Number Theory Seminar: Evaluating the wild Brauer group

SEMINARS, NUMBER THEORY

Speaker:

Rachel Newton - King's College London

The local global approach to the study of rational points over number fields begins by embedding the set of rational points on a variety X into the set of its adelic points. The Brauer-Manin pairing cuts out a subset of the adelic points, called the Brauer-Manin set, that contains the rational points. If the set of adelic points is non-empty but the Brauer–Manin set is empty then we say there's a Brauer–Manin obstruction to the existence of rational points on X.  Computing the Brauer–Manin pairing involves evaluating elements of the Brauer group of X at local points.  If an element of the Brauer group has order coprime to p, then its evaluation at a p-adic point factors via reduction of the point modulo p. For elements of order a power of p this is no longer the case: in order to compute the evaluation map one must know the point to a higher p-adic precision. Classifying Brauer group elements according to the precision required to evaluate them at p-adic points gives a filtration which we describe using work of Kato. Applications of our work include addressing Swinnerton-Dyer's question about which places can play a role in the Brauer–Manin obstruction. This is joint work with Martin Bright.