Numerical computation of the Schwarz function

An analytic function can be continued across an analytic arc $\mathrm{\Gamma }$ with the help of the Schwarz function $S(z)$, the analytic function satisfying $S(z)=\bar{z}$ for $z\in \mathrm{\Gamma }$. We show how $S(z)$ can be computed with the AAA algorithm of rational approximation, an operation that is the basis of the AAALS method for solution of Laplace and related PDE problems in the plane. We discuss the challenge of computing $S(z)$ further away from from $\mathrm{\Gamma }$, where it becomes multi-valued.