2013 | 2014 | 2015 | 2016 | 2017 | 2018 | 2019
Current contact: Gerardo Mendoza
The seminar takes place Mondays 2:40 - 3:30 pm in Wachman 617. Click on title for abstract.
Tomasz Z. Szarek, Rutgers University
The classical spherical heat kernel is an important object in analysis, probability and physics, among other fields. It is the integral kernel of the spherical heat semigroup and thus provides solutions to the heat equation based on the Laplace-Beltrami operator on the sphere. It is also a transition probability density of the spherical Brownian motion. In this talk we prove sharp two-sided global estimates for the heat kernel associated with a Euclidean sphere of arbitrary dimension. If time permits, we will present a generalization of this result to the compact rank-one symmetric spaces. The talk is based on joint papers with Adam Nowak and Peter Sjögren
Yury Grabovsky, Temple University
The Cauchy-Born principle in physics says that macroscopic affine deformations cause microscopic affine deformations. Mathematically this principle can be formulated in the language of Calculus of Variations: $y(x)=F_{0}x$ is the minimizer of the (energy) functional \[ E[y]=\int_{\Omega}W(\nabla y(x))dx\qquad(\Omega\subset\mathbf{R}^{d}-\mbox{a Lipschitz domain}) \] among all Lipschitz functions $y:\Omega\to\mathbf{R}^{m}$, such that $y(x)=F_{0}x$ on $\partial\Omega$. In this form the Cauchy-Born principle can be viewed as a version of Jensen's inequality for convex functions: \[ \frac{1}{|\Omega|}\int_{\Omega}W(\nabla y(x))dx\ge W(F_{0})= W\left(\frac{1}{|\Omega|}\int_{\Omega}\nabla y(x)dx\right), \] since $y(x)=F_{0}x$ on $\partial\Omega$. When the above inequality holds, we say that $W(F)$ is quasiconvex at $F_{0}\in\mathbf{R}^{m\times d}$. The boundary of the set of points of quasiconvexity is called the elastic binodal. When quasiconvexity fails, the gradients of minimizers of $E[y]$ can become discontinuous or even cease to exist, while minimizing sequences develop fine scale oscillations that people call the microstructure.
In this talk I will discuss my joint work with Lev Truskinovsky, aiming to understand when and why such spontaneous discontinuities and microstructures form. This lecture is geared towards graduate students and is meant to be widely accessible.
Gianmarco Molino, University of Connecticut
Sub-Riemannian geometry is a generalization of Riemannian geometry to spaces that have a notion of distance, but have restrictions on the valid directions of motion. These arise in a natural way in remarkably many settings.
This talk will include a review of Riemannian geometry and an introduction to sub-Riemannian geometry; we'll then introduce the notion of H-type foliations; these are a family of sub-Riemannian manifolds that generalize both the K-contact structures arising in contact geometry and the H-type group structures. Our main focus will be recent results giving uniform comparison theorems for the Hessian and Laplacian on a family of Riemannian metrics converging to sub-Riemannian ones. From this we can conclude a sharp sub-Riemannian Bonnet-Myers type theorem.
Laurent Stolovitch, Université Nice Sophia Antipolis
We prove that if two real-analytic hypersurfaces in $\mathbb C^2$ are equivalent formally, then they are also $C^\infty$ CR-equivalent at the respective point. As a corollary, we prove that all formal equivalences between real-algebraic Levi-nonflat hypersurfaces in $\mathbb C^2$ are algebraic (in particular are convergent). The result is obtained by using the recent CR - DS technique, connecting degenerate CR-manifolds and Dynamical Systems, and employing subsequently the multisummability theory of divergent power series used in the Dynamical Systems theory. This is a joint work with I. Kossovskiy and B. Lamel.
Wissam Raji, American University of Beirut
Modular forms are analytic functions defined on the upper half-plane with a specific transformation law under elements of the full modular group $\mathrm{SL}_2(\mathbb{Z})$. In this talk, we give different motivations to the theory and then give an explicit introduction about the main definitions in the theory of modular forms. Interesting series called $L$-series, constructed using the Fourier coefficients of modular forms have important connections to elliptic curves. We show that, on average, the $L$-functions of cuspidal Hilbert modular forms (a generalization of classical modular forms) with sufficiently large weight $k$ do not vanish on the line segments $ \Im(s)=t_0, \ \Re(s) \in (\frac{k-1}{2},\frac{k}{2}-\epsilon)\cup (\frac{k}{2}+\epsilon,\frac{k+1}{2})$.
Francisco Villarroya, Temple University
We introduce a new local $Tb$ Theorem for Calder\'on-Zygmund operators \begin{equation*} Tf(x)=\int f(t)K(t,x)d\mu (t) \end{equation*} that extend compactly on $L^{p}(\mathbb R^{n},\mu)$ for $1< p<\infty$ and $\mu $ in a class of non-homogeneous measures. In the main result, compactness is deduced from the following two hypotheses:
$\bullet$ appropriate decay estimates satisfied by either the operator kernel or the operator measure, and
$\bullet$ the action of the operator over families of testing functions $(b_{Q})_{Q\in \mathcal D}$ supported on dyadic cubes, which in general may not be accretive.
As an application we describe the measures $\mu $ such that the Cauchy integral defines a compact operator.
Samuel Cogar, University of Delaware
In this talk I will introduce a new modified transmission eigenvalue problem for scattering by a partially coated crack. Rather than study this problem in isolation, I will present a generalized Robin eigenvalue problem depending on a bounded linear operator that encodes the information for a given scattering medium. Results obtained in this general setting will then be applied to the case of scattering by a partially coated crack, including a new proof that finitely many eigenvalues exist when the surface impedance of the crack is sufficiently small. I will conclude with some numerical examples that both verify the theoretical results and demonstrate the sensitivit
Zachary Bailey, Temple University
We consider two inverse problems for hyperbolic PDE in three space dimensions. The two problems are associated with a single hyperbolic PDE with a zero order coefficient and the goal is the recovery of this coefficient from two different types of "backscattering data" - backscattering data coming from a fixed offset distribution of sources and receivers on the boundary or backscattering data coming from a single incoming spherical wave. For these problems we prove a stability result provided the difference of the two coefficients is horizontally or angularly controlled respectively. Our work adapts the techniques used by Eemeli Bl&cira;sten, Rakesh and Gunther Uhlmann to solve problems similar to theirs.
Maxim Braverman, Northeastern University
We study the index of the APS boundary value problem for a strongly Callias-type operator $D$ on a complete Riemannian manifold $M$. We use this index to define the relative eta-invariant of two strongly Callias-type operators $A$ and $A'$, which are equal outside of a compact set. Even though in our situation the $\eta$-invariants of $A$ and $A'$ are not defined, the relative $\eta$-invariant behaves as if it were the difference of the $\eta$-invariants of $A$ and $A'$. We also define the spectral flow of a family of such operators and use it compute the variation of the relative $\eta$-invariant. (Joint work with Pengshuai Shi.)
Wanke Yin, Wuhan University and Rutgers University
Let $M$ be a smooth real hypersurface in $\mathbb C^n$ with $n\geq 2$. For any $p\in M$ and any integer $s\in [1,n-1]$, Bloom in 1981 defined the following three kinds of integral invariants: invariant $a^{(s)}(M,p)$ defined in terms of contact order by complex submanifolds, invariant $t^{(s)}(M,p)$ defined by the iterated Lie bracket of vector fields and invariant $c^{(s)}(M,p)$ defined through the degeneracy of the trace of the Levi form. When $M$ is pseudoconvex, Bloom conjectured that these three invariants are equal. Bloom and Graham gave a complete solution of the conjecture for $s=n-1$. Bloom showed that the conjecture is true for $a^{(1)}(M,p)=c^{(1)}(M,p)$ when $n=3$. In this talk, I will present a recent joint work with Xiaojun Huang, in which we gave a solution of the conjecture for $s=n-2$. In particular, this gave a complete solution of the Bloom conjecture for $n=3$.
Jose Maria Martell, ICMAT, Madrid, Spain
In nice environments, such as Lipschitz or chord-arc domains, it is well-known that the Dirichlet problem for the Laplacian with data in Lebesgue spaces $L^p$ is solvable for some finite $p$. This property is equivalent to the fact that the associated harmonic measure is absolutely continuous, in a quantitative way, with respect to the surface measure on the boundary. In this talk we will study under what circumstances the harmonic measure for a rough domain is a well-behaved object. We will also present some results for the converse, in which case good properties for the domain and its boundary can be proved by knowing that the harmonic measure satisfies a quantitative absolute continuity property with respect to the surface measure. We will describe the two main features appearing in this context: one related to the regularity of the boundary, expressed via its uniform rectifiablity, and another one related to the connectivity of the domain, written in terms of some quantitative connectivity towards the boundary using non-tangential paths. The results that we will present are higher dimensional scale-invariant extensions of the F. and M. Riesz theorem and its converse. That classical result says that, in the complex plane, the harmonic measure is absolutely continuous with respect to the arc-length measure for simply connected domains (a strong connectivity condition) with rectifiable boundary (a regularity condition).
Murat Akman, University of Connecticut
The classical Minkowski problem consists in finding a convex polyhedron from data consisting of normals to their faces and their surface areas. In the smooth case, the corresponding problem for convex bodies is to find the convex body given the Gauss curvature of its boundary, as a function of the unit normal. The proof consists of three parts: existence, uniqueness and regularity.
In this talk, we study a Minkowski problem for certain measure, called $p$-capacitary surface area measure, associated to a compact convex set with nonempty interior and its $p$-harmonic capacitary function. We will discuss existence, uniqueness, and regularity of this problem under this setting and see connections with the Brunn-Minkowski inequality and Monge-Ampere equation.
Narek Hovsepyan, Temple University
Analytic continuation problems are notoriously ill-posed without additional regularizing constraints, even though every analytic function has a rigidity property of unique continuation from every curve inside the domain of analyticity. In fact, well known theorems, guarantee that every continuous function can be uniformly approximated by analytic functions (polynomials or rational functions, for example). We consider several analytic continuation problems with typical global boundedness constraints. All such problems exhibit a power law precision deterioration as one moves away from the source of data. In this talk we demonstrate the effectiveness of our general Hilbert space-based approach for determining these exponents. The method identifies the ``worst case'' function as a solution of a linear equation with a compact operator. In special geometries, such as the circular annulus this equation can be solved explicitly. The obtained solution is then used to determine the power law exponent for the analytic continuation from an interval between the foci of a Bernstein ellipse to the entire ellipse. In those cases where such exponents have been determined in prior work our results reproduce them faithfully.
This is joint work with Yury Grabovsky.
Spring break, no meeting
Joseph Feneuil, Temple University
The Riesz transform $\nabla \Delta^{-1/2}$ on $\mathbb R^n$ is bounded on $L^p$ for all $p\in (1,+\infty)$. This well known fact can quickly be proved by using the Fourier transform. Strichartz asked then whether this property is transmitted to Riemannian manifold, more exactly, what are the geometric conditions needed on our manifold to get the boundedness of the Riesz transform.
We shall present (part of) the literature on the topic, including the results of the speaker (together with Li Chen, Thierry Coulhon, and Emmanuel Russ) on fractal-like spaces. We shall also talk about the case of graphs, that can be seen as discrete version of Riemannian manifolds, which will allow us to give concrete examples of application of our work.
If time permits, we will provide equivalent statements for an assumption frequently met when working on graphs (which implies $L^2$-analyticity of the Markov operator). In particular, we will see a way to weaken this assumption to $L^2$-analyticity.
Atilla Yilmaz, Temple University
I will present joint work with Elena Kosygina and Ofer Zeitouni in which we prove the homogenization of a class of one-dimensional viscous Hamilton-Jacobi equations with random Hamiltonians that are nonconvex in the gradient variable. Due to the special form of the Hamiltonians, the solutions of these PDEs with linear initial conditions have representations involving exponential expectations of controlled Brownian motion in a random potential. The effective Hamiltonian is the asymptotic rate of growth of these exponential expectations as time goes to infinity and is explicit in terms of the tilted free energy of (uncontrolled) Brownian motion in a random potential. The proof involves large deviations, construction of correctors which lead to exponential martingales, and identification of asymptotically optimal policies.
Nestor Guillen, University of Massachusetts, Amherst
A large number of problems involve mappings with a prescribed Jacobian, from optimal transport mappings to problems of lenses and antenna design in geometric optics. Many of these problems originate from what is now known as a "generating function", e.g. the cost function in optimal transport, in which case the equation is known as Generated Jacobian Equation. This class of equations has been proposed by Trudinger, and it covers not only optimal transport problems, but also near-field problems in optics. In this talk I will discuss work with Jun Kitagawa were we prove Holder continuity for the gradient of weak solutions to GJE, under natural assumptions. The results are in the spirit of, and extend, Caffarelli's theory for the real Monge-Ampere equation. The key observation is that a quasiconvexity property of the underlying generating function (related to MTW tensor) guarantees the validity of an estimate akin to Aleksandrov's estimate for convex functions.
Francisco Villarroya, Temple University
I will introduce a Tb Theorem that characterizes all Calderón-Zygmund operators that extend compactly on $L^p(\mathbb R^n)$ by means of testing functions as general as possible. In the classical theory of boundedness, the testing functions satisfy a non-degeneracy property called accretivity, which essentially implies the existence of a positive lower bound for the absolute value of the averages of the testing functions over all dyadic cubes. However, in the the setting of compact operators, due to their better properties, the hypothesis of accretivity can be relaxed to a large extend. As a by-product, the results also describe those Calderon-Zygmund operators whose boundedness can be checked with non-accretive testing functions.
Alessia Elisabetta Kogoj, University of Urbino "Carlo Bo"
Several Liouville-type theorems are presented, related to evolution equations on Lie Groups and to their stationary counterpart. Our results apply in particular to the heat operator on Carnot groups, to linearized Kolmogorov operators and to operators of Fokker-Planck-type like the Mumford operator. An application to the uniqueness for the Cauchy problem is also shown.
These results are based on joint publications with A. Bonfiglioli, E. Lanconelli, Y. Pinchover and S. Polidoro.