# GeA

##### Semiario di Geometria e Analisi a Padova

Martedì 10 Marzo 2020, ore 14:30, Aula 430

**Soft construction of Floer Homology**

** Michele Stecconi** (SISSA, Trieste)

*Abstract:*
Invented by Andreas Floer in 1988 to solve Arnold’s conjecture, (symplectic) Floer Homology is a machinery to relate the existence of periodic trajectories of an Hamiltonian flow on a symplectic manifold M with the homology groups of M, analougously to Morse Homology. Indeed this is done by developing an infinite dimensional Morse theoretic framework adapted to a certain functional (the action functional) on the loop space of M, whose critical points are the periodic trajectories of the given hamiltonian flow. Despite the topological nature of the results, the construction is technically quite heavy, since involves elliptic systems of PDEs.

Together with Andrei Agrachev and Antonio Lerario we are developing a method to construct such infinite dimensional homology invariants using only soft and essentially finite dimensional tools. In my talk I will present our approach. The main feature consists in approximating the loop space with finite dimensional submanifolds of increasing dimension, we do this with the language of control theory, and then interpret asymptotically the information provided by classical Morse theory.

## Past Talks

Lunedì 28 Ottobre 2019, ore 11, Aula 2AB45

**On the geometry of soap films and soap bubbles**

** Andrea Marchese** (Università di Pavia)

*Abstract:*
I will discuss a recent result establishing that area minimizing
m-dimensional currents modulo p in any codimension are regular submanifolds
outside a singular set of Hausdorff dimension at most m-1, moreover the
singular set is rectifiable and of locally finite Hausdorff measure for odd
values of p. Joint work with Camillo De Lellis, Jonas Hirsh, and Salvatore
Stuvard.

Lunedì 11 Novembre 2019, ore 12 (non 11!), Aula 2AB45

**Some results on the flow of vector fields of BV class**

** Stefano Bianchini** (Sissa, Trieste)

*Abstract:*
This seminar will try to present 3 results.

1) A general theory for uniqueness of Lagrangian representations

Given a vector field `$\rho (1,{\mathbf b}) \in L^1_{\mathrm{loc}}({\mathbb R}^+\times {\mathbb R}^{d},{\mathbb R}^{d+1})$`

such that `${\mathrm{div}}_{t,x} (\rho (1,{\mathbf b}))$`

is a measure, we consider the problem of uniqueness of the representation `$\eta$`

of `$\rho (1,{\mathbf b}) \mathcal L^{d+1}$`

as a superposition of characteristics `$\gamma : (t^-_\gamma,t^+_\gamma) \to {\mathbb R}^d$`

, `$\dot \gamma (t)= \b(t,\gamma(t))$`

. We give conditions in terms of a local structure of the representation `$\eta$`

on suitable sets in order to prove that there is a partition of `${\mathbb R}^{d+1}$`

into disjoint trajectories `$\wp_{\mathfrak a}$`

, `${\mathfrak a} \in {\mathfrak A}$`

, such that the PDE

`$$`

`{\mathrm{div}}_{t,x} \big( u \rho (1,{\mathbf b}) \big) \in \mathcal M({\mathbb R}^{d+1}), \qquad u \in L^\infty({\mathbb R}^+\times {\mathbb R}^{d}),`

`$$`

can be disintegrated into a family of ODEs along `$\wp_{\mathfrak a}$`

with measure r.h.s.. The decomposition `$\wp_{\mathfrak a}$`

is essentially unique.

2) The application to BV vector fields
We show that `${\mathbf b} \in L^1_t({\mathrm{BV}}_x)_{\mathrm{loc}}$`

satisfies this local structural assumption and this yields, in particular, the renormalization property for nearly incompressible `${\mathrm{BV}}$`

vector fields.

3) Differentiability of the flow

The unique flow generated in point 2 enjoys a weak differentiability property, and its weak derivative satisfies the equation for the Jacobian matrix in this weak setting.

Lunedì 25 Novembre 2019, ore 11, Aula 2AB45

**Geometry of 1-codimensional measures in the Heisenberg groups**

** Andrea Merlo** (Pisa)

*Abstract:*
Characterisation of rectifiable measures in Euclidean spaces through the existence of the density has
been a longstanding problem for Geometric Measure Theory until the complete answer by D. Preiss
in 1987. The question of how in more general metric spaces existence of density can affect any kind
of gain in terms of regularity of the measure is a completely open problem.
In this talk I will discuss how the mere existence of the 1-codimensional density for a measure in
the Heisenberg groups endowed with the Koranyi metric implies that almost everywhere the tangents
to the measure are flat.

Lunedì 9 Dicembre 2019, ore 11, Aula 2AB45

**Minimal bubble clusters in the plane with double density**

** Valentina Franceschi** (Sorbonne Université, Parigi)

*Abstract:*
We present some results about minimal bubble clusters in the plane with double density. This amounts to find the best configuration of $m\in \mathbb N$ regions in the plane enclosing given volumes, in order to minimize their total perimeter, where perimeter and volume are defined by suitable densities. We focus on a particular structure of such densities, which is inspired by a sub-Riemannian model, called the Grushin plane.

After an overview concerning existence of minimizers, we focus on their Steiner regularity, i.e., the fact that their boundaries are made of regular curves meeting at 120 degrees. We will show that this holds in a wide generality.

Although our initial motivation came from the study of the particular sub-Riemannian framework of the Grushin plane, our approach works in wide generality and is new even in the classical Euclidean case.

Mercoledì 18 Dicembre 2019, ore 11, Aula 430

(Attenzione: non di lunedì e in un’aula diversa dal solito)

**A rectifiability result for sets of finite perimeter in Carnot groups**

** Sebastiano Don** (University of Jyväskylä)

*Abstract:*
After an introduction to the regularity problem for sets of finite perimeter in Carnot groups, we prove that the reduced boundary of a set of finite perimeter in a Carnot group can be covered by a countable union of sets satisfying a “cone property”. We show that this weak notion of rectifiability implies the intrinsic Lipschitz rectifiability in a class of Carnot groups including all filiform groups.
This is a joint work with Enrico Le Donne, Terhi Moisala and Davide Vittone.

Venerdì 24 Gennaio 2020, ore 11, Aula 430

**Gradient of the single layer potential and rectifiability**

** Carmelo Puliatti** (Universitat Autònoma de Barcelona, puliatti@mat.uab.cat)

*Abstract:*
An important theorem by Nazarov, Tolsa and Volberg asserts that a measure on which has -polynomial growth both from above and from below (also known as -Ahlfors-David regular measure) and whose associated -Riesz transform is bounded on is -uniformly rectifiable.

In this talk I will discuss an analogous result for the operator

where is an -uniformly elliptic matrix with Hölder continuous coefficients and is the fundamental solution to the equation

This is a joint work with Laura Prat and Xavier Tolsa, and it is also motivated by an application to the study of elliptic measure.

I will also present a recent closely related article which contains a quantitative rectifiability criterion for more general Radon measures in terms of . This applies to a two-phase problem for the elliptic measure.