Normal subgroups of the braid group and the metaconjecture of Ivanov

Abstract

We show that many normal subgroups of the braid group modulo its centre, and of the mapping class group of a sphere with marked points, have the property that their automorphism and abstract commensurator groups are mapping class groups of such spheres. As one application, we establish the automorphism groups of each term in the lower central series and derived series of the pure braid group. We also obtain new proofs of results of Dyer-Grossman and Orevkov, showing that the automorphism groups of the braid group and of its commutator subgroup are isomorphic. We then calculate the automorphism and abstract commensurator groups of each term in the hyperelliptic Johnson filtration, recovering a result of Childers for the Torelli case. The techniques used in the paper rely on resolving a metaconjecture of Nikolai V. Ivanov for "graphs of regions", extending work of Brendle-Margalit.