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Series: Analysis Seminar

I will survey recent results about the convergence of the Wash-Fourier series near L1. Joint work with Michael Lacey.

Series: Analysis Seminar

Let C[-1, 1] be the space of continuous functions on [-1, 1], and denote by \Delta^2
the set of convex functions f \in C[-1, 1]. Also, let E_n(f) and En^{(2)}_n(f) denote
the degrees of best unconstrained and convex approximation of f \in \Delta^2 by
algebraic polynomials of degree < n, respectively.
Clearly, E_n(f) \le E^{(2)}_n (f), and Lorentz and Zeller proved that the
opposite inequality E^{(2)}_n(f) \le CE_n(f) is invalid even with the constant
C = C(f) which depends on the function f \in \Delta^2.
We prove, for every \alpha > 0 and function f \in \Delta^2, that
sup{n^\alpha E^{(2)}_n(f) : n \ge 1} \le c(\alpha)sup{n^\alpha E_n(f): n \ge 1},
where c(\alpha) is a constant depending only on \alpha.
Validity of similar results for the class of piecewise convex functions having
s convexity changes inside (-1,1) is also investigated. It turns out that there
are substantial differences between the cases s \le 1 and s \ge 2.

Series: Analysis Seminar

The Seiberg-Witten equations, introduced by Edward Witten in 1994, are a first-order semilinear geometric PDE that have led to manyimportant developments in low-dimensional topology. In this talk,we study these equations on cylindrical 4-manifolds with boundary, which we supplement with (Lagrangian) boundary conditions that have a natural Morse-Floer theoretic interpretation. These boundary conditions, however, are nonlinear and nonlocal, and so the resulting PDE is highlyunusual and nontrivial. After motivating and describing the underlying geometry for the Seiberg-Witten equations with Lagrangian boundary conditions, we discuss some of the intricate analysis involved in establishing elliptic regularity for these equations, including tools from the pseudodifferential analysis ofelliptic boundary value problems and nonlinear functional analysis.

Series: Analysis Seminar

When Calderón studied his commutators, in connection with the Cauchy
integral on Lipschitz curves, he ran into the bilinear Hilbert
transform by dropping an average in his first commutator. He posed the
question whether this new operator satisfied any L^p estimates. Lacey
and Thiele showed a wide range of L^p estimates in two papers from 1997
and 1999. By dropping two averages in the second Calderón commutator
one bumps into the trilinear Hilbert transform. Finding L^p estimates
for this operator is still an open question.
In my talk I will discuss L^p estimates for a singular integral
operator motivated by Calderón's second commutator by dropping one
average instead of two. I will motivate this operator from a historical
perspective and give some comments on potential applications to partial
differential equations motivated by recent results on the water wave
problem.

Series: Analysis Seminar

The quest for a suitable geometric description of major analyticproperties of sets has largely motivated the development of GeometricMeasure Theory in the XXth theory. In particular, the 1880 Painlev\'eproblem and the closely related conjecture of Vitushkin remained amongthe central open questions in the field. As it turns out, their higherdimensional versions come down to the famous conjecture of G. Davidrelating the boundedness of the Riesz transform and rectifiability. Upto date, it remains unresolved in all dimensions higher than 2.However, we have recently showed with A. Volberg that boundedness ofthe square function associated to the Riesz transform indeed impliesrectifiability of the underlying set. Hence, in particular,boundedness of the singular operators obtained via truncations of theRiesz kernel is sufficient for rectifiability. I will discuss thisresult, the major methods involved, and the connections with the G.David conjecture.

Series: Analysis Seminar

In this talk, we will discuss the theory of Hardy spacesassociated with a number of different multiparamter structures andboundedness of singular integral operators on such spaces. Thesemultiparameter structures include those arising from the Zygmunddilations, Marcinkiewcz multiplier. Duality and interpolation theoremsare also discussed. These are joint works with Y. Han, E. Sawyer.

Series: Analysis Seminar

We discuss a global weighted estimate for a class of divergence form elliptic operators with BMO coefficients on Reifenbergflat domains. Such an estimate implies new global regularity results in Morrey, Lorentz, and H\"older spaces for solutionsof certain nonlinear elliptic equations. Moreover, it can also be used to obtain a capacitary estimate to treat a measuredatum quasilinear Riccati type equations with nonstandard growth in the gradient.

Series: Analysis Seminar

We consider the influence of an incompressible drift on the expected exit time of a diffusing particle from a bounded domain. Mixing resulting from an incompressible drift typically enhances diffusion so one might think it always decreases the expected exit time. Nevertheless, we show that in two dimensions, the only simply connected domains for which the expected exit time is maximized by zero drift are the discs.

Series: Analysis Seminar

The Schur-Agler class is a subclass of the bounded analytic functions on the polydisk with close ties to operator theory. We shall describe our recent investigations into the properties of rational inner functions in this class. Non-minimality of transfer function realization, necessary and sufficient conditions for membership (in special cases), and low degree examples are among the topics we will discuss.

Series: Analysis Seminar

A separated sequence of real numbers is called a Polya sequence if the only entire functions of zero type which are bounded on this sequence are the constants. The Polya-Levinson problem asks for a description of all Polya sequences. In this talk, I will present some points of the recently obtained solution. The approach is based on the use of Toeplitz operators and de Branges spaces of entire functions. I will also present some partial results about the related Beurling gap problem.