The Analysis and PDE seminar will take place Monday, February 12, in 740 Evans from 4:10 to 5 pm.

Title: Edge (resonant) states for 1D bi-periodic systems

Abstract: We study the bifurcation of Dirac points under the introduction of edges in certain periodic systems. For honeycomb Schrodinger operators, Fefferman, Lee-Thorp and Weinstein showed that if introducing the edge opens an essential spectral gap near the Dirac energy, then the perturbed operator has an edge-localized eigenstate. This state is associated to the topologically protected zero-mode of a Dirac operator, which emerges from a formal multiscale approach (one of the two scales being the size of the edge).

We consider 1D models where the introduction of a large edge does not necessarily open an essential spectral gap near the Dirac energy. We approach such systems with Fredholm analytic tools and show that Dirac points bifurcate to resonant states. When the edge perturbation happens to open an essential spectral gap, this improves a previous result of Fefferman–Lee-Thorp–Weinstein by (a) proving the validity of the multiscale approach; (b) relating each eigenvalue in the gap to an eigenvalue of the above Dirac operator; (c) deriving full expansions of the associated states.

Joint work with Michael Weinstein and Charles Fefferman.

The Analysis and PDE seminar will take place Monday, February 5, in 740 Evans from 4:10 to 5 pm.

Title: Fredholm theory and the resolvent of the Laplacian near zero energy on asymptotically conic spaces

Abstract: We consider geometric generalizations of Euclidean low energy resolvent estimates, such as estimates for the resolvent of the Euclidean Laplacian plus a decaying potential, in a Fredholm framework. More precisely, the setting is that of perturbations $P(\sigma)$ of the spectral family of the Laplacian $\Delta_g-\sigma^2$ on asymptotically conic spaces $(X,g)$ of dimension at least $3$, and the main result is uniform estimates for $P(\sigma)^{-1}$ as $\sigma\to 0$ on microlocal variable order spaces under an assumption on the nullspace of $P(0)$ on the appropriate function space (which in the Euclidean case translates to $0$ not being an $L^2$-eigenvalue or having a half-bound state). These spaces capture the limiting absorption principle for $\sigma\neq 0$ in a lossless, in terms of decay, manner.

The Analysis and PDE seminar will take place Monday, January 29, in 740 Evans from 4:10 to 5 pm.

Title: Fourier dimension for limit sets

Abstract: For a finite measure $\mu$ on the real line, its Fourier dimension is defined using the rate of polynomial decay of the Fourier transform $\hat \mu$. The Fourier dimension of $\mu$ may be much smaller than the Hausdorff dimension of the support of $\mu$: a classical example is the Cantor measure on the mid-third Cantor set which has Fourier dimension equal to 0.

I will present a joint result with J. Bourgain showing that the Patterson-Sullivan measure on the limit set of a convex co-compact group of fractional linear transformations has positive Fourier dimension. The proof uses advanced tools from additive combinatorics (the discretized sum-product theorem) and exploits the fact that fractional linear transformations are (generally) not linear. An application is a new spectral gap result for convex co-compact hyperbolic surfaces.

The Analysis and PDE seminar will take place Monday, January 15, in 740 Evans from 4:10 to 5pm.

Title: Concentration of eigenfunctions: Averages and Sup-norms

Abstract: In this talk, we relate microlocal concentration of eigenfunctions to sup-norms and sub-manifold averages. In particular, we characterize the microlocal concentration of eigenfunctions with maximal sup-norm and average growth. We then exploit this characterization to derive geometric conditions under which maximal growth cannot occur. This talk is based on joint works with Yaiza Canzani and John Toth.

The Analysis and PDE seminar will take place on Monday, November 27, from 4:10 to 5pm, in 740 Evans.

Title: Fine properties of Dirichlet energy minimizing multi-valued functions

Abstract: I will discuss the fine structure of the branch set of multivalued Dirichlet energy minimizing functions as developed by Almgren. It is well-known that the dimension of the interior singular set of a Dirichlet energy minimizing function on an $n$-dimensional domain is at most $n-2$. We show that the singular set is countably $(n-2)$-rectifiable and also prove the uniqueness of homogeneous tangent functions at almost every singular point. Our approach involves adapting a “blow up” method due to Leon Simon, which was originally applied to multiplicity one classes of minimal submanifolds. We apply Simon’s method in the higher multiplicity setting of multivalued energy minimizers using techniques from prior work of Neshan Wickramasekera together with new estimates. This is joint work with Neshan Wickramasekera.

The Analysis and PDE seminar will take place Monday, October 30, in 740 Evans from 4:10 to 5pm.

Title: Inverse scattering and the Davey-Stewartson II equation

Abstract: The aim of this talk is to describe a complete implementation of the inverse scattering approach to the study of the defocusing Davey-Stewartson equation.
This will involve dispersive quations, dbar pde’s, microlocal analysis and other fun stuff. This is joint work with Adrian Nachman and Idan Regev.

The Analysis and PDE seminar will take place Monday, October 16, in 740 Evans from 4:10 to 5pm.

Title: Fractal uncertainty for transfer operators

Abstract: I will present a new explanation of the connection between
the fractal uncertainty principle
of Bourgain–Dyatlov, a statement in harmonic analysis, and the
existence of zero free strips for Selberg zeta functions, which is a
statement in geometric scattering/dynamical systems. The connection is
proved using (relatively) elementary methods via the Ruelle transfer
operator which is a well known object in thermodynamical formalism of
chaotic dynamics. (Joint work with S Dyatlov.)

The Analysis and PDE seminar will take place Monday, October 9nd, in 740 Evans from 4:10 to 5pm.

Title: Gravity water waves and emerging bottom

Abstract: To understand the behavior of waves at a fluid surface in configurations where the surface and the bottom meet (islands, beaches…), one encounters a difficulty: the presence in the bulk of the fluid of an edge, at the triple line. To solve the Cauchy problem, we need to study elliptic regularity in such domains, understand the linearized operator around an arbitrary solution, and construct an appropriate procedure to quasi-linearize the equations. Using those tools, I will present some a priori estimates, a first step to a local existence result.

The Analysis and PDE seminar will take place Monday, October 2nd, in 740 Evans from 4:10 to 5pm.

Title: Convergence of phase-field models and thresholding schemes for multi-phase mean curvature flow

Abstract: The thresholding scheme is a time discretization for mean curvature flow. Recently, Esedoglu and Otto showed that thresholding can be interpreted as minimizing movements for an energy that Gamma-converges to the total interfacial area. In this talk I’ll present new convergence results, in particular in the multi-phase case with arbitrary surface tensions. The main result establishes convergence to a weak formulation of (multi-phase) mean curvature flow in the BV-framework of sets of finite perimeter. Furthermore, I will present a similar result for the vector-valued Allen-Cahn equation.

This talk encompasses joint works with Felix Otto, Thilo Simon, and Drew Swartz.

The Bay Area Microlocal Analysis Seminar will take place on Monday, September 25, in room 740, Evans Hall, with two talks, given by Kiril Datchev at 2:40 pm and Charles Hadfield at 4:10 pm.

Speaker: Kiril Datchev (2:40 pm)

Title: Semiclassical resolvent estimates away from trapping

Abstract: Semiclassical resolvent estimates relate dynamics of a particle scattering problem to regularity and decay of waves in a corresponding wave scattering problem. Roughly speaking, more trapping of particles corresponds to a larger resolvent near the trapping. If the trapping is mild, then propagation estimates imply that the larger norm occurs only there. However, in this talk I will show how the effects of heavy trapping can tunnel over long distances, implying that the resolvent can be very large far away as well. This is joint work with Long Jin.

Speaker: Charles Hadfield (4:10 pm)

Title: Resonances on asymptotically hyperbolic manifolds; the ambient metric approach

Abstract: On an asymptotically hyperbolic manifold, the Laplacian has essential spectrum. Since work of Mazzeo and Melrose, this essential spectrum has been studied via the theory of resonances; poles of the meromorphic continuation of the resolvent of the Laplacian (with modified spectral parameter). A recent technique of Vasy provides an alternative construction of this meromorphic continuation which dovetails the ambient metric approach to conformal geometry initiated by Fefferman and Graham. I will discuss the ambient geometry present in this construction, use it to define quantum resonances for the Laplacian acting on natural tensor bundles (forms, symmetric tensors), and mention an application showing a correspondence between Ruelle resonances and quantum resonances on convex cocompact hyperbolic manifolds.