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Victor Ivrii:
For the system of differential equations
&x' =\tan (y) - \frac{1}{2}\tan (x)  \,,\\
&y' = \tan (x) - \frac{1}{2}\tan (y) \,.

(a) Linearize the system at a critical point $(x_0 ,y_0)$ of your choice;

(b) Describe the type of the critical point $(x_0,y_0)$ of the linearized and of the original system;

(c) Sketch the phase portraits of the linearized and of the  original system near this critical point $(x_0,y_0)$.

Michal Staszewski:
Solution attached.

Victor Ivrii:

--- Quote from: Michal Staszewski on April 17, 2013, 08:22:58 PM ---Solution attached.

--- End quote ---

Few remarks.
1. Field is obviously $\pi$-periodic with respect to both $x$ and $y$ and it is singular as $x=(m+\frac{1}{2})\pi$  or $y=(n+\frac{1}{2})\pi$ with $m,n\in \mathbb{Z}$ so one needs to consider only square $\{ -\frac{\pi}{2}<x < \frac{\pi}{2},  -\frac{\pi}{2}<y < \frac{\pi}{2}\}$ where $(0,0)$ is an only equilibrium point.

2. Missing: eigenvectors (so directions of separatrices have not been found.

3. Global phase portrait would be appreciated.

Michal Staszewski:
A follow-up.

Michal Staszewski:
And the computer-generated phase portrait.


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