Department of

# Mathematics

Seminar Calendar
for Harmonic Analysis and Differential Equations Seminar events the year of Tuesday, December 3, 2019.

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events for the
events containing

Questions regarding events or the calendar should be directed to Tori Corkery.
    November 2019          December 2019           January 2020
Su Mo Tu We Th Fr Sa   Su Mo Tu We Th Fr Sa   Su Mo Tu We Th Fr Sa
1  2    1  2  3  4  5  6  7             1  2  3  4
3  4  5  6  7  8  9    8  9 10 11 12 13 14    5  6  7  8  9 10 11
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Tuesday, January 22, 2019

1:00 pm in 347 Altgeld Hall,Tuesday, January 22, 2019

#### Singular limits of sign-changing weighted eigenproblems

###### Derek Kielty   [email] (Illinois Math)

Abstract: Eigenvalue problems with positive weights are related to heat flow and wave propagation in inhomogeneous media. Sign-changing weights have ecological interpretations, and generate spectra that accumulate at both positive and negative infinity. This talk will discuss recent results on limits of such eigenvalue problems when a negative portion of the weight is made arbitrarily large.

Tuesday, March 19, 2019

1:00 pm in 347 Altgeld Hall,Tuesday, March 19, 2019

#### The Steklov and Laplacian spectra of Riemannian manifolds with boundary

###### Alexandre Girouard   [email] (Université Laval)

Abstract: The Dirichlet-to-Neumann map is a first order pseudodifferential operator acting on the smooth functions of the boundary of a compact Riemannian manifold M. Its spectrum is known as the Steklov spectrum of M. The asymptotic behaviour (as j tends to infinity) of the Steklov eigenvalues s_j is determined by the geometry of the boundary of M. Neverthless, each individual eigenvalue can become arbitrarily big if the Riemannian metric is perturbed adequately. This can be achieved while keeping the geometry of the boundary unchanged, but it requires wild perturbations in arbitrarily small neighborhoods of the boundary. In recent work with Bruno Colbois and Asma Hassannezhad, we impose constraints on the geometry of M on and near its boundary. This allows the comparison of each Steklov eigenvalue s_j with the corresponding eigenvalues l_j of the Laplace operator acting on the boundary. This control is uniform in the index j. The proof is based on a generalized Pohozaev identity and on comparison results for the principal curvatures of hypersurfaces that are parallel to the boundary.

Tuesday, August 27, 2019

1:00 pm in Altgeld Hall,Tuesday, August 27, 2019

#### Deflated Continuation: A bifurcation analysis tool for Nonlinear Schrodinger (NLS) Systems

###### Stathis Charalampidis (Mathematics Department, California Polytechnic State University)

Abstract: Continuation methods are numerical algorithmic procedures for tracing out branches of fixed points/roots to nonlinear equations as one (or more) of the free parameters of the underlying system is varied. On top of standard continuation techniques such as the sequential and pseudo-arclength continuation, we will present a new and powerful continuation technique called the deflated continuation method which tries to find/construct undiscovered/disconnected branches of solutions by eliminating known branches. In this talk we will employ this method and apply it to the one-component Nonlinear Schrodinger (NLS) equation in two spatial dimensions. We will present novel nonlinear steady states that have not been reported before and discuss bifurcations involving such states. Next, we will focus on a two-component NLS system and discuss about recent developments by using the deflated continuation method where the landscape of solutions of such a system is far richer. A discussion about the challenges in the two-component setting will be offered and a summary of open problems will be emphasized.

Tuesday, September 3, 2019

12:50 pm in 347 Altgeld Hall,Tuesday, September 3, 2019

#### Multi-frequency class averaging for three-dimensional cryo-electron microscopy

###### Zhizhen Jane Zhao (Illinois ECE)

Abstract: We introduce a novel intrinsic classification algorithm--multi-frequency class averaging (MFCA)--for clustering noisy projection images obtained from three-dimensional cryo-electron microscopy (cryo-EM) by the similarity among their viewing directions. This new algorithm leverages multiple irreducible representations of the unitary group to introduce additional redundancy into the representation of the transport data, extending and outperforming the previous class averaging algorithm that uses only a single representation. We will discuss the formal algebraic model and representation theoretic patterns of the proposed MFCA algorithm. We conceptually establish the consistency and stability of MFCA by inspecting the spectral properties of a generalized localized parallel transport operator on the two-dimensional unit sphere through the lens of Wigner D-matrices. We will also show how this algorithm can be applied to directly denoise the real data.

Tuesday, September 17, 2019

1:00 pm in 347 Altgeld Hall,Tuesday, September 17, 2019

#### Finite Elements for Curvature

###### Kaibo Hu   [email] (University of Minnesota)

Abstract: We review the elasticity (linearized Calabi) complex, its cohomology and potential applications in differential geometry and continuum defect theory. We construct discrete finite element complexes. In particular, this leads to new finite element discretization for the 2D linearized curvature operator. Compared with classical discrete geometric approaches, e.g., the Regge calculus, the new finite elements are conforming. The construction is based on a Bernstein-Gelfand-Gelfand type diagram chase with various finite element de Rham complexes. This is a joint work with Snorre H. Christiansen.

Tuesday, October 15, 2019

1:00 pm in 347 Altgeld Hall,Tuesday, October 15, 2019

#### Angled crested type water waves

###### Siddhant Agrawal (U Mass Amherst)

Abstract: We consider the two-dimensional water wave equation which is a model of ocean waves. The water wave equation is a free boundary problem for the Euler equation where we assume that the fluid is inviscid, incompressible, irrotational and the air density is zero. In the case of zero surface tension, we show that the singular solutions recently constructed by Wu (19) are rigid. In the case of non-zero surface tension, we construct an energy functional and prove a local wellposedness result without assuming the Taylor sign condition. This energy reduces to the energy obtained by Kinsey and Wu (18) in the zero surface tension case and allows angled crest interfaces. For non zero surface tension, the energy does not allow singularities in the interface but allows interfaces with large curvature. We show that in an appropriate regime, the zero surface tension limit of our solutions is a solution of the gravity water wave equation which includes waves with angled crests.

Tuesday, October 22, 2019

1:00 pm in 347 Altgeld Hall,Tuesday, October 22, 2019

#### The Dirichlet problem for elliptic operators having a BMO antisymmetric part

###### Linhan Li (UMN Math)

Abstract: In this talk, we are going to introduce our result on the absolute continuity of elliptic measure with respect to the Lebesgue measure for a divergence form elliptic operator with non-smooth coefficients that have a BMO antisymmetric part. In particular, the coefficients of the operator are not necessarily bounded. Our method relies on kernel estimates and off-diagonal estimates for the semigourp e^{-tL}, solution to the Kato problem, and various estimates for the Hardy norms of certain commutators. This is a joint work with S. Hofmann, S. Mayboroda, and J. Pipher.

Tuesday, November 12, 2019

1:00 pm in 347 Altgeld Hall,Tuesday, November 12, 2019

#### Turing patterns in the Schnakenberg equations: From normal to anomalous diffusion

###### Yanzhi Zhang (Department of Mathematics and Statistics, Missouri University of Science and Technology)

Abstract: In recent years, anomalous diffusion has been observed in many biological experiments. Instead of classical Laplace operator, the anomalous diffusion is described by the fractional Laplacian. In this talk, we will discuss the Turing patterns of the Schnakenberg equation and compare the pattern selection under normal and anomalous diffusion. Our analysis shows that the wave number of the Turing instability increases with the exponent of the fractional Laplacian. The interplay of the nonlinearity and long-range diffusion are studied numerically, and especially comparisons are provided to understand the nonlocal effects of the fractional Laplacian.

Tuesday, November 19, 2019

1:00 pm in 347 Altgeld Hall,Tuesday, November 19, 2019

#### Global Strichartz estimates for the semiperiodic Schrodinger equation.

###### Alex Barron (illinois Math)

Abstract: We will discuss some recent results related to space-time estimates for solutions to the linear Schrodinger equation on manifolds which are products of tori and Euclidean space (e.g. a cylinder embedded in R^3 ). On these manifolds it is possible to prove certain analogues of the classical Euclidean Strichartz estimates which are scale-invariant and global-in-time. These estimates are strong enough to prove small-data scattering for solutions to the critical quintic NLS on R × T and the critical cubic NLS on R^2 × T (where T is the one-dimensional torus).

Tuesday, December 3, 2019

1:00 pm in 347 Altgeld Hall,Tuesday, December 3, 2019

#### Steklov Spectral Asymptotics for Polygons

###### David Sher   [email] (DePaul Mathematics)

Abstract: We consider the Steklov eigenvalue problem on curvilinear polygons in the plane, with all interior angles measuring less than pi. In this setting, we formulate and prove precise spectral asymptotics, with error converging to zero as the spectral parameter increases. These asymptotics have a surprising dependence on arithmetic properties of the angles. Moreover, the problem turns out to have an interesting relationship to a scattering-type eigenvalue problem on the one-dimensional boundary of the polygon, viewed as a quantum graph.