Teorı́a de bordismo y acciones de grupos finitos

Organizadores: Andrés Ángel (ja.angel908@uniandes.edu.co), Ana González de los Santos (anagon@fing.edu.uy), Rita Jiménez Rolland (rita@im.unam.mx csegovia@matem.unam.mx), Carlos Segovia (csegovia@matem.unam.mx)

• Miércoles 15

15:00 - 15:45

Diffeomorphisms of reducible three manifolds and bordisms of group actions on torus

Sam Nariman (Purdue University, Estados Unidos)

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.

15:45 - 16:30

$$Z_p$$-bordism and the mod($$p$$)-Borsuk-Ulam Theorem

Alice Kimie Miwa Libardi (Universidad Estatal Paulista, Brasil)

Crabb-Gonçalves-Libardi-Pergher classified for given integers $$m,n, \geq 1$$ the bordism class of a closed smooth $$m$$-manifold $$X^m$$ with a free smooth involution $$\tau$$ with respect to the validity of the Borsuk-Ulam property that for every continuous map $$\varphi: X^m \to R^n$$, there exists a point $$x \in X^m$$ such that $$\varphi(x)= \varphi(\tau(x))$$.

In this work together with Barbaresco-de Mattos-dos Santos-da Silva, we are considering the same problem for free $$Z_p$$ action.

16:45 - 17:30

Extending free group action on surfaces

Carlos Segovia (Universidad Nacional Autónoma de México, México)

We are interested in the question of when a free action of a finite group on a closed oriented surface extends to a non-necessarily free action on a 3-manifold. We show the answer to this question is affirmative for abelian, dihedral, symmetric and alternating groups. We present also the proof for finite Coxeter groups.

17:30 - 18:15

Stolz' Positive scalar curvature surgery exact sequence and low dimensional group homology

Noé Bárcenas (Universidad Nacional Autónoma de México, México)

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.

• Jueves 16

15:00 - 15:45

On the evenness conjecture for equivariant unitary bordism

Bernardo Uribe (Universidad del Norte, Colombia)

The evenness conjecture for equivariant unitary bordism states that these homology groups are free modules of even degree with respect to the unitary bordism ring. This conjecture is known to be true for all finite abelian groups and some semidirect products of abelian groups. In this talk I will talk about some progress done with the group of quaternions of order 8, together with an approach of Eric Samperton and Carlos Segovia to construct a free action on a surface which does not bound equivariantly. This last construction might provide a counterexample of the evenness conjecture in dimension 2.

15:45 - 16:30

The Loch Ness Monster as Homology Covers

Rubén A. Hidalgo (Universidad de la Frontera, Chile)

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.

16:45 - 17:30

When does a free action of a finite group on a surface extend to a (possibly non-free) action on a 3-manifold?

Eric G. Samperton (University of Illinois, Estados Unidos)

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.

17:30 - 18:15

Signature of manifold with single fixed point set of an abelian normal group

Quitzeh Morales (Universidad Pedagógica de Oaxaca, México)

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.