When solving 2 ODEs by eliminating time is valid?
$begingroup$
When solving a 2nd order ODE, say
begin{equation}tag{*}begin{cases}frac{dx}{dt}=f(x,y)\frac{dy}{dt}=g(x,y),end{cases}end{equation}
it is common to eliminate time and solve the resulting 1st order ODE
begin{equation}tag{**}frac{dy}{dx}=frac{g(x,y)}{f(x,y)}end{equation}
that gives a dependence $y=y(x)$.
I wonder what are the conditions for this approach to be valid at an equilibrium point $(x^*,y^*)$? At this point, the fraction $frac{g(x^*,y^*)}{f(x^*,y^*)}=frac{0}{0}$. Potentially, this indeterminacy can be resolved using a sort of multivalued L'Hopital rule, but that is quite tricky.
Intuitively, I understand that the answer depends on the structure of the eigenvalues of the linearization of (*) at $(x^*,y^*)$, but I cannot formulate this quite well.
Let, say, the linearized system have a saddle at $(x^*,y^*)$. The equation ($**$) has two solutions corresponding to the stable and the unstable manifolds (they seem to be both unstable as they go away from $(0,0)$). Does it imply that the DE ($**$) isn't well posed?
ordinary-differential-equations dynamical-systems
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add a comment |
$begingroup$
When solving a 2nd order ODE, say
begin{equation}tag{*}begin{cases}frac{dx}{dt}=f(x,y)\frac{dy}{dt}=g(x,y),end{cases}end{equation}
it is common to eliminate time and solve the resulting 1st order ODE
begin{equation}tag{**}frac{dy}{dx}=frac{g(x,y)}{f(x,y)}end{equation}
that gives a dependence $y=y(x)$.
I wonder what are the conditions for this approach to be valid at an equilibrium point $(x^*,y^*)$? At this point, the fraction $frac{g(x^*,y^*)}{f(x^*,y^*)}=frac{0}{0}$. Potentially, this indeterminacy can be resolved using a sort of multivalued L'Hopital rule, but that is quite tricky.
Intuitively, I understand that the answer depends on the structure of the eigenvalues of the linearization of (*) at $(x^*,y^*)$, but I cannot formulate this quite well.
Let, say, the linearized system have a saddle at $(x^*,y^*)$. The equation ($**$) has two solutions corresponding to the stable and the unstable manifolds (they seem to be both unstable as they go away from $(0,0)$). Does it imply that the DE ($**$) isn't well posed?
ordinary-differential-equations dynamical-systems
$endgroup$
add a comment |
$begingroup$
When solving a 2nd order ODE, say
begin{equation}tag{*}begin{cases}frac{dx}{dt}=f(x,y)\frac{dy}{dt}=g(x,y),end{cases}end{equation}
it is common to eliminate time and solve the resulting 1st order ODE
begin{equation}tag{**}frac{dy}{dx}=frac{g(x,y)}{f(x,y)}end{equation}
that gives a dependence $y=y(x)$.
I wonder what are the conditions for this approach to be valid at an equilibrium point $(x^*,y^*)$? At this point, the fraction $frac{g(x^*,y^*)}{f(x^*,y^*)}=frac{0}{0}$. Potentially, this indeterminacy can be resolved using a sort of multivalued L'Hopital rule, but that is quite tricky.
Intuitively, I understand that the answer depends on the structure of the eigenvalues of the linearization of (*) at $(x^*,y^*)$, but I cannot formulate this quite well.
Let, say, the linearized system have a saddle at $(x^*,y^*)$. The equation ($**$) has two solutions corresponding to the stable and the unstable manifolds (they seem to be both unstable as they go away from $(0,0)$). Does it imply that the DE ($**$) isn't well posed?
ordinary-differential-equations dynamical-systems
$endgroup$
When solving a 2nd order ODE, say
begin{equation}tag{*}begin{cases}frac{dx}{dt}=f(x,y)\frac{dy}{dt}=g(x,y),end{cases}end{equation}
it is common to eliminate time and solve the resulting 1st order ODE
begin{equation}tag{**}frac{dy}{dx}=frac{g(x,y)}{f(x,y)}end{equation}
that gives a dependence $y=y(x)$.
I wonder what are the conditions for this approach to be valid at an equilibrium point $(x^*,y^*)$? At this point, the fraction $frac{g(x^*,y^*)}{f(x^*,y^*)}=frac{0}{0}$. Potentially, this indeterminacy can be resolved using a sort of multivalued L'Hopital rule, but that is quite tricky.
Intuitively, I understand that the answer depends on the structure of the eigenvalues of the linearization of (*) at $(x^*,y^*)$, but I cannot formulate this quite well.
Let, say, the linearized system have a saddle at $(x^*,y^*)$. The equation ($**$) has two solutions corresponding to the stable and the unstable manifolds (they seem to be both unstable as they go away from $(0,0)$). Does it imply that the DE ($**$) isn't well posed?
ordinary-differential-equations dynamical-systems
ordinary-differential-equations dynamical-systems
asked Dec 25 '18 at 15:41
DmitryDmitry
716618
716618
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Indeed, it doesn't work at an equilibrium point, but you don't really need it there: $(x,y) = (x^*, y^*)$ is the solution with initial conditions $x(0)=x^*, y(0)=y^*$.
It's also not defined on the curve $f(x,y) = 0$ (although there, when $g(x,y) ne 0$, you could look at $x$ as a function of $y$). However, it is OK everywhere else, and it can be useful to study limits of these solutions as $x to x^*$.
$endgroup$
$begingroup$
Dear @Robert, I ask because I often saw this approach used for computing stable/unstable manifolds of a system of nonlinear DEs. In this case the solution goes exactly from the equilibrium. Sometimes this seems to be a trick rather than a formal approach.
$endgroup$
– Dmitry
Dec 25 '18 at 16:24
$begingroup$
I don't think so: the stable and unstable manifolds would have the same initial condition $y(x^*) = y^*$. The equilibrium should correspond to a limit. For example, the linear saddle-point system $dot{x} = y$, $dot{y} = x$ leads to $dfrac{dy}{dx} = dfrac{x}{y}$, not defined at $(0,0)$, but the solutions $y=x$ and $y = -x$ have limits $(0,0)$ as $x to 0$.
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– Robert Israel
Dec 26 '18 at 0:52
add a comment |
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1 Answer
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1 Answer
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$begingroup$
Indeed, it doesn't work at an equilibrium point, but you don't really need it there: $(x,y) = (x^*, y^*)$ is the solution with initial conditions $x(0)=x^*, y(0)=y^*$.
It's also not defined on the curve $f(x,y) = 0$ (although there, when $g(x,y) ne 0$, you could look at $x$ as a function of $y$). However, it is OK everywhere else, and it can be useful to study limits of these solutions as $x to x^*$.
$endgroup$
$begingroup$
Dear @Robert, I ask because I often saw this approach used for computing stable/unstable manifolds of a system of nonlinear DEs. In this case the solution goes exactly from the equilibrium. Sometimes this seems to be a trick rather than a formal approach.
$endgroup$
– Dmitry
Dec 25 '18 at 16:24
$begingroup$
I don't think so: the stable and unstable manifolds would have the same initial condition $y(x^*) = y^*$. The equilibrium should correspond to a limit. For example, the linear saddle-point system $dot{x} = y$, $dot{y} = x$ leads to $dfrac{dy}{dx} = dfrac{x}{y}$, not defined at $(0,0)$, but the solutions $y=x$ and $y = -x$ have limits $(0,0)$ as $x to 0$.
$endgroup$
– Robert Israel
Dec 26 '18 at 0:52
add a comment |
$begingroup$
Indeed, it doesn't work at an equilibrium point, but you don't really need it there: $(x,y) = (x^*, y^*)$ is the solution with initial conditions $x(0)=x^*, y(0)=y^*$.
It's also not defined on the curve $f(x,y) = 0$ (although there, when $g(x,y) ne 0$, you could look at $x$ as a function of $y$). However, it is OK everywhere else, and it can be useful to study limits of these solutions as $x to x^*$.
$endgroup$
$begingroup$
Dear @Robert, I ask because I often saw this approach used for computing stable/unstable manifolds of a system of nonlinear DEs. In this case the solution goes exactly from the equilibrium. Sometimes this seems to be a trick rather than a formal approach.
$endgroup$
– Dmitry
Dec 25 '18 at 16:24
$begingroup$
I don't think so: the stable and unstable manifolds would have the same initial condition $y(x^*) = y^*$. The equilibrium should correspond to a limit. For example, the linear saddle-point system $dot{x} = y$, $dot{y} = x$ leads to $dfrac{dy}{dx} = dfrac{x}{y}$, not defined at $(0,0)$, but the solutions $y=x$ and $y = -x$ have limits $(0,0)$ as $x to 0$.
$endgroup$
– Robert Israel
Dec 26 '18 at 0:52
add a comment |
$begingroup$
Indeed, it doesn't work at an equilibrium point, but you don't really need it there: $(x,y) = (x^*, y^*)$ is the solution with initial conditions $x(0)=x^*, y(0)=y^*$.
It's also not defined on the curve $f(x,y) = 0$ (although there, when $g(x,y) ne 0$, you could look at $x$ as a function of $y$). However, it is OK everywhere else, and it can be useful to study limits of these solutions as $x to x^*$.
$endgroup$
Indeed, it doesn't work at an equilibrium point, but you don't really need it there: $(x,y) = (x^*, y^*)$ is the solution with initial conditions $x(0)=x^*, y(0)=y^*$.
It's also not defined on the curve $f(x,y) = 0$ (although there, when $g(x,y) ne 0$, you could look at $x$ as a function of $y$). However, it is OK everywhere else, and it can be useful to study limits of these solutions as $x to x^*$.
answered Dec 25 '18 at 16:00
Robert IsraelRobert Israel
331k23220475
331k23220475
$begingroup$
Dear @Robert, I ask because I often saw this approach used for computing stable/unstable manifolds of a system of nonlinear DEs. In this case the solution goes exactly from the equilibrium. Sometimes this seems to be a trick rather than a formal approach.
$endgroup$
– Dmitry
Dec 25 '18 at 16:24
$begingroup$
I don't think so: the stable and unstable manifolds would have the same initial condition $y(x^*) = y^*$. The equilibrium should correspond to a limit. For example, the linear saddle-point system $dot{x} = y$, $dot{y} = x$ leads to $dfrac{dy}{dx} = dfrac{x}{y}$, not defined at $(0,0)$, but the solutions $y=x$ and $y = -x$ have limits $(0,0)$ as $x to 0$.
$endgroup$
– Robert Israel
Dec 26 '18 at 0:52
add a comment |
$begingroup$
Dear @Robert, I ask because I often saw this approach used for computing stable/unstable manifolds of a system of nonlinear DEs. In this case the solution goes exactly from the equilibrium. Sometimes this seems to be a trick rather than a formal approach.
$endgroup$
– Dmitry
Dec 25 '18 at 16:24
$begingroup$
I don't think so: the stable and unstable manifolds would have the same initial condition $y(x^*) = y^*$. The equilibrium should correspond to a limit. For example, the linear saddle-point system $dot{x} = y$, $dot{y} = x$ leads to $dfrac{dy}{dx} = dfrac{x}{y}$, not defined at $(0,0)$, but the solutions $y=x$ and $y = -x$ have limits $(0,0)$ as $x to 0$.
$endgroup$
– Robert Israel
Dec 26 '18 at 0:52
$begingroup$
Dear @Robert, I ask because I often saw this approach used for computing stable/unstable manifolds of a system of nonlinear DEs. In this case the solution goes exactly from the equilibrium. Sometimes this seems to be a trick rather than a formal approach.
$endgroup$
– Dmitry
Dec 25 '18 at 16:24
$begingroup$
Dear @Robert, I ask because I often saw this approach used for computing stable/unstable manifolds of a system of nonlinear DEs. In this case the solution goes exactly from the equilibrium. Sometimes this seems to be a trick rather than a formal approach.
$endgroup$
– Dmitry
Dec 25 '18 at 16:24
$begingroup$
I don't think so: the stable and unstable manifolds would have the same initial condition $y(x^*) = y^*$. The equilibrium should correspond to a limit. For example, the linear saddle-point system $dot{x} = y$, $dot{y} = x$ leads to $dfrac{dy}{dx} = dfrac{x}{y}$, not defined at $(0,0)$, but the solutions $y=x$ and $y = -x$ have limits $(0,0)$ as $x to 0$.
$endgroup$
– Robert Israel
Dec 26 '18 at 0:52
$begingroup$
I don't think so: the stable and unstable manifolds would have the same initial condition $y(x^*) = y^*$. The equilibrium should correspond to a limit. For example, the linear saddle-point system $dot{x} = y$, $dot{y} = x$ leads to $dfrac{dy}{dx} = dfrac{x}{y}$, not defined at $(0,0)$, but the solutions $y=x$ and $y = -x$ have limits $(0,0)$ as $x to 0$.
$endgroup$
– Robert Israel
Dec 26 '18 at 0:52
add a comment |
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