Appendix to a paper [arXiv:1809.08623] of B. Williams

Abstract

In this appendix to a paper [arXiv:1809.08623] by B. Williams, we give birational equivalences between the models of the Hilbert modular surfaces for ${\mathbb Q}(\sqrt{29})$ and ${\mathbb Q}(\sqrt{37})$ given there and those previously found by Elkies and Kumar.