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The Lorenz attractor is the following surprisingly simple system of three nonlinear ordinary differential equations:
$$ \frac{dx}{dt}= \sigma(y-x)$$ $$\frac{dy}{dt}=x(\rho-z)-y$$ $$\frac{dz}{dt}=xy-\beta z$$
This system exhibits chaotic behavior for parameters $(\sigma,\beta,\rho)$ in a neighborhood of $(10,8/3,28)$.