|Name||Jae Choon Cha|
|Time||Jan 11 13:00~13:40|
|Title||Hidden torsion in 3-manifold groups and homology cobordism|
We introduce a new type of torsion in 3-manifold groups we call hidden torsion, and an algebraic approximation we call local hidden torsion, regarding 4-dimensional homology cobordism. We construct hyperbolic 3-manifolds which have local hidden torsion in the transfinite lower central subgroup, and we show that our hyperbolic 3-manifolds are not pairwise homology cobordant, yet remain indistinguishable by any prior known homology cobordism invariants. We also give an answer to a question of T. D. Cochran and M. H. Freedman about transfinite lower central series of 3-manifolds.