I am studying stability for non linear control systems, and I am focusing on the Center manifold theory .
In particular, I am trying to understand an example which is also in the Hassan K.Khalil book at pag. 311, and it is the following:
Consider the system:
$\dot{y} = yz$
$\dot{z}=-z+ay^{2}$
we have that the center manifold equation is:
$\dot{h}(y)[yh(y)]+h(y)-ay^{2}=0$
(1)
with boundary conditions :
$\dot{h}(y)=h(y)=0$
now, on the book it is said that this is hard to solve, so it is performed an approximation, and from this point I have doubts on how to proceed. I will say what I have understood so far so to explain better my doubts.
Since the equation of the center manifold is hard to solve, it will be done an approximation, by choosing:
$\dot{h}(y)=h_2(y)y^{2}+h_3(y)y^{3}+…$
and we will start firsr by considering $\dot{h}(y)\approx 0$ and if we cannot do considerations about the stability, we will use as approximation $\dot{h}(y)\approx h_2(y)y^{2}+O(|y|^3)$ and so on until we can say something about the stability at the origin.
In the example on the book, it is said that if I use $\dot{h}(y)\approx 0$, the reduced system is:
$\dot{y}=O(|y|^3)$
which, as far as I have understood, is obtaining sunstituting $\dot{h}(y)\approx 0$ into the center manifold equation (1), ans so the only non zero term that remains is $-ay^2$, so:
$\dot{y}=-ay^2+O(|y|^3)$
and it says that we cannot conclude nothing on the stability of the origin from here.
Why we cannot conclude anything?
Then, since we cannot conclude anything, it chooses $\dot{h}(y)\approx h_2(y)y^{2}+O(|y|^3)$,(2), and it says that if we substitute this into the center manifold equation (1), the reduced system is:
$\dot{y}=ay^3+O(|y|^4)$
the words used in the book to explain this are:
we substitute (2) into the center manifold equation and calculate $h_2$, by matching coefficients of $y^2$, to obtain $h_2=a$.
but, how did he get this result?
after this it says that for $a<0$ the origin is stable and for $a>0$ is unstable, but why?
I dont' understand some parts of this example, can somebody please help me?
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