How should I show that these systems have no periodic solutions?

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Homework Statement
Show that the following systems have no periodic solutions.
a) ## \dot{x}=x+x^3-y^2, \dot{y}=x^2-x^4+y^5 ##
b) ## \dot{x}=yx^2-x^3-3xy^2-2y+y^2, \dot{y}=2xy^2-x^2+4x^3-2y ##
Relevant Equations
Bendixson's negative criterion: There are no closed paths in a simply connected region of the phase plane on which ## \frac{\partial X}{\partial x}+\frac{\partial Y}{\partial y} ## is of one sign.
a) Proof:

Consider the system ## \dot{x}=x+x^3-y^2 ## and ## \dot{y}=x^2-x^4+y^5 ##.
By theorem, Bendixson's negative criterion states that there are no closed paths in a simply connected region of the phase plane on which ## \frac{\partial X}{\partial x}+\frac{\partial Y}{\partial y} ## is of one sign.
Let ## \dot{x}=X(x, y)=x+x^3-y^2 ## and ## \dot{y}=Y(x, y)=x^2-x^4+y^5 ##.
Then ## \frac{\partial X}{\partial x}=3x^2+1 ## and ## \frac{\partial Y}{\partial y}=5y^4 ##.
This gives ## \frac{\partial X}{\partial x}+\frac{\partial Y}{\partial y}=5y^4+3x^2+1>0, \forall x, y ##.
Since ## \frac{\partial X}{\partial x}+\frac{\partial Y}{\partial y} ## is always positive ## \forall x, y ## and is of one sign,
it follows that there are no closed paths in a simply connected region of the phase plane.
Therefore, the system ## \dot{x}=x+x^3-y^2 ## and ## \dot{y}=x^2-x^4+y^5 ## has no periodic solutions.

b) Proof:

Consider the system ## \dot{x}=yx^2-x^3-3xy^2-2y+y^2 ## and ## \dot{y}=2xy^2-x^2+4x^3-2y ##.
By theorem, Bendixson's negative criterion states that there are no closed paths in a simply connected region of the phase plane on which ## \frac{\partial X}{\partial x}+\frac{\partial Y}{\partial y} ## is of one sign.
Let ## \dot{x}=X(x, y)=yx^2-x^3-3xy^2-2y+y^2 ## and ## \dot{y}=Y(x, y)=2xy^2-x^2+4x^3-2y ##.
Then ## \frac{\partial X}{\partial x}=2xy-3x^2-3y^2 ## and ## \frac{\partial Y}{\partial y}=4xy-2 ##.
This gives ## \frac{\partial X}{\partial x}+\frac{\partial Y}{\partial y}=6xy-3x^2-3y^2-2=6xy-3(x^2+y^2)-2<0, \forall x, y ##.
Since ## \frac{\partial X}{\partial x}+\frac{\partial Y}{\partial y} ## is always negative ## \forall x, y ## and is of one sign,
it follows that there are no closed paths in a simply connected region of the phase plane.
Therefore, the system ## \dot{x}=yx^2-x^3-3xy^2-2y+y^2 ## and ## \dot{y}=2xy^2-x^2+4x^3-2y ## has no periodic solutions.

Above is my work for these two proofs. May anyone please check/verify/confirm to see if they are correct and accurate?
 
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You need to do more to show that 6xy - 3(x^2 + y^2) - 2 does not change sign. For example, complete the square to show that <br /> 6xy - 3(x^2 + y^2) - 2 = -3 (x^2 - 2xy + y^2) - 2 = -3(x - y)^2 - 2 &lt; 0.
 
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pasmith said:
You need to do more to show that 6xy - 3(x^2 + y^2) - 2 does not change sign. For example, complete the square to show that <br /> 6xy - 3(x^2 + y^2) - 2 = -3 (x^2 - 2xy + y^2) - 2 = -3(x - y)^2 - 2 &lt; 0.
Thank you so much, that was very helpful!
 
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