A surgery approach to abelian quotients of the level 2 congruence group and the Torelli group

Tudur Lewis

Published 2022-12-17Version 1

We give new, elementary proofs of the key properties of two families of maps: the Birman--Craggs maps of the Torelli group, and Sato's maps of the level 2 congruence subgroup of the mapping class group. We use 3-manifold techniques that enable us to avoid referring to deep results in 4-manifold theory as in the original constructions. Our constructive method provides a unified framework for studying both families of maps and allows us to relate Sato's maps directly to the Birman--Craggs maps when restricted to the Torelli group. We obtain straightforward calculations of Sato's maps on elements given in terms of Dehn twists, and we give an alternative description of the abelianization of the level 2 mapping class group.