Add a NL direction field

[kangwonlee]

ref : https://youtu.be/cdAQZjt4SyM

$$\begin{array}{rl}\frac{d}{dt}{x}_1& =x_2-x_1({x_1}^2+{x_2}^2)\\ \frac{d}{dt}{x}_2& =-x_1-x_2({x_1}^2+{x_2}^2)\end{array}$$

[kangwonlee]

Consider adding a collection of direction fields

[kangwonlee]

Balthasar van der Pol oscillator
limit cycle attractor

ref : https://youtu.be/uzJXeluCKMs&t=338

$$\begin{array}{rl}\frac{d}{dt}x& =\mu (x-\frac{1}{3}{x}^3-y)\\ \frac{d}{dt}y& =\frac{1}{\mu }x\end{array}$$