A new filtration of the Magnus kernel

[摘要] For a oriented genus g surface with one boundary component, S_g, the Torelli group is the group of orientation preserving homeomorphisms of S_g that induce the identity on homology.The Magnus representation of the Torelli group represents the action on F/F'' where F=π_1(S_g) and F'' is the second term of thederived series. I show that the kernel of the Magnus representation, Mag(S_g), is highly non-trivial and has a rich structure as a group.Specifically, I define an infinite filtration of Mag(S_g) by subgroups,called the higher order Magnus subgroups, M_k(S_g).I develop methods for generating nontrivial mapping classes in M_k(S_g) for all k and g≥2.I show that for each k the quotient M_k(S_g)/M_{k+1}(S_g) contains a subgroup isomorphic to a lower central series quotient of free groups E(g-1)_k/E(g-1)_{k+1}.Finally I show that for g≥3 the quotient M_k(S_g)/M_{k+1}(S_g) surjects onto an infinite rank torsion free abelian group. To do this, I define a Johnson-type homomorphism on each higher order Magnus subgroup quotient and show it has a highly non-trivial image.

[发布机构] Rice University

[效力级别] central