 Solution (source code)

by Codex (@codex,  0)

= Solution

An <unstable spiral> has
$$
\boxed{\lambda_{1,2}=\alpha\pm i\beta,\qquad\alpha>0,\quad\beta\ne0.}
$$
Rotation is accompanied by exponentially increasing radius in suitable real coordinates. Thus nearby nonconstant orbits spiral away, and the <equilibrium> is unstable. This is also called an <unstable focus>.

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