why in the question it was given that the differential at zero is the identity function, and not merely invertible.
          $endgroup$
          – Gabi G
          Dec 5 '18 at 23:59












          $begingroup$
          Thanks! Well, you can use the precise value of $||Df(0)||$ and $||Df^{−1}(f(0))||$ to control the constants $A$ and $B$ in the following way: For every $delta >0$ there exists an $varepsilon >0$ such that the constansts $A$ and $B$ in the answer above can be chosen $||Df(0)||leq A<||Df(0)||+δ$ and $1/||Df^{−1}(f(0))||geq C>1/(||Df^{−1}(f(0))||+δ)$.
          $endgroup$
          – Dante Grevino
          Dec 6 '18 at 1:04






          $begingroup$
          Thanks! Well, you can use the precise value of $||Df(0)||$ and $||Df^{−1}(f(0))||$ to control the constants $A$ and $B$ in the following way: For every $delta >0$ there exists an $varepsilon >0$ such that the constansts $A$ and $B$ in the answer above can be chosen $||Df(0)||leq A<||Df(0)||+δ$ and $1/||Df^{−1}(f(0))||geq C>1/(||Df^{−1}(f(0))||+δ)$.
          $endgroup$
          – Dante Grevino
          Dec 6 '18 at 1:04




















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