MAT244-2013S > Final Exam

FE-5

**Victor Ivrii**:

For the system of differential equations

\begin{equation*}

\left\{\begin{aligned}

&x' =\tan (y) - \frac{1}{2}\tan (x) \,,\\

&y' = \tan (x) - \frac{1}{2}\tan (y) \,.

\end{aligned}\right.

\end{equation*}

(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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