# Finding the separatrix

• Mar 26th 2012, 09:05 PM
cshanholtzer
Finding the separatrix
I have a pair of differential equations:
$\displaystyle \frac{dx}{dt} = x (3 - x - 2 y)$
$\displaystyle \frac{dy}{dt} = y (2 - x - y)$

We can see that there are equilibrium points at $\displaystyle (0,0), (0,3), (2,0), (1,1)$ which we can further determine are respectively unstable, stable, stable and a saddle point. Depending on the initial conditions, as $\displaystyle t \to \infty$, we expect the system to settle at one of the stable points (i.e. x or y goes 'extinct'). The separatrix is the curve which will determine the end behavior depending on which side of it the initial conditions lie. It should pass through the origin and the saddle point. I am wondering if it is possible to find the exact curve? I have tried a lot of things at this point and am beginning to think that it might not always be possible. I've attained a good approximation numerically, but would love to be able to find the exact curve so if anyone knows how to do so or if it's not possible I'd greatly appreciate the help!

Thanks
• Mar 29th 2012, 04:39 PM
cshanholtzer
Re: Finding the separatrix
At this point I'm beginning to feel like my efforts are futile. It may just be the case that the exact curve cannot be found and my numerical approximation will have to cut it. A bit of a letdown in a way, but also learned a lot making the effort :)