\[W_{\pm 1}\left(x\mp 0\mathrm{i}\right)\sim-\eta-\ln\eta+\sum_{n=1}^{\infty}%
\frac{1}{\eta^{n}}\sum_{m=1}^{n}\genfrac{[}{]}{0.0pt}{}{n}{n-m+1}\frac{\left(-%
\ln\eta\right)^{m}}{m!},\]