APS
Speaker: Soumyajit Acharyya, IIT Bombay
Host: Chandan Biswas
Title: Besicovitch Compression and a Quantitative Failure of Endpoint Fourier Restriction
Time, day and date: 10:00:00 AM – 11:00:00 AM, Friday, September 25
Venue: Online (https://us06web.zoom.us/j/82255856730?pwd=55smRtn0conmA2vRyLwGSKmNQxVEr4.1)
Meeting ID: 822 5585 6730
Passcode: 116874
Abstract: A classical theorem of Beckner, Carbery, Semmes, and Soria states that the Fourier restriction operator for the sphere does not satisfy the restricted weak-type estimate at the conjectured endpoint. Their proof uses the Besicovitch compression phenomenon and proceeds by contradiction. As a result, it reveals little about how badly the estimate fails. We develop a general framework that converts Besicovitch compression into a quantitative failure at an endpoint. Given a family of sets that compresses, our framework produces an explicit set that violates the bound, with a rate governed by the amount of compression. The rate is sharp within the admissible family of sets. We apply this to the sphere and the paraboloid and construct a deterministic sequence of sets along which the restricted weak-type inequality degrades at an explicitly computed rate. We will also discuss why our construction does not provide blowup for the moment curve in dimensions three and higher, where the (much stronger) restricted strong-type bound is known to hold at the endpoint.

APS
Speaker: Surajit Pal, IIT Bombay
Host: Debanjana Mitra
Title: Characterization of the Reachable Space for Coupled Parabolic System with Boundary Controls.
Time, day and date: 2:30:00 PM – 3:30:00 PM, Friday, September 25
Venue: Ramanujan Hall
Abstract: We are interested in the characterization of the reachable space of a coupled parabolic system with boundary controls. For uniform diffusion coefficients, we have shown that the reachable space is a certain Bergman Space. Further, using this result, we have derived local reachability results for a coupled nonlinear system. For distinct diffusion coefficients, using Fourier series technique, we have obtained a partial characterization of the reachable space for a coupled parabolic linear system.