Agler, Lykova, and Young determine the holomorphic retractions of the symmetrized bidisc and its subsets, which permit the extension of holomorphic functions without an increase of the supremum norm. The methods they use are of independent interest, they say, and they analyze the complex geodesics of the symmetrized bidisc, and show that there are five qualitatively different types of them. Their topics include extremal problems in the symmetrized bidisc, purely unbalanced and exceptional datums, geodesics and sets with the norm-preserving extension property, proof of the main theorem, and applications to the theory of spectral sets. Annotation ©2019 Ringgold, Inc., Portland, OR (protoview.com)