WKB correction term
= WKB correction term
{c}
{title2=$F=f\prime\prime/(4f^2)-5(f\prime)^2/(16f^3)$}
The <Liouville transformation of a second-order equation> with $t=\int^x\sqrt f\,dx$ gives $\ddot U-(1+F)U=0$. Neglecting this correction yields amplitude $f^{-1/4}$ and phase or exponent $\int\sqrt f$. Its smallness measures a local slowly-varying-coefficient approximation, away from zeros of $f$.