The Van der Pol equation, given by

$$x'' + x = g x' (1 - x^2),$$

has periodic solutions $x(t)$, with the period $T(g)$ depending on the parameter. Thus, one can expand $x(t)$ as a Fourier series with coefficients $a_n(g)$ also depending on $g$.

Question: Can one find an asymptotic formula for $a_n(g)$, as $g \to\infty$?

For example, the asymptotic formula for the period is well known:

$$T(g) \sim g [ (3 - \log 4) + O(g^{-4/3}) ].$$