Method of characteristics for $f f_x + f_y = 1$. Where is the solution valid?












1












$begingroup$


Suppose we have a PDE that can be solved with the method of characteristics



begin{align}
F(nabla u, u , x) = 0 text{ in $U$}\
u|_Gamma = g text{ on $Gamma$ }
end{align}



Where $Gamma subset partial U$. Suppose the characteristic curves starting on $Gamma$ don't span the whole set $U$, what can we say about the solution in this points outside the span? Do we lack information to solve the problem or can this be somehow avoided?



To have a precise example I solved the following problem,



$$f(x,y)frac{partial f}{partial x} + frac{partial f}{partial y}= 1 qquadtext{with}qquad f(t,t) = frac{t}{2} quadtext{for}quad 0 < t < 1.$$



In the above notation we have $Gamma = {(t,t): 0 < t < 1}$ and $g(t) = t/2$. I got the solution $f(x,y) = z(s) + g(x(0)) =z(s) + frac{x_0}{2} = y - frac{x - 1/2y^2}{2-y}$ where $y neq 2$, what can I say about the validity of my solution? Is it valid for $mathbb{R^2}$ or just for points $x, y$ such that the characteristic curve trough them, $(x(s),y(s))$ lies on $Gamma$ for $s=0$?










share|cite|improve this question











$endgroup$

















    1












    $begingroup$


    Suppose we have a PDE that can be solved with the method of characteristics



    begin{align}
    F(nabla u, u , x) = 0 text{ in $U$}\
    u|_Gamma = g text{ on $Gamma$ }
    end{align}



    Where $Gamma subset partial U$. Suppose the characteristic curves starting on $Gamma$ don't span the whole set $U$, what can we say about the solution in this points outside the span? Do we lack information to solve the problem or can this be somehow avoided?



    To have a precise example I solved the following problem,



    $$f(x,y)frac{partial f}{partial x} + frac{partial f}{partial y}= 1 qquadtext{with}qquad f(t,t) = frac{t}{2} quadtext{for}quad 0 < t < 1.$$



    In the above notation we have $Gamma = {(t,t): 0 < t < 1}$ and $g(t) = t/2$. I got the solution $f(x,y) = z(s) + g(x(0)) =z(s) + frac{x_0}{2} = y - frac{x - 1/2y^2}{2-y}$ where $y neq 2$, what can I say about the validity of my solution? Is it valid for $mathbb{R^2}$ or just for points $x, y$ such that the characteristic curve trough them, $(x(s),y(s))$ lies on $Gamma$ for $s=0$?










    share|cite|improve this question











    $endgroup$















      1












      1








      1





      $begingroup$


      Suppose we have a PDE that can be solved with the method of characteristics



      begin{align}
      F(nabla u, u , x) = 0 text{ in $U$}\
      u|_Gamma = g text{ on $Gamma$ }
      end{align}



      Where $Gamma subset partial U$. Suppose the characteristic curves starting on $Gamma$ don't span the whole set $U$, what can we say about the solution in this points outside the span? Do we lack information to solve the problem or can this be somehow avoided?



      To have a precise example I solved the following problem,



      $$f(x,y)frac{partial f}{partial x} + frac{partial f}{partial y}= 1 qquadtext{with}qquad f(t,t) = frac{t}{2} quadtext{for}quad 0 < t < 1.$$



      In the above notation we have $Gamma = {(t,t): 0 < t < 1}$ and $g(t) = t/2$. I got the solution $f(x,y) = z(s) + g(x(0)) =z(s) + frac{x_0}{2} = y - frac{x - 1/2y^2}{2-y}$ where $y neq 2$, what can I say about the validity of my solution? Is it valid for $mathbb{R^2}$ or just for points $x, y$ such that the characteristic curve trough them, $(x(s),y(s))$ lies on $Gamma$ for $s=0$?










      share|cite|improve this question











      $endgroup$




      Suppose we have a PDE that can be solved with the method of characteristics



      begin{align}
      F(nabla u, u , x) = 0 text{ in $U$}\
      u|_Gamma = g text{ on $Gamma$ }
      end{align}



      Where $Gamma subset partial U$. Suppose the characteristic curves starting on $Gamma$ don't span the whole set $U$, what can we say about the solution in this points outside the span? Do we lack information to solve the problem or can this be somehow avoided?



      To have a precise example I solved the following problem,



      $$f(x,y)frac{partial f}{partial x} + frac{partial f}{partial y}= 1 qquadtext{with}qquad f(t,t) = frac{t}{2} quadtext{for}quad 0 < t < 1.$$



      In the above notation we have $Gamma = {(t,t): 0 < t < 1}$ and $g(t) = t/2$. I got the solution $f(x,y) = z(s) + g(x(0)) =z(s) + frac{x_0}{2} = y - frac{x - 1/2y^2}{2-y}$ where $y neq 2$, what can I say about the validity of my solution? Is it valid for $mathbb{R^2}$ or just for points $x, y$ such that the characteristic curve trough them, $(x(s),y(s))$ lies on $Gamma$ for $s=0$?







      pde characteristics






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      edited Dec 7 '18 at 18:12









      Harry49

      7,49431341




      7,49431341










      asked Dec 5 '18 at 10:41









      h3h325h3h325

      19610




      19610






















          2 Answers
          2






          active

          oldest

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          2












          $begingroup$

          This PDE is a non-homogeneous inviscid Burgers' equation. Let us apply the method of characteristics:





          • $frac{text d y}{text ds} = 1$, letting $y(0) = t$, we know $y = t + s$


          • $frac{text d f}{text ds} = 1$, letting $f(0) = t/2$, we know $f = t/2 + s$


          • $frac{text d x}{text ds} = f$, letting $x(0) = t$, we know $x = t + s(t + s)/2$


          Therefore, the characteristics are the curves $x = t + y(y-t)/2$ for $0<t<1$, along which $f$ is given by
          $$
          f(x,y) = y - frac{x-y^2/2}{2-y} , .
          $$

          One can note that this expression is not defined at $y=2$, where the denominator vanishes. The solution is only valid over the domain in yellow
          $$
          (x,y) in lbrace (t + y(y-t)/2, y), 0<t<1, y<2rbrace
          $$



          characteristics



          Indeed, we start following characteristic curves at some point $(t,t)$ with $0<t<1$. On the one hand, we can decrease $y$ without restriction. But on the other hand, we cannot increase $y$ further than $y=2^-$. At the point $(x,y) = (2,2)$, all characteristic curves coming from $lbrace(t,t), 0<t<1rbrace$ intersect. The classical solution collapses (similar case to this one).





          The reason why the domain of validity is restricted to $y<2$ can be compared to the case of the Riccati ODE initial-value problem
          $$
          x'(t) = -x(t)^2 qquadtext{with}qquad x(0) = -1, .
          $$

          Indeed, the solution $x(t) = frac{1}{t-1}$ blows up at $t=1$ and cannot be used for larger times (even if the solution function is defined for $t>1$). Note that this simple ODE problem can be linked to the PDE problem by following a derivation by Lax, leading to Eq. (6.10) p. 36 of (1).



          (1) P.D. Lax, Hyperbolic systems of conservation laws and the mathematical theory of shock waves, SIAM, 1973 doi:10.1137/1.9781611970562.ch1






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Why is the solution only valid on the domain in yellow when $f(x,y)$ is defined for all $x$, $y neq 2$ ? It seems to me that the validity of the solution still holds outside although we might lose unicity since the associated system of ODE does not have initial conditions.
            $endgroup$
            – h3h325
            Dec 5 '18 at 15:45












          • $begingroup$
            I understand why the solution is not valid for $y =2$, I also see why the idea behind the method should fail if the characteristic curves intersect. But as I see it for $y>2$ the function $f(x,y)$ satisfies the pde and also the initial condition, so why can we not take it as a solution? Thanks a lot for the answer by the way.
            $endgroup$
            – h3h325
            Dec 5 '18 at 16:12












          • $begingroup$
            I see! So the conclusion is that the solution still holds but we cannot guarantee uniqueness, right?
            $endgroup$
            – h3h325
            Dec 5 '18 at 16:22










          • $begingroup$
            Sorry if im beeing too picky but I still don't get your point. The system of ODEs does have a solution by Picard-Lindelöf (even if it is not unique), since 1, 1 and $f$ are lipschitz, for any $[S, infty)$, for some $S$ corresponding to $y >2$, so why are we losing existence? Also $f(x,y)$ satisfies the PDE everywhere where it is defined, and also the initial conditions. So how is $f$ not a solution? Could you maybe refer me somewhere were this is explained?
            $endgroup$
            – h3h325
            Dec 5 '18 at 16:43








          • 1




            $begingroup$
            @h3h325 I'm troubled by your statement about Picard Lindelof, please note that it is a local existence and uniqueness theorem
            $endgroup$
            – qbert
            Dec 7 '18 at 18:24



















          0












          $begingroup$

          $$f(x,y)frac{partial f}{partial x} + frac{partial f}{partial y}= 1 tag 1$$
          Charpit-Lagrange equations :
          $$frac{dx}{f}=frac{dy}{1}=frac{df}{1}$$
          First family of characteristic curves, from $frac{dy}{1}=frac{df}{1}$ :
          $$f-y=c_1$$
          Second family of characteristic curves, from $frac{dx}{f}=frac{df}{1}$
          $$frac12 f^2-x=c_2$$
          General solution of the PDE expressed on the form of implicit equation :
          $$frac12 f^2-x=Phi(f-y) tag 2$$
          Where $Phi$ is an arbitrary function, to be determined according to boundary condition.



          Condition : $f(x,x)=frac{x}{2}$



          $frac12 (frac{x}{2})^2-x=Phi(frac{x}{2}-x)quad;quad frac{x^2}{8}-x=Phi(-frac{x}{2})$



          Let $X=-frac{x}{2}quad;quad x=-2X$



          $ frac{(-2X)^2}{8}-(-2X)=Phi(X)$
          $$Phi(X)=frac12 X^2+2X$$
          So, the function $Phi$ is determined. We put it into the general solution Eq.$(2)$ where $X=f-y$ :
          $$frac12 f^2-x=frac12 (f-y)^2+2(f-y)$$
          After simplification :
          $$f(x,y)=frac{y^2-4y+2x}{2y-4}qquad yneq 2.$$
          This is the solution of the PDE which satisfies the boundary condition.



          One can check that this solution is valid in putting it into the PDE.






          share|cite|improve this answer









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            2 Answers
            2






            active

            oldest

            votes








            2 Answers
            2






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

            votes









            2












            $begingroup$

            This PDE is a non-homogeneous inviscid Burgers' equation. Let us apply the method of characteristics:





            • $frac{text d y}{text ds} = 1$, letting $y(0) = t$, we know $y = t + s$


            • $frac{text d f}{text ds} = 1$, letting $f(0) = t/2$, we know $f = t/2 + s$


            • $frac{text d x}{text ds} = f$, letting $x(0) = t$, we know $x = t + s(t + s)/2$


            Therefore, the characteristics are the curves $x = t + y(y-t)/2$ for $0<t<1$, along which $f$ is given by
            $$
            f(x,y) = y - frac{x-y^2/2}{2-y} , .
            $$

            One can note that this expression is not defined at $y=2$, where the denominator vanishes. The solution is only valid over the domain in yellow
            $$
            (x,y) in lbrace (t + y(y-t)/2, y), 0<t<1, y<2rbrace
            $$



            characteristics



            Indeed, we start following characteristic curves at some point $(t,t)$ with $0<t<1$. On the one hand, we can decrease $y$ without restriction. But on the other hand, we cannot increase $y$ further than $y=2^-$. At the point $(x,y) = (2,2)$, all characteristic curves coming from $lbrace(t,t), 0<t<1rbrace$ intersect. The classical solution collapses (similar case to this one).





            The reason why the domain of validity is restricted to $y<2$ can be compared to the case of the Riccati ODE initial-value problem
            $$
            x'(t) = -x(t)^2 qquadtext{with}qquad x(0) = -1, .
            $$

            Indeed, the solution $x(t) = frac{1}{t-1}$ blows up at $t=1$ and cannot be used for larger times (even if the solution function is defined for $t>1$). Note that this simple ODE problem can be linked to the PDE problem by following a derivation by Lax, leading to Eq. (6.10) p. 36 of (1).



            (1) P.D. Lax, Hyperbolic systems of conservation laws and the mathematical theory of shock waves, SIAM, 1973 doi:10.1137/1.9781611970562.ch1






            share|cite|improve this answer











            $endgroup$













            • $begingroup$
              Why is the solution only valid on the domain in yellow when $f(x,y)$ is defined for all $x$, $y neq 2$ ? It seems to me that the validity of the solution still holds outside although we might lose unicity since the associated system of ODE does not have initial conditions.
              $endgroup$
              – h3h325
              Dec 5 '18 at 15:45












            • $begingroup$
              I understand why the solution is not valid for $y =2$, I also see why the idea behind the method should fail if the characteristic curves intersect. But as I see it for $y>2$ the function $f(x,y)$ satisfies the pde and also the initial condition, so why can we not take it as a solution? Thanks a lot for the answer by the way.
              $endgroup$
              – h3h325
              Dec 5 '18 at 16:12












            • $begingroup$
              I see! So the conclusion is that the solution still holds but we cannot guarantee uniqueness, right?
              $endgroup$
              – h3h325
              Dec 5 '18 at 16:22










            • $begingroup$
              Sorry if im beeing too picky but I still don't get your point. The system of ODEs does have a solution by Picard-Lindelöf (even if it is not unique), since 1, 1 and $f$ are lipschitz, for any $[S, infty)$, for some $S$ corresponding to $y >2$, so why are we losing existence? Also $f(x,y)$ satisfies the PDE everywhere where it is defined, and also the initial conditions. So how is $f$ not a solution? Could you maybe refer me somewhere were this is explained?
              $endgroup$
              – h3h325
              Dec 5 '18 at 16:43








            • 1




              $begingroup$
              @h3h325 I'm troubled by your statement about Picard Lindelof, please note that it is a local existence and uniqueness theorem
              $endgroup$
              – qbert
              Dec 7 '18 at 18:24
















            2












            $begingroup$

            This PDE is a non-homogeneous inviscid Burgers' equation. Let us apply the method of characteristics:





            • $frac{text d y}{text ds} = 1$, letting $y(0) = t$, we know $y = t + s$


            • $frac{text d f}{text ds} = 1$, letting $f(0) = t/2$, we know $f = t/2 + s$


            • $frac{text d x}{text ds} = f$, letting $x(0) = t$, we know $x = t + s(t + s)/2$


            Therefore, the characteristics are the curves $x = t + y(y-t)/2$ for $0<t<1$, along which $f$ is given by
            $$
            f(x,y) = y - frac{x-y^2/2}{2-y} , .
            $$

            One can note that this expression is not defined at $y=2$, where the denominator vanishes. The solution is only valid over the domain in yellow
            $$
            (x,y) in lbrace (t + y(y-t)/2, y), 0<t<1, y<2rbrace
            $$



            characteristics



            Indeed, we start following characteristic curves at some point $(t,t)$ with $0<t<1$. On the one hand, we can decrease $y$ without restriction. But on the other hand, we cannot increase $y$ further than $y=2^-$. At the point $(x,y) = (2,2)$, all characteristic curves coming from $lbrace(t,t), 0<t<1rbrace$ intersect. The classical solution collapses (similar case to this one).





            The reason why the domain of validity is restricted to $y<2$ can be compared to the case of the Riccati ODE initial-value problem
            $$
            x'(t) = -x(t)^2 qquadtext{with}qquad x(0) = -1, .
            $$

            Indeed, the solution $x(t) = frac{1}{t-1}$ blows up at $t=1$ and cannot be used for larger times (even if the solution function is defined for $t>1$). Note that this simple ODE problem can be linked to the PDE problem by following a derivation by Lax, leading to Eq. (6.10) p. 36 of (1).



            (1) P.D. Lax, Hyperbolic systems of conservation laws and the mathematical theory of shock waves, SIAM, 1973 doi:10.1137/1.9781611970562.ch1






            share|cite|improve this answer











            $endgroup$













            • $begingroup$
              Why is the solution only valid on the domain in yellow when $f(x,y)$ is defined for all $x$, $y neq 2$ ? It seems to me that the validity of the solution still holds outside although we might lose unicity since the associated system of ODE does not have initial conditions.
              $endgroup$
              – h3h325
              Dec 5 '18 at 15:45












            • $begingroup$
              I understand why the solution is not valid for $y =2$, I also see why the idea behind the method should fail if the characteristic curves intersect. But as I see it for $y>2$ the function $f(x,y)$ satisfies the pde and also the initial condition, so why can we not take it as a solution? Thanks a lot for the answer by the way.
              $endgroup$
              – h3h325
              Dec 5 '18 at 16:12












            • $begingroup$
              I see! So the conclusion is that the solution still holds but we cannot guarantee uniqueness, right?
              $endgroup$
              – h3h325
              Dec 5 '18 at 16:22










            • $begingroup$
              Sorry if im beeing too picky but I still don't get your point. The system of ODEs does have a solution by Picard-Lindelöf (even if it is not unique), since 1, 1 and $f$ are lipschitz, for any $[S, infty)$, for some $S$ corresponding to $y >2$, so why are we losing existence? Also $f(x,y)$ satisfies the PDE everywhere where it is defined, and also the initial conditions. So how is $f$ not a solution? Could you maybe refer me somewhere were this is explained?
              $endgroup$
              – h3h325
              Dec 5 '18 at 16:43








            • 1




              $begingroup$
              @h3h325 I'm troubled by your statement about Picard Lindelof, please note that it is a local existence and uniqueness theorem
              $endgroup$
              – qbert
              Dec 7 '18 at 18:24














            2












            2








            2





            $begingroup$

            This PDE is a non-homogeneous inviscid Burgers' equation. Let us apply the method of characteristics:





            • $frac{text d y}{text ds} = 1$, letting $y(0) = t$, we know $y = t + s$


            • $frac{text d f}{text ds} = 1$, letting $f(0) = t/2$, we know $f = t/2 + s$


            • $frac{text d x}{text ds} = f$, letting $x(0) = t$, we know $x = t + s(t + s)/2$


            Therefore, the characteristics are the curves $x = t + y(y-t)/2$ for $0<t<1$, along which $f$ is given by
            $$
            f(x,y) = y - frac{x-y^2/2}{2-y} , .
            $$

            One can note that this expression is not defined at $y=2$, where the denominator vanishes. The solution is only valid over the domain in yellow
            $$
            (x,y) in lbrace (t + y(y-t)/2, y), 0<t<1, y<2rbrace
            $$



            characteristics



            Indeed, we start following characteristic curves at some point $(t,t)$ with $0<t<1$. On the one hand, we can decrease $y$ without restriction. But on the other hand, we cannot increase $y$ further than $y=2^-$. At the point $(x,y) = (2,2)$, all characteristic curves coming from $lbrace(t,t), 0<t<1rbrace$ intersect. The classical solution collapses (similar case to this one).





            The reason why the domain of validity is restricted to $y<2$ can be compared to the case of the Riccati ODE initial-value problem
            $$
            x'(t) = -x(t)^2 qquadtext{with}qquad x(0) = -1, .
            $$

            Indeed, the solution $x(t) = frac{1}{t-1}$ blows up at $t=1$ and cannot be used for larger times (even if the solution function is defined for $t>1$). Note that this simple ODE problem can be linked to the PDE problem by following a derivation by Lax, leading to Eq. (6.10) p. 36 of (1).



            (1) P.D. Lax, Hyperbolic systems of conservation laws and the mathematical theory of shock waves, SIAM, 1973 doi:10.1137/1.9781611970562.ch1






            share|cite|improve this answer











            $endgroup$



            This PDE is a non-homogeneous inviscid Burgers' equation. Let us apply the method of characteristics:





            • $frac{text d y}{text ds} = 1$, letting $y(0) = t$, we know $y = t + s$


            • $frac{text d f}{text ds} = 1$, letting $f(0) = t/2$, we know $f = t/2 + s$


            • $frac{text d x}{text ds} = f$, letting $x(0) = t$, we know $x = t + s(t + s)/2$


            Therefore, the characteristics are the curves $x = t + y(y-t)/2$ for $0<t<1$, along which $f$ is given by
            $$
            f(x,y) = y - frac{x-y^2/2}{2-y} , .
            $$

            One can note that this expression is not defined at $y=2$, where the denominator vanishes. The solution is only valid over the domain in yellow
            $$
            (x,y) in lbrace (t + y(y-t)/2, y), 0<t<1, y<2rbrace
            $$



            characteristics



            Indeed, we start following characteristic curves at some point $(t,t)$ with $0<t<1$. On the one hand, we can decrease $y$ without restriction. But on the other hand, we cannot increase $y$ further than $y=2^-$. At the point $(x,y) = (2,2)$, all characteristic curves coming from $lbrace(t,t), 0<t<1rbrace$ intersect. The classical solution collapses (similar case to this one).





            The reason why the domain of validity is restricted to $y<2$ can be compared to the case of the Riccati ODE initial-value problem
            $$
            x'(t) = -x(t)^2 qquadtext{with}qquad x(0) = -1, .
            $$

            Indeed, the solution $x(t) = frac{1}{t-1}$ blows up at $t=1$ and cannot be used for larger times (even if the solution function is defined for $t>1$). Note that this simple ODE problem can be linked to the PDE problem by following a derivation by Lax, leading to Eq. (6.10) p. 36 of (1).



            (1) P.D. Lax, Hyperbolic systems of conservation laws and the mathematical theory of shock waves, SIAM, 1973 doi:10.1137/1.9781611970562.ch1







            share|cite|improve this answer














            share|cite|improve this answer



            share|cite|improve this answer








            edited Dec 8 '18 at 11:05

























            answered Dec 5 '18 at 14:09









            Harry49Harry49

            7,49431341




            7,49431341












            • $begingroup$
              Why is the solution only valid on the domain in yellow when $f(x,y)$ is defined for all $x$, $y neq 2$ ? It seems to me that the validity of the solution still holds outside although we might lose unicity since the associated system of ODE does not have initial conditions.
              $endgroup$
              – h3h325
              Dec 5 '18 at 15:45












            • $begingroup$
              I understand why the solution is not valid for $y =2$, I also see why the idea behind the method should fail if the characteristic curves intersect. But as I see it for $y>2$ the function $f(x,y)$ satisfies the pde and also the initial condition, so why can we not take it as a solution? Thanks a lot for the answer by the way.
              $endgroup$
              – h3h325
              Dec 5 '18 at 16:12












            • $begingroup$
              I see! So the conclusion is that the solution still holds but we cannot guarantee uniqueness, right?
              $endgroup$
              – h3h325
              Dec 5 '18 at 16:22










            • $begingroup$
              Sorry if im beeing too picky but I still don't get your point. The system of ODEs does have a solution by Picard-Lindelöf (even if it is not unique), since 1, 1 and $f$ are lipschitz, for any $[S, infty)$, for some $S$ corresponding to $y >2$, so why are we losing existence? Also $f(x,y)$ satisfies the PDE everywhere where it is defined, and also the initial conditions. So how is $f$ not a solution? Could you maybe refer me somewhere were this is explained?
              $endgroup$
              – h3h325
              Dec 5 '18 at 16:43








            • 1




              $begingroup$
              @h3h325 I'm troubled by your statement about Picard Lindelof, please note that it is a local existence and uniqueness theorem
              $endgroup$
              – qbert
              Dec 7 '18 at 18:24


















            • $begingroup$
              Why is the solution only valid on the domain in yellow when $f(x,y)$ is defined for all $x$, $y neq 2$ ? It seems to me that the validity of the solution still holds outside although we might lose unicity since the associated system of ODE does not have initial conditions.
              $endgroup$
              – h3h325
              Dec 5 '18 at 15:45












            • $begingroup$
              I understand why the solution is not valid for $y =2$, I also see why the idea behind the method should fail if the characteristic curves intersect. But as I see it for $y>2$ the function $f(x,y)$ satisfies the pde and also the initial condition, so why can we not take it as a solution? Thanks a lot for the answer by the way.
              $endgroup$
              – h3h325
              Dec 5 '18 at 16:12












            • $begingroup$
              I see! So the conclusion is that the solution still holds but we cannot guarantee uniqueness, right?
              $endgroup$
              – h3h325
              Dec 5 '18 at 16:22










            • $begingroup$
              Sorry if im beeing too picky but I still don't get your point. The system of ODEs does have a solution by Picard-Lindelöf (even if it is not unique), since 1, 1 and $f$ are lipschitz, for any $[S, infty)$, for some $S$ corresponding to $y >2$, so why are we losing existence? Also $f(x,y)$ satisfies the PDE everywhere where it is defined, and also the initial conditions. So how is $f$ not a solution? Could you maybe refer me somewhere were this is explained?
              $endgroup$
              – h3h325
              Dec 5 '18 at 16:43








            • 1




              $begingroup$
              @h3h325 I'm troubled by your statement about Picard Lindelof, please note that it is a local existence and uniqueness theorem
              $endgroup$
              – qbert
              Dec 7 '18 at 18:24
















            $begingroup$
            Why is the solution only valid on the domain in yellow when $f(x,y)$ is defined for all $x$, $y neq 2$ ? It seems to me that the validity of the solution still holds outside although we might lose unicity since the associated system of ODE does not have initial conditions.
            $endgroup$
            – h3h325
            Dec 5 '18 at 15:45






            $begingroup$
            Why is the solution only valid on the domain in yellow when $f(x,y)$ is defined for all $x$, $y neq 2$ ? It seems to me that the validity of the solution still holds outside although we might lose unicity since the associated system of ODE does not have initial conditions.
            $endgroup$
            – h3h325
            Dec 5 '18 at 15:45














            $begingroup$
            I understand why the solution is not valid for $y =2$, I also see why the idea behind the method should fail if the characteristic curves intersect. But as I see it for $y>2$ the function $f(x,y)$ satisfies the pde and also the initial condition, so why can we not take it as a solution? Thanks a lot for the answer by the way.
            $endgroup$
            – h3h325
            Dec 5 '18 at 16:12






            $begingroup$
            I understand why the solution is not valid for $y =2$, I also see why the idea behind the method should fail if the characteristic curves intersect. But as I see it for $y>2$ the function $f(x,y)$ satisfies the pde and also the initial condition, so why can we not take it as a solution? Thanks a lot for the answer by the way.
            $endgroup$
            – h3h325
            Dec 5 '18 at 16:12














            $begingroup$
            I see! So the conclusion is that the solution still holds but we cannot guarantee uniqueness, right?
            $endgroup$
            – h3h325
            Dec 5 '18 at 16:22




            $begingroup$
            I see! So the conclusion is that the solution still holds but we cannot guarantee uniqueness, right?
            $endgroup$
            – h3h325
            Dec 5 '18 at 16:22












            $begingroup$
            Sorry if im beeing too picky but I still don't get your point. The system of ODEs does have a solution by Picard-Lindelöf (even if it is not unique), since 1, 1 and $f$ are lipschitz, for any $[S, infty)$, for some $S$ corresponding to $y >2$, so why are we losing existence? Also $f(x,y)$ satisfies the PDE everywhere where it is defined, and also the initial conditions. So how is $f$ not a solution? Could you maybe refer me somewhere were this is explained?
            $endgroup$
            – h3h325
            Dec 5 '18 at 16:43






            $begingroup$
            Sorry if im beeing too picky but I still don't get your point. The system of ODEs does have a solution by Picard-Lindelöf (even if it is not unique), since 1, 1 and $f$ are lipschitz, for any $[S, infty)$, for some $S$ corresponding to $y >2$, so why are we losing existence? Also $f(x,y)$ satisfies the PDE everywhere where it is defined, and also the initial conditions. So how is $f$ not a solution? Could you maybe refer me somewhere were this is explained?
            $endgroup$
            – h3h325
            Dec 5 '18 at 16:43






            1




            1




            $begingroup$
            @h3h325 I'm troubled by your statement about Picard Lindelof, please note that it is a local existence and uniqueness theorem
            $endgroup$
            – qbert
            Dec 7 '18 at 18:24




            $begingroup$
            @h3h325 I'm troubled by your statement about Picard Lindelof, please note that it is a local existence and uniqueness theorem
            $endgroup$
            – qbert
            Dec 7 '18 at 18:24











            0












            $begingroup$

            $$f(x,y)frac{partial f}{partial x} + frac{partial f}{partial y}= 1 tag 1$$
            Charpit-Lagrange equations :
            $$frac{dx}{f}=frac{dy}{1}=frac{df}{1}$$
            First family of characteristic curves, from $frac{dy}{1}=frac{df}{1}$ :
            $$f-y=c_1$$
            Second family of characteristic curves, from $frac{dx}{f}=frac{df}{1}$
            $$frac12 f^2-x=c_2$$
            General solution of the PDE expressed on the form of implicit equation :
            $$frac12 f^2-x=Phi(f-y) tag 2$$
            Where $Phi$ is an arbitrary function, to be determined according to boundary condition.



            Condition : $f(x,x)=frac{x}{2}$



            $frac12 (frac{x}{2})^2-x=Phi(frac{x}{2}-x)quad;quad frac{x^2}{8}-x=Phi(-frac{x}{2})$



            Let $X=-frac{x}{2}quad;quad x=-2X$



            $ frac{(-2X)^2}{8}-(-2X)=Phi(X)$
            $$Phi(X)=frac12 X^2+2X$$
            So, the function $Phi$ is determined. We put it into the general solution Eq.$(2)$ where $X=f-y$ :
            $$frac12 f^2-x=frac12 (f-y)^2+2(f-y)$$
            After simplification :
            $$f(x,y)=frac{y^2-4y+2x}{2y-4}qquad yneq 2.$$
            This is the solution of the PDE which satisfies the boundary condition.



            One can check that this solution is valid in putting it into the PDE.






            share|cite|improve this answer









            $endgroup$


















              0












              $begingroup$

              $$f(x,y)frac{partial f}{partial x} + frac{partial f}{partial y}= 1 tag 1$$
              Charpit-Lagrange equations :
              $$frac{dx}{f}=frac{dy}{1}=frac{df}{1}$$
              First family of characteristic curves, from $frac{dy}{1}=frac{df}{1}$ :
              $$f-y=c_1$$
              Second family of characteristic curves, from $frac{dx}{f}=frac{df}{1}$
              $$frac12 f^2-x=c_2$$
              General solution of the PDE expressed on the form of implicit equation :
              $$frac12 f^2-x=Phi(f-y) tag 2$$
              Where $Phi$ is an arbitrary function, to be determined according to boundary condition.



              Condition : $f(x,x)=frac{x}{2}$



              $frac12 (frac{x}{2})^2-x=Phi(frac{x}{2}-x)quad;quad frac{x^2}{8}-x=Phi(-frac{x}{2})$



              Let $X=-frac{x}{2}quad;quad x=-2X$



              $ frac{(-2X)^2}{8}-(-2X)=Phi(X)$
              $$Phi(X)=frac12 X^2+2X$$
              So, the function $Phi$ is determined. We put it into the general solution Eq.$(2)$ where $X=f-y$ :
              $$frac12 f^2-x=frac12 (f-y)^2+2(f-y)$$
              After simplification :
              $$f(x,y)=frac{y^2-4y+2x}{2y-4}qquad yneq 2.$$
              This is the solution of the PDE which satisfies the boundary condition.



              One can check that this solution is valid in putting it into the PDE.






              share|cite|improve this answer









              $endgroup$
















                0












                0








                0





                $begingroup$

                $$f(x,y)frac{partial f}{partial x} + frac{partial f}{partial y}= 1 tag 1$$
                Charpit-Lagrange equations :
                $$frac{dx}{f}=frac{dy}{1}=frac{df}{1}$$
                First family of characteristic curves, from $frac{dy}{1}=frac{df}{1}$ :
                $$f-y=c_1$$
                Second family of characteristic curves, from $frac{dx}{f}=frac{df}{1}$
                $$frac12 f^2-x=c_2$$
                General solution of the PDE expressed on the form of implicit equation :
                $$frac12 f^2-x=Phi(f-y) tag 2$$
                Where $Phi$ is an arbitrary function, to be determined according to boundary condition.



                Condition : $f(x,x)=frac{x}{2}$



                $frac12 (frac{x}{2})^2-x=Phi(frac{x}{2}-x)quad;quad frac{x^2}{8}-x=Phi(-frac{x}{2})$



                Let $X=-frac{x}{2}quad;quad x=-2X$



                $ frac{(-2X)^2}{8}-(-2X)=Phi(X)$
                $$Phi(X)=frac12 X^2+2X$$
                So, the function $Phi$ is determined. We put it into the general solution Eq.$(2)$ where $X=f-y$ :
                $$frac12 f^2-x=frac12 (f-y)^2+2(f-y)$$
                After simplification :
                $$f(x,y)=frac{y^2-4y+2x}{2y-4}qquad yneq 2.$$
                This is the solution of the PDE which satisfies the boundary condition.



                One can check that this solution is valid in putting it into the PDE.






                share|cite|improve this answer









                $endgroup$



                $$f(x,y)frac{partial f}{partial x} + frac{partial f}{partial y}= 1 tag 1$$
                Charpit-Lagrange equations :
                $$frac{dx}{f}=frac{dy}{1}=frac{df}{1}$$
                First family of characteristic curves, from $frac{dy}{1}=frac{df}{1}$ :
                $$f-y=c_1$$
                Second family of characteristic curves, from $frac{dx}{f}=frac{df}{1}$
                $$frac12 f^2-x=c_2$$
                General solution of the PDE expressed on the form of implicit equation :
                $$frac12 f^2-x=Phi(f-y) tag 2$$
                Where $Phi$ is an arbitrary function, to be determined according to boundary condition.



                Condition : $f(x,x)=frac{x}{2}$



                $frac12 (frac{x}{2})^2-x=Phi(frac{x}{2}-x)quad;quad frac{x^2}{8}-x=Phi(-frac{x}{2})$



                Let $X=-frac{x}{2}quad;quad x=-2X$



                $ frac{(-2X)^2}{8}-(-2X)=Phi(X)$
                $$Phi(X)=frac12 X^2+2X$$
                So, the function $Phi$ is determined. We put it into the general solution Eq.$(2)$ where $X=f-y$ :
                $$frac12 f^2-x=frac12 (f-y)^2+2(f-y)$$
                After simplification :
                $$f(x,y)=frac{y^2-4y+2x}{2y-4}qquad yneq 2.$$
                This is the solution of the PDE which satisfies the boundary condition.



                One can check that this solution is valid in putting it into the PDE.







                share|cite|improve this answer












                share|cite|improve this answer



                share|cite|improve this answer










                answered Dec 8 '18 at 8:11









                JJacquelinJJacquelin

                44.2k21854




                44.2k21854






























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