These notes are included by kind permission of Alexander Ritter. They are based on lectures he gave for the masterclasses at Wadham College, Oxford, in August 2014, organised by Wadham College and the CMI-PROMYS Alliance. Their use is licensed by Alexander Ritter under Creative Commons Licence BY-NC-ND (Attribution-NonCommercial-NoDerivs).
Speaker: Alexander Ritter
Venue: 2014 Masterclasses in Geometry, University of Oxford