= Nonlinear crossover delayed by a small unstable-mode coefficient
{title2=$y\sim Ax^2,\quad\varepsilon y'\sim1\ \Longrightarrow\ x\sim(2\varepsilon A)^{-1}$}
When the early growing solution is $Ax^2$ and the nonlinear coefficient depends on $\varepsilon y'$, its crossover occurs at the displayed scale. If $A$ is order one this is $x=O(\varepsilon^{-1})$; if the initial slope suppresses the leading mode and $A=O(\varepsilon)$, it is delayed to $x=O(\varepsilon^{-2})$. The first nonzero perturbative seed must therefore be computed before constructing the global leading approximation; setting the small parameter to zero can erase the mode that ultimately dominates.
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