Using polar coordinates to find the critical points
$begingroup$
I have written the following DE system
$$dot{x} = x+y-x(x^2+y^2)\
dot{y} = -x+3y-y(x^2+y^2) $$
as follows in Polar form:
$$dot{r} = -2rcos^2theta+r(3-r^2) $$
$$dot{theta} = 2sinthetacostheta-1$$
What I wonder is could I also find all the critical points of this system by setting $dot{theta}$ and $dot{r}$ equal to zero? Since these two systems are actually equivalent to each other this seemed like a logical thing to me and at the same time finding the critical points through setting $dot{x}$ and $dot{y}$ equal to zero seems like much more tedious in this state.
Solving for $dot{theta}=0$ leads to $2sinthetacostheta = 1$ which is the same as $sin2theta=1$ leading to $theta = frac{pi}{4}$ or $theta = -frac{3pi}{4}$. Substituting these within $dot{r}$ leads to:
$$-2rcos^2theta+r(3-r^2) = -2r(pmfrac{sqrt{2}}{2})^2 +r(3-r^2)=r(2-r^2)$$
So $dot{r}=0$ if $r=0$ or $r^2=2$. This leads us to the following critical points: $(0,0)$, $(sqrt{2},sqrt{2})$, $(-sqrt{2},-sqrt{2})$.
However what bugs me is that entering these points into the non-polar system does not lead to $dot{x}=dot{y}=0$ for $(sqrt{2},sqrt{2})$, $(-sqrt{2},-sqrt{2})$.
What am I missing here? Is the whole thought of trying to find critical points through putting $dot{theta}$ and $dot{r}$ equal to zero unwise? Any help or hint with this will be much appreciated.
ordinary-differential-equations systems-of-equations polar-coordinates
$endgroup$
add a comment |
$begingroup$
I have written the following DE system
$$dot{x} = x+y-x(x^2+y^2)\
dot{y} = -x+3y-y(x^2+y^2) $$
as follows in Polar form:
$$dot{r} = -2rcos^2theta+r(3-r^2) $$
$$dot{theta} = 2sinthetacostheta-1$$
What I wonder is could I also find all the critical points of this system by setting $dot{theta}$ and $dot{r}$ equal to zero? Since these two systems are actually equivalent to each other this seemed like a logical thing to me and at the same time finding the critical points through setting $dot{x}$ and $dot{y}$ equal to zero seems like much more tedious in this state.
Solving for $dot{theta}=0$ leads to $2sinthetacostheta = 1$ which is the same as $sin2theta=1$ leading to $theta = frac{pi}{4}$ or $theta = -frac{3pi}{4}$. Substituting these within $dot{r}$ leads to:
$$-2rcos^2theta+r(3-r^2) = -2r(pmfrac{sqrt{2}}{2})^2 +r(3-r^2)=r(2-r^2)$$
So $dot{r}=0$ if $r=0$ or $r^2=2$. This leads us to the following critical points: $(0,0)$, $(sqrt{2},sqrt{2})$, $(-sqrt{2},-sqrt{2})$.
However what bugs me is that entering these points into the non-polar system does not lead to $dot{x}=dot{y}=0$ for $(sqrt{2},sqrt{2})$, $(-sqrt{2},-sqrt{2})$.
What am I missing here? Is the whole thought of trying to find critical points through putting $dot{theta}$ and $dot{r}$ equal to zero unwise? Any help or hint with this will be much appreciated.
ordinary-differential-equations systems-of-equations polar-coordinates
$endgroup$
$begingroup$
As I state in my answer, I'm sorry I didn't answer what you were asking. Thus, I suggest you change your accepted answer to a different one so I may then delete my answer. Thanks.
$endgroup$
– John Omielan
Jan 4 at 5:12
add a comment |
$begingroup$
I have written the following DE system
$$dot{x} = x+y-x(x^2+y^2)\
dot{y} = -x+3y-y(x^2+y^2) $$
as follows in Polar form:
$$dot{r} = -2rcos^2theta+r(3-r^2) $$
$$dot{theta} = 2sinthetacostheta-1$$
What I wonder is could I also find all the critical points of this system by setting $dot{theta}$ and $dot{r}$ equal to zero? Since these two systems are actually equivalent to each other this seemed like a logical thing to me and at the same time finding the critical points through setting $dot{x}$ and $dot{y}$ equal to zero seems like much more tedious in this state.
Solving for $dot{theta}=0$ leads to $2sinthetacostheta = 1$ which is the same as $sin2theta=1$ leading to $theta = frac{pi}{4}$ or $theta = -frac{3pi}{4}$. Substituting these within $dot{r}$ leads to:
$$-2rcos^2theta+r(3-r^2) = -2r(pmfrac{sqrt{2}}{2})^2 +r(3-r^2)=r(2-r^2)$$
So $dot{r}=0$ if $r=0$ or $r^2=2$. This leads us to the following critical points: $(0,0)$, $(sqrt{2},sqrt{2})$, $(-sqrt{2},-sqrt{2})$.
However what bugs me is that entering these points into the non-polar system does not lead to $dot{x}=dot{y}=0$ for $(sqrt{2},sqrt{2})$, $(-sqrt{2},-sqrt{2})$.
What am I missing here? Is the whole thought of trying to find critical points through putting $dot{theta}$ and $dot{r}$ equal to zero unwise? Any help or hint with this will be much appreciated.
ordinary-differential-equations systems-of-equations polar-coordinates
$endgroup$
I have written the following DE system
$$dot{x} = x+y-x(x^2+y^2)\
dot{y} = -x+3y-y(x^2+y^2) $$
as follows in Polar form:
$$dot{r} = -2rcos^2theta+r(3-r^2) $$
$$dot{theta} = 2sinthetacostheta-1$$
What I wonder is could I also find all the critical points of this system by setting $dot{theta}$ and $dot{r}$ equal to zero? Since these two systems are actually equivalent to each other this seemed like a logical thing to me and at the same time finding the critical points through setting $dot{x}$ and $dot{y}$ equal to zero seems like much more tedious in this state.
Solving for $dot{theta}=0$ leads to $2sinthetacostheta = 1$ which is the same as $sin2theta=1$ leading to $theta = frac{pi}{4}$ or $theta = -frac{3pi}{4}$. Substituting these within $dot{r}$ leads to:
$$-2rcos^2theta+r(3-r^2) = -2r(pmfrac{sqrt{2}}{2})^2 +r(3-r^2)=r(2-r^2)$$
So $dot{r}=0$ if $r=0$ or $r^2=2$. This leads us to the following critical points: $(0,0)$, $(sqrt{2},sqrt{2})$, $(-sqrt{2},-sqrt{2})$.
However what bugs me is that entering these points into the non-polar system does not lead to $dot{x}=dot{y}=0$ for $(sqrt{2},sqrt{2})$, $(-sqrt{2},-sqrt{2})$.
What am I missing here? Is the whole thought of trying to find critical points through putting $dot{theta}$ and $dot{r}$ equal to zero unwise? Any help or hint with this will be much appreciated.
ordinary-differential-equations systems-of-equations polar-coordinates
ordinary-differential-equations systems-of-equations polar-coordinates
asked Jan 3 at 21:04
BluedogBluedog
257
257
$begingroup$
As I state in my answer, I'm sorry I didn't answer what you were asking. Thus, I suggest you change your accepted answer to a different one so I may then delete my answer. Thanks.
$endgroup$
– John Omielan
Jan 4 at 5:12
add a comment |
$begingroup$
As I state in my answer, I'm sorry I didn't answer what you were asking. Thus, I suggest you change your accepted answer to a different one so I may then delete my answer. Thanks.
$endgroup$
– John Omielan
Jan 4 at 5:12
$begingroup$
As I state in my answer, I'm sorry I didn't answer what you were asking. Thus, I suggest you change your accepted answer to a different one so I may then delete my answer. Thanks.
$endgroup$
– John Omielan
Jan 4 at 5:12
$begingroup$
As I state in my answer, I'm sorry I didn't answer what you were asking. Thus, I suggest you change your accepted answer to a different one so I may then delete my answer. Thanks.
$endgroup$
– John Omielan
Jan 4 at 5:12
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
If $r=sqrt 2$ and $theta=pi/4$ you get $x=y=sqrt 2frac 1{sqrt2}=1$. Then $(1,1)$ is a critical point. Similarly, you can check the other solution $(-1,-1)$.
$endgroup$
add a comment |
$begingroup$
It is not that difficult to proceed from $dot x=0=dot y$. Multiply the first with $y$, the second equation with $x$ and subtract. Then
$$
0=xy+y^2+x^2-3xy=(x-y)^2
$$
has solutions only for $x=y$. Insert that in either equation to get
$$
0=2x -x(x^2+x^2)=2x(1-x^2)
$$
to conclude that the values taken are $x=yin{-1,0,1}$.
$endgroup$
$begingroup$
I proceded to find the same points in your answer by inserting $frac{x+y}{x}=x^2+y^2$ (rewritten from $dot{x}=0$) into $dot{y}$. I thought that for two equations, one cannot simply combine them into one and say the original system is equivalent to the result and that one must keep at least one of the original two equations. So I tried going the "find a solution in one equation, insert in the other one and so on" and it was a gruesome process. But from your answer I am to understand that combining the equations in this situation is fine?
$endgroup$
– Bluedog
Jan 4 at 6:04
$begingroup$
@Bluedog : This is effectively the same as my first step. Yes, any combination of equations is again an equation that must be satisfied, in the algebra this is the idea of ideals generated by some polynomials whose roots one is seeking. You must only be careful in declaring a previous equation no longer needed, that is, a new reduced set of equations equivalent to the original one. But the set of all obtained and original equations is obviously equivalent to the original set.
$endgroup$
– LutzL
Jan 4 at 9:03
add a comment |
Your Answer
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
If $r=sqrt 2$ and $theta=pi/4$ you get $x=y=sqrt 2frac 1{sqrt2}=1$. Then $(1,1)$ is a critical point. Similarly, you can check the other solution $(-1,-1)$.
$endgroup$
add a comment |
$begingroup$
If $r=sqrt 2$ and $theta=pi/4$ you get $x=y=sqrt 2frac 1{sqrt2}=1$. Then $(1,1)$ is a critical point. Similarly, you can check the other solution $(-1,-1)$.
$endgroup$
add a comment |
$begingroup$
If $r=sqrt 2$ and $theta=pi/4$ you get $x=y=sqrt 2frac 1{sqrt2}=1$. Then $(1,1)$ is a critical point. Similarly, you can check the other solution $(-1,-1)$.
$endgroup$
If $r=sqrt 2$ and $theta=pi/4$ you get $x=y=sqrt 2frac 1{sqrt2}=1$. Then $(1,1)$ is a critical point. Similarly, you can check the other solution $(-1,-1)$.
edited Jan 3 at 21:46
answered Jan 3 at 21:39
AndreiAndrei
11.8k21026
11.8k21026
add a comment |
add a comment |
$begingroup$
It is not that difficult to proceed from $dot x=0=dot y$. Multiply the first with $y$, the second equation with $x$ and subtract. Then
$$
0=xy+y^2+x^2-3xy=(x-y)^2
$$
has solutions only for $x=y$. Insert that in either equation to get
$$
0=2x -x(x^2+x^2)=2x(1-x^2)
$$
to conclude that the values taken are $x=yin{-1,0,1}$.
$endgroup$
$begingroup$
I proceded to find the same points in your answer by inserting $frac{x+y}{x}=x^2+y^2$ (rewritten from $dot{x}=0$) into $dot{y}$. I thought that for two equations, one cannot simply combine them into one and say the original system is equivalent to the result and that one must keep at least one of the original two equations. So I tried going the "find a solution in one equation, insert in the other one and so on" and it was a gruesome process. But from your answer I am to understand that combining the equations in this situation is fine?
$endgroup$
– Bluedog
Jan 4 at 6:04
$begingroup$
@Bluedog : This is effectively the same as my first step. Yes, any combination of equations is again an equation that must be satisfied, in the algebra this is the idea of ideals generated by some polynomials whose roots one is seeking. You must only be careful in declaring a previous equation no longer needed, that is, a new reduced set of equations equivalent to the original one. But the set of all obtained and original equations is obviously equivalent to the original set.
$endgroup$
– LutzL
Jan 4 at 9:03
add a comment |
$begingroup$
It is not that difficult to proceed from $dot x=0=dot y$. Multiply the first with $y$, the second equation with $x$ and subtract. Then
$$
0=xy+y^2+x^2-3xy=(x-y)^2
$$
has solutions only for $x=y$. Insert that in either equation to get
$$
0=2x -x(x^2+x^2)=2x(1-x^2)
$$
to conclude that the values taken are $x=yin{-1,0,1}$.
$endgroup$
$begingroup$
I proceded to find the same points in your answer by inserting $frac{x+y}{x}=x^2+y^2$ (rewritten from $dot{x}=0$) into $dot{y}$. I thought that for two equations, one cannot simply combine them into one and say the original system is equivalent to the result and that one must keep at least one of the original two equations. So I tried going the "find a solution in one equation, insert in the other one and so on" and it was a gruesome process. But from your answer I am to understand that combining the equations in this situation is fine?
$endgroup$
– Bluedog
Jan 4 at 6:04
$begingroup$
@Bluedog : This is effectively the same as my first step. Yes, any combination of equations is again an equation that must be satisfied, in the algebra this is the idea of ideals generated by some polynomials whose roots one is seeking. You must only be careful in declaring a previous equation no longer needed, that is, a new reduced set of equations equivalent to the original one. But the set of all obtained and original equations is obviously equivalent to the original set.
$endgroup$
– LutzL
Jan 4 at 9:03
add a comment |
$begingroup$
It is not that difficult to proceed from $dot x=0=dot y$. Multiply the first with $y$, the second equation with $x$ and subtract. Then
$$
0=xy+y^2+x^2-3xy=(x-y)^2
$$
has solutions only for $x=y$. Insert that in either equation to get
$$
0=2x -x(x^2+x^2)=2x(1-x^2)
$$
to conclude that the values taken are $x=yin{-1,0,1}$.
$endgroup$
It is not that difficult to proceed from $dot x=0=dot y$. Multiply the first with $y$, the second equation with $x$ and subtract. Then
$$
0=xy+y^2+x^2-3xy=(x-y)^2
$$
has solutions only for $x=y$. Insert that in either equation to get
$$
0=2x -x(x^2+x^2)=2x(1-x^2)
$$
to conclude that the values taken are $x=yin{-1,0,1}$.
answered Jan 3 at 21:48
LutzLLutzL
57.7k42054
57.7k42054
$begingroup$
I proceded to find the same points in your answer by inserting $frac{x+y}{x}=x^2+y^2$ (rewritten from $dot{x}=0$) into $dot{y}$. I thought that for two equations, one cannot simply combine them into one and say the original system is equivalent to the result and that one must keep at least one of the original two equations. So I tried going the "find a solution in one equation, insert in the other one and so on" and it was a gruesome process. But from your answer I am to understand that combining the equations in this situation is fine?
$endgroup$
– Bluedog
Jan 4 at 6:04
$begingroup$
@Bluedog : This is effectively the same as my first step. Yes, any combination of equations is again an equation that must be satisfied, in the algebra this is the idea of ideals generated by some polynomials whose roots one is seeking. You must only be careful in declaring a previous equation no longer needed, that is, a new reduced set of equations equivalent to the original one. But the set of all obtained and original equations is obviously equivalent to the original set.
$endgroup$
– LutzL
Jan 4 at 9:03
add a comment |
$begingroup$
I proceded to find the same points in your answer by inserting $frac{x+y}{x}=x^2+y^2$ (rewritten from $dot{x}=0$) into $dot{y}$. I thought that for two equations, one cannot simply combine them into one and say the original system is equivalent to the result and that one must keep at least one of the original two equations. So I tried going the "find a solution in one equation, insert in the other one and so on" and it was a gruesome process. But from your answer I am to understand that combining the equations in this situation is fine?
$endgroup$
– Bluedog
Jan 4 at 6:04
$begingroup$
@Bluedog : This is effectively the same as my first step. Yes, any combination of equations is again an equation that must be satisfied, in the algebra this is the idea of ideals generated by some polynomials whose roots one is seeking. You must only be careful in declaring a previous equation no longer needed, that is, a new reduced set of equations equivalent to the original one. But the set of all obtained and original equations is obviously equivalent to the original set.
$endgroup$
– LutzL
Jan 4 at 9:03
$begingroup$
I proceded to find the same points in your answer by inserting $frac{x+y}{x}=x^2+y^2$ (rewritten from $dot{x}=0$) into $dot{y}$. I thought that for two equations, one cannot simply combine them into one and say the original system is equivalent to the result and that one must keep at least one of the original two equations. So I tried going the "find a solution in one equation, insert in the other one and so on" and it was a gruesome process. But from your answer I am to understand that combining the equations in this situation is fine?
$endgroup$
– Bluedog
Jan 4 at 6:04
$begingroup$
I proceded to find the same points in your answer by inserting $frac{x+y}{x}=x^2+y^2$ (rewritten from $dot{x}=0$) into $dot{y}$. I thought that for two equations, one cannot simply combine them into one and say the original system is equivalent to the result and that one must keep at least one of the original two equations. So I tried going the "find a solution in one equation, insert in the other one and so on" and it was a gruesome process. But from your answer I am to understand that combining the equations in this situation is fine?
$endgroup$
– Bluedog
Jan 4 at 6:04
$begingroup$
@Bluedog : This is effectively the same as my first step. Yes, any combination of equations is again an equation that must be satisfied, in the algebra this is the idea of ideals generated by some polynomials whose roots one is seeking. You must only be careful in declaring a previous equation no longer needed, that is, a new reduced set of equations equivalent to the original one. But the set of all obtained and original equations is obviously equivalent to the original set.
$endgroup$
– LutzL
Jan 4 at 9:03
$begingroup$
@Bluedog : This is effectively the same as my first step. Yes, any combination of equations is again an equation that must be satisfied, in the algebra this is the idea of ideals generated by some polynomials whose roots one is seeking. You must only be careful in declaring a previous equation no longer needed, that is, a new reduced set of equations equivalent to the original one. But the set of all obtained and original equations is obviously equivalent to the original set.
$endgroup$
– LutzL
Jan 4 at 9:03
add a comment |
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$begingroup$
As I state in my answer, I'm sorry I didn't answer what you were asking. Thus, I suggest you change your accepted answer to a different one so I may then delete my answer. Thanks.
$endgroup$
– John Omielan
Jan 4 at 5:12