= Periodically reversing Parker dynamo coupling
{title2=$\operatorname{tr}N=\cosh(\sqrt2DT)+\cos(\sqrt2DT)$}
For $\dot A=D^2f(t)B$ and $\dot B=iA$, with $f$ alternating between $+1$ and $-1$ for intervals of length $T$, the <monodromy matrix of a periodic linear system> has determinant one and trace $\cosh(\sqrt2DT)+\cos(\sqrt2DT)>2$. Its reciprocal <Floquet multipliers> give positive dominant <Floquet growth rate> for $D,T>0$, despite the zero mean of $f$. An exceptional initial state in the stable <eigenspace> decays.
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