Bryant-Salamon G2 manifolds and coassociative fibrations

Speaker: 

Spiro Karigiannis

Institution: 

University of Waterloo

Time: 

Tuesday, November 10, 2020 - 4:00pm to 5:00pm

Location: 

Remotely

I will discuss joint work with Jason Lotay from arXiv:2002.06444. We show how the three Bryant-Salamon G2 manifolds can be viewed as coassociative fibrations. In all cases the coassociative fibres are invariant under a 3-dimensional group and are thus of cohomogeneity one, In general there are both generic smooth fibres and degenerate singular fibres. The induced Riemannian geometry on the fibres turns out to exhibit asymptotically conical and conically singular behaviour. In some cases we also explicitly determine the induced hypersymplectic structure. In all three cases we show that the "flat limits" of these coassociative fibrations are well-known calibrated fibrations of Euclidean space. Finally, we establish connections with the multimoment maps of Madsen-Swann, the new compact construction of G2 manifolds of Joyce-Karigiannis, and recent work of Donaldson involving vanishing cycles and "thimbles".
 

Witten deformation on noncompact manifolds

Speaker: 

Xianzhe Dai

Institution: 

UC Santa Barbara

Time: 

Monday, October 28, 2019 - 4:00pm

Location: 

RH 340P

Motivated by considerations from the mirror symmetry and
Landau-Ginzburg model, we consider Witten deformation on noncompact
manifolds.

Witten deformation is a deformation of the de Rham complex introduced by
Witten in an influential paper and has had many important applications,
mostly on compact manifolds. We will discuss some recent work with my
student Junrong Yan on the spectral theory of Witten Laplacian, the
cohomology of the deformation as well as its index theory.

 

 

A joint seminar with the Geometry & Topology Seminar series.

Comparing gauge theoretic invariants of homology S1 cross S3

Speaker: 

Jianfeng Lin

Institution: 

UC San Diego

Time: 

Tuesday, October 1, 2019 - 4:00pm

Location: 

RH 306

While classical gauge theoretic invariants for 4-manifolds are usually
defined in the setting that the intersection form has nontrivial positive
part, there are several invariants for a 4-manifold X with the homology S1
cross S3. The first one is the Casson-Seiberg-Witten invariant LSW(X)
defined by Mrowka-Ruberman-Saveliev; the second one is the Fruta-Ohta
invariant LFO(X). It is conjectured that these two invariants are equal to
each other (This is an analogue of Witten’s conjecture relating Donaldson and
Seiberg-Witten invariants.)

In this talk, I will recall the definition of these two invariants, give
some applications of them (including a new obstruction for metric with
positive scalar curvature), and sketch a prove of this conjecture for
finite-order mapping tori. This is based on a joint work with Danny Ruberman
and Nikolai Saveliev.

A joint seminar with the Geometry & Topology Seminar series.

Pointwise lower scalar curvature bounds for C^0 metrics via regularizing Ricci flow

Speaker: 

Paula Burkhardt-Guim

Institution: 

UC Berkeley

Time: 

Tuesday, October 15, 2019 - 4:00pm

Host: 

Location: 

RH306

We propose a class of local definitions of weak lower scalar curvature bounds that is well defined for C^0 metrics. We show the following: that our definitions are stable under greater-than-second-order perturbation of the metric, that there exists a reasonable notion of a Ricci flow starting from C^0 initial data which is smooth for positive times, and that the weak lower scalar curvature bounds are preserved under evolution by the Ricci flow from C^0 initial data.

Generic Multiplicity One Singularities of Mean Curvature Flow of Surfaces

Speaker: 

Ao Sun

Institution: 

MIT

Time: 

Friday, November 8, 2019 - 4:00pm to 5:00pm

Host: 

Location: 

RH 306

One of the central topics in mean curvature flow is understanding the singularities. In 1995, Ilmanen conjectured that the first singularity appeared in a smooth mean curvature flow of surfaces must have multiplicity one. Following the theory of generic mean curvature flow developed by Colding-Minicozzi, we prove that a closed singularity with high multiplicity is not generic, in the sense that we may perturb the flow so that this singularity with high multiplicity can never show up. One of the main techniques is the local entropy, which is an extension of the entropy used by Colding-Minicozzi to study the generic mean curvature flow.​

Complete noncompact Kaehler manifolds with positive curvature

Speaker: 

Xi-Ping Zhu

Institution: 

Sun Yat-sen University

Time: 

Tuesday, August 6, 2019 - 4:00pm

Location: 

RH 306

The well-known Yau’s uniformization conjecture states that any
complete noncompact Kaehler manifold with positive bisectional curvature is
bi-holomorphic to the complex Euclidean space. The conjecture for the case
of maximal volume growth has been recently confirmed by G. Liu. In this
talk, we will consider the conjecture for manifolds with non-maximal volume
growth. We will show that the finiteness of the first Chern number is an
essential condition to solve Yau’s conjecture by using algebraic embedding
method. Furthermore, we can verify the finiteness in the case of minimal
volume growth. In particular, we obtain a partial answer to Yau’s
uniformization conjecture on complex two-dimensional Kaehler manifolds with
minimal volume growth. This is a joint work with Bing-Long Chen.

Location of hot spots in thin curved strips

Speaker: 

David Krejcirik

Institution: 

Czech Technical University in Prague

Time: 

Tuesday, June 25, 2019 - 4:00pm

Host: 

Location: 

RH 306

According to the conjecture of Rauch’s from 1974, any eigenfunction corresponding to the principal eigenvalue of the Neumann Laplacian attains its extrema on the boundary of planar domains. After giving an account on the history and validity of the conjecture, we present our own new results for tubular neighbourhoods of curves on surfaces. This is joint work with Matej Tusek.

Entropy, noncollapsing, and a gap theorem for ancient solutions to the Ricci flow

Speaker: 

Yongjia Zhang

Institution: 

UC San Diego

Time: 

Tuesday, May 21, 2019 - 4:00pm

Location: 

RH 306

Entropy has been an important topic in the study of Ricci flow
ever since it was invented by Perelman. We consider Perelman's entropy
defined on an ancient solution, and prove a gap theorem for its backward
limit: If Perelman's entropy limits to a number too close to zero as time
approaches negative infinity, then the ancient solution must be the trivial
Euclidean space.

UCI-UCR-UCSD Joint Differential Geometry Seminar

Institution: 

Joint seminar

Time: 

Tuesday, June 4, 2019 - 4:00pm to 6:00pm

Location: 

UC Riverside Skye 347

Location: Skye Hall 347

 

4:00-4:50pm

Speaker: Yu-Shen Lin (Boston University)

Title: Special Lagrangian fibrations in weak Del Pezzo Surfaces

Abstract: Motivated by the study of mirror symmetry, Strominger-Yau-Zaslow (SYZ) conjectured that Calabi-Yau manifolds admit certain minimal Lagrangian fibrations. These minimal Lagrangians are the special Lagrangian submanifolds studied earlier by Harvey-Lawson. Many of the implication of the SYZ conjecture is proved and it has been the guiding principle for studying mirror symmetry for a long time. However, not many special Lagrangians are known in the literature. In this talk, I will prove the existence of special Lagrangian fibration on the complement of a smooth anti-canonical divisor in a (weak) Del Pezzo surface. If the time allows, I will explain its impact to mirror symmetry. This is joint work with Tristan Collins and Adam Jacob.

 

5:00-5:50pm

Speaker: Yannis Angelopoulos (UCLA)
Title: Linear and nonlinear waves on extremal Reissner-Nordstrom spacetimes 

Abstract:  I will present several results (that have been obtained jointly with Stefanos Aretakis and Dejan Gajic) from the analysis of solutions of linear and nonlinear wave equations on extremal Reissner-Nordstrom spacetimes, including sharp asymptotics on the horizon and at infinity for linear waves, and instability phenomena for nonlinear waves. These results can be seen as stepping stones to the fully nonlinear problem of stability/instability of extremal black holes.

 

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