PALESTRA: Esferas com equadores mínimos
Resumo: Discutiremos em detalhes a classificação das métricas Riemannianas na esfera com respeito às quais os equadores (ou grandes esferas) são todos hipersuperfícies mínimas.
PALESTRA: On the harmonic flow of geometric structures
RESUMO: In this talk, I will report on recent results of an ongoing collaboration with Éric Loubeau, Andrés Moreno and Henrique Sá Earp on the study of the harmonic flow of H-structures. This is the negative gradient flow of a natural Dirichlet-type energy functional on a given isometric class of H-structures on a closed Riemannian n-manifold, where H is the stabilizer in SO(n) of a finite collection of tensors in Rn. Using general Bianchi-type identities of H-structures, we are able to prove monotonicity formulas for scale-invariant local versions of the energy, similar to the classic formulas proved by Struwe and Chen (1988-89) in the theory of harmonic map heat flow. We then deduce a general epsilon-regularity result along the harmonic flow and, more importantly, we get long-time existence and finite-time singularity results in parallel to the classical results proved by Chen-Ding (1990) in harmonic map theory. In particular, we show that if the energy of the initial H-structure is small enough, depending on the C^0-norm of its torsion, then the harmonic flow exists for all time and converges to a torsion-free H-structure. Moreover, we prove that the harmonic flow of H-structures develops a finite time singularity if the initial energy is sufficiently small but there is no torsion-free H-structure in the homotopy class of the initial H-structure. Finally, based on the analogous work of He-Li (2021) for almost complex structures, we give a general construction of examples where the later finite-time singularity result applies on the flat n-torus, provided the n-th homotopy group of the quotient SO(n)/H is non-trivial; e.g. when n=7 and H=G2, or when n=8 and H=Spin(7).
PALESTRA: Laplacian coflow of G2-structures on 7-manifolds
Abstract: Flows of G2-structures have been used as tools in the study of G2-geometry. The talk will focus on some principal results of the Laplacian coflow. We will give some general preliminaries on Contact Calabi-Yau 7-manifolds which was used in the Laplacian coflow with the initial coclosed G2-structure given by Habib and Vezzoni finding a singularity and show that the metric and the volume collapse at this singularity. Finally, the Almost abelian Lie group also studied the Laplacian coflow, finding that the solution converges to G2-structure torsion free.
PALESTRA: Superfícies de Ricci rotacionais.
RESUMO: clique aqui.