 Solution

On the slit plane $\mathbb{C}\setminus [-a,a]$, chooseso that $F(z)\sim z$ as $z\to \infty$. This branch cut gives, for $-a<x<a$, the boundary values

Integrate $F$ over the boundary of a large disk with a thin slit removed. The upper bank is traversed from $-a$ to $a$ and the lower bank from $a$ to $-a$, so together they contributeAt infinity the Laurent series isand hence the counterclockwise large-circle integral tends to $-\pi i{a}^2$. By the Cauchy integral theorem, the large-circle and slit-boundary contributions sum to zero. Thereforewhich yields