Bordism Theory and finite group actions
This session was part of the CLAM VI located virtually in Montevideo Uruguay and was held during the days 15 and 16 of September 2021.
Organizers
Conferences
Title:
Diffeomorphisms of reducible three manifolds and bordisms of group actions on torus
Abstract
I first talk about the joint work with K. Mann on certain rigidity results on group actions on torus. In particular, we show that if the torus action on itself extends to a C^0 action on a three manifold M that bounds the torus, then
M is homeomorphic to the solid torus. This also leads to the first example of a smooth action on the torus via diffeomorphisms that isotopic to the identity that is nontrivial in the bordisms of group actions. Time permitting, I will also talk about certain finiteness results about classifying space of reducible three manifolds which came out of the cohomological aspect of the above project.
Title:
Zp-bordism and the mod(p)-Borsuk-Ulam Theorem
Abstract
Crabb-Gonçalves-Libardi-Pergher classified for given integers m,n,≥1 the bordism class of a closed smooth m-manifold X^m with a free smooth involution τ with respect to the validity of the Borsuk-Ulam property that for every continuous map φ:X^m→R^n, there exists a point x∈X^m such that φ(x)=φ(τ(x)) .
In this work together with Barbaresco-de Mattos-dos Santos-da Silva, we are considering the same problem for free Z_p action.
Title:
Extending free group action on surfaces
Abstract
Crabb-Gonçalves-Libardi-Pergher classified for given integers m,n,≥1 the bordism class of a closed smooth m-manifold X^m with a free smooth involution τ with respect to the validity of the Borsuk-Ulam property that for every continuous map φ:X^m→R^n, there exists a point x∈X^m such that φ(x)=φ(τ(x)) .
In this work together with Barbaresco-de Mattos-dos Santos-da Silva, we are considering the same problem for free Z_p action.
Title:
Stolz' Positive scalar curvature surgery exact sequence and low dimensional group homology
Abstract
We will show how positive knowledge about the Baum-Connes Conjecture for a group, together with a Pontrjagyn character and knowledge about the conjugacy classes of finite subgroups and their low dimensional homology provide an estimation of the degree of non rigidity of positive scalar curvature metrics of spin high dimensional manifolds with the given fundamental group.
Title:
On the evenness conjecture for equivariant unitary bordism
Abstract
We will show how positive knowledge about the Baum-Connes Conjecture for a group, together with a Pontrjagyn character and knowledge about the conjugacy classes of finite subgroups and their low dimensional homology provide an estimation of the degree of non rigidity of positive scalar curvature metrics of spin high dimensional manifolds with the given fundamental group.
Title:
The Loch Ness Monster as Homology Covers
Abstract
The Loch Ness Monster (LNM) is, up to homeomorphisms, the unique orientable, second countable, connected and Hausdorff surface of infinite genus and exactly one end. In this talk I would like to discuss some properties on the LNM. In particular, we note that LNM is the homology cover of most of the surfaces and also it is the derived cover of uniformizations of Riemann surfaces (with some few exceptions). This is a joint work, in progress, with Ara Basmajian.
Title:
When does a free action of a finite group on a surface extend to a (possibly non-free) action on a 3-manifold?
Abstract
Dominguez and Segovia recently asked the question in the title, and showed that the answer is “always” for many examples of finite groups, including symmetric groups, alternating groups and abelian. Surprisingly, in joint work with Segovia, we have found the first examples of finite groups that admit free actions on surfaces that do NOT extend to actions on 3-manifolds. Even more surprisingly, these groups are already known in algebraic geometry as counterexamples to the Noether conjecture over the complex numbers. In this talk, I will explain how to find these groups, and, more generally, how to decide algorithmically if any fixed finite group admits a non-extending action.
Title:
Signature of manifold with single fixed point set of an abelian normal group
Abstract
In this talk we will report progress made in the task of reducing the computation of the signature of a (possibly non compact) oriented smooth dimensional manifold with an orientation preserving co-compact smooth proper action of a discrete group with a single non empty fixed point submanifold of fixed dimension with respect to the action of a (finite) abelian normal subgroup. The aim of this reduction is to give a formula for the signature in terms of the signature of a manifold with free action and the signature of a disc neighborhood of its fixed-point set with quasi-free action. A necessary task in this reduction is the generalization of Novikov’s additivity to this context.