In the recent preprint with A. Aptekarev and M. Yattselev, we found the missing link between the spectral theory of self-adjoint operators and the theory of multiple orthogonal polynomials. Recall that given a measure on the real line with compact support, we can construct a sequence of orthonormal polynomials which satisfy three-term recurrence that defines one-sided Jacobi matrix. This matrix is actually a self-adjoint operator in for which many standard quantities of operator theory (e.g., Green’s function, spectral measures, etc) can be computed through the measure we started with and the associated orthogonal polynomials. Conversely, given a bounded self-adjoint Jacobi matrix, we can uniquely find the measure of orthogonality that generates it. This correspondence proved to be very useful both in spectral theory and approximation theory (however, to my knowledge never played a crucial role in proving deeper analytical results). The multiple orthogonal polynomials (MOPs) can be of two types and they are defined given measures through some orthogonality conditions (see the paper for details). These polynomials depend on multi-index of dimension and satisfy recurrence relations on the integer lattice. It turns out that, to define the corresponding operator, one has to untwine these recurrences to the rooted homogeneous tree. The resulting operator is a self-adjoint Jacobi matrix which is defined on that tree. We obtained, among other things, the formulas connecting the Green’s function to MOPs and found its spatial asymptotics for analytic weights using matrix Riemann-Hilbert analysis. The usefulness of this connection is illustrated by, e.g., reproving some known results.

In the recent preprint “On the growth of the support of positive vorticity for 2D Euler equation in an infinite cylinder”, Kyudong Choi and I obtained an upper bound for the diameter of the support of positive vorticity in the 2D Euler dynamics

We considered the problem on the infinite cylinder which is equivalent to -periodic initial data in one direction. If one takes as nonnegative bounded function with compact support, then the weak solution exists globally in time. If denotes the diameter of its support, then the trivial bound reads

In the paper, we improve it to

Our argument is based on controlling the sequence of specially chosen moments on the dyadic spatial scale. The crucial part of the argument was to exploit the uniform in time estimate on the first moment of vorticity. For the problem on the whole plain, similar results were obtained previously in the paper by Iftimie, Sideris, and Gamblin “On the evolution of compactly supported planar vorticity”. Our method is different from the one used previously in that it uses another conserved quantity and different set of moments.

The 2D Euler evolution in infinite cylinder is remarkable model because the kernel in the Biot-Savart law that expresses velocity in terms of has the exponentially decaying first component. That means two distant parts of vorticity are essentially decoupled. One would hope that this, along with conservation of horizontal center of mass, should yield much stronger bound on , for example, . This, however, seems difficult to achieve due to possible “diffusive dynamics” of .

One possible interesting direction is to study the evolution of patch of vorticity in the active scalar equation when no conserved quantities are known but the kernel in the Biot-Savart law is short-range and has some basic symmetries. One would think that this should be enough to prove strong confinement results.

Roman Bessonov and I just posted the paper “A spectral Szego theorem on the real line” on arxiv. You can also read it here. Given a probability measure on the unit circle, one can ask when the analytic polynomials are NOT dense in . The theorem of Szego claims that this is so iff any of the following conditions holds:

- The sequence of recurrence parameters (or Schur parameters) of polynomials orthogonal with respect to belongs to .

Given a measure on the real line that satisfies normalization

,

we can ask the question when the set of functions

is NOT dense in . The answer is given by the theorem of Kolmogorov-Krein-Wiener: it is iff

However, the spectral characterization of this condition has been missing. In the paper, we consider the Krein string, – the “mother of all non-negative self-adjoint operators with simple spectrum”. It is given by the formal differential operator

where is any non-decreasing function on . The corresponding self-adjoint operator can be defined and its spectral measure along with one additional real parameter determines completely. In the paper, we characterize all strings for which the logarithmic integral of converges. This is done by proving analogous statement for diagonal De Branges canonical systems. The existence of the entropy is important for the prediction theory of stationary Gaussian processes with continuous time. It is likely that the obtained characterization will allow one to quantify some statements in this theory.

In the recent preprint, I study the wave equation for the elliptic operator in divergence form. In , define

where oscillates and decays at infinity. More precisely, where and the norm is defined as

I also assume to make sure that is non-negative operator. The wave equation for is

The main result of the paper states that the following wave operators

exist for every and the limit is understood in norm. The condition on is optimal in some sense, i.e., the rate of decay is sharp and the oscillation is necessary if the potential is not short-range.

The proof is based on the analysis of the asymptotical behavior of the Green’s function where , is fixed, and . The result about asymptotics is similar to that for the orthogonal polynomials on the circle in the Szego case. The main difference with the one-dimensional situation is that the resulting “Szego” function belongs to the vector-valued Hardy space. The operator can be written through the resolvent by the contour integral and this is how the Green’s function enters the proof. The method is quite general and can be adapted to wave equations for the Schrodinger equation and other problems.

In the recent preprint with Jen Beichman we considered the 2D Euler evolution on the tube . Each rectangle is a steady state. We proved that if is sufficiently large, then these steady states are stable for all time. For example, if one takes a patch such that is small, then the Euler evolution of this patch will have small for arbitrary . This result generalizes analogous statement for the stability of the disc on the plane (proved by Sideris-Vega). As Sideris and Vega, we used the method of V. Arnold. The idea of this method is to study the variational problem associated to the conserved quantities. In our case, these are

where is a stream function given by Then, we set up a variational problem with constraint

We proved that the global minimizer is and if is close to the minimum value for some patch , then is close to in a weak topology. This essentially gives the required stability.

**March 3, 2014. **

If one considers the 2d Euler equation of incompressible inviscid fluids on the plane in the vorticity form and takes the initial data as the characteristic function of a certain domain, then the Yudovich theory guarantees that the solution will exist globally and will be equal to the characteristic function of a time-dependent domain which is homeomorphic to the original one for all times. The numerical experiments dating back to the works of P. Saffman and Zabusky et al. indicate the existence of the centrally-symmetric V-states, i.e. a symmetric pair of patches that rotates with constant angular velocity around the origin without changing shape. If the distance between the patches in the pair equals to and , then the boundary of the V-state seems to be smooth. However, when , the both patches form a 90 degrees angle at the point of contact. The analytical proof for the existence of these V-states has never been obtained and this is an interesting problem. In the recent preprint, I considered the analogous equation with the cut-off. Loosely speaking, this corresponds to looking at the window around the origin where the contact of the patches is supposed to happen. Mathematically, the model with cut-off is important as it possesses the explicit singular solution: . Then, I addressed the problem of existence of the curve of smooth solutions that converge to in the uniform metric when the parameter . Technically, this boils down to application of the implicit function theorem and is somewhat tedious. This technique might be important to better understand the mechanism of the merging and the sharp corner formation in the Euler dynamics. Another important problem is to prove that the merging in finite time is possible for the -model when and is close to .

**May 2011.** Consider . Going on the Fourier side one can see that is equivalent to multiplication by and so the spectrum of is purely absolutely continuous (a.c.). From the physics perspective, the presence of a.c. spectrum is an indication that the wave propagation governed by

does have a transport effect (though without much specifics).

Perturb the Laplacian as follows where is potential and ask the question what is a minimal assumptions on to guarantee that the a.c. spectrum is preserved. Make it a perturbation theory question, assume that is in some weighted Lebesgue space . Then, what are the critical and ? In one-dimensional case, one answer is . This is a critical space. This result was proved for Schrodinger in a great paper by Deift and Killip but for Dirac it was known for at least half a century and dates back to the works by Mark Krein. Krein’s result on Dirac, however, is only a continuous analog of the classical results for polynomials orthogonal on the unit circle (Szego case).

For , the conjecture by Barry Simon is that

is sufficient for the preservation of a.c. spectrum. Very little is known so far. Only the case of Schrodinger on the Cayley tree is well-understood. Take a rooted Cayley tree with the origin at and assume that each vertex has exactly three neighbors while has only two. Consider the Laplacian on defined at each point as the sum over the neighbors and then a simple calculation shows that the spectrum of is and it is purely a.c. Then, perturb by . The multidimensional result reads as follows. Consider all paths that go from to infinity without self-intersections (rays). Put the probability measure on them by tossing a Bernoulli coin at each vertex. Then, the claim is that the a.c. spectrum contains the a.c. spectrum of unperturbed operator if with positive probability the potential where denotes the path from the origin. There is more quantitative version, of course, which implies Simon’s conjecture if the Jensen inequality is applied. The condition we have here is more general and more physically appealing: it says that we only need enough directions were the potential is small for the particle to propagate.

For , sparse or slowly decaying and oscillating potentials can be handled. If the potential does not oscillate, then the scattering process is quite complicated, it is governed by very intricate evolution equation (that captures semiclassical WKB correction as very special case). This evolution equation is poorly studied and much work is needed in this direction. Soft one-dimensional methods seem to be of little help.

In Euclidean case, what would be the analog of the probability space on the set of paths that escape to infinity? This question was addressed here. It turns out that there is a natural Ito’s stochastic equation that describes these paths. The statement we have is somewhat weak though. It says that the a.c. spectrum contains the positive half-line if with positive probability and we can not yet replace summability by the square summability over the path . Nevertheless, even this result gives rise to interesting questions like how one computes probabilities given by this Ito’s calculus? That conventionally can be reduced to the analysis of the corresponding potential theory and the modified harmonic measure. The potential theory one encounters in this case is somewhat in between elliptic and the parabolic one: on the large scale it is parabolic and on the small scale it is elliptic. The estimates on the harmonic measure in terms of the geometric properties of support can be found in my paper with Kupin.

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**May 2012.** I recently finished writing a survey for the Nikolskii conference volume, it contains more details.

**January 2012.** This problem is motivated by the multidimensional conjecture posed by Barry Simon. It deserves a post on its own but the crux of the matter is this. Take a model case:

where and the parameter is real as well as the potential . We define the Cauchy problem by taking . Then, assuming is sufficiently regular, we can define the unique solution. Its norm is preserved in time. However, how fast do the Sobolev norms can grow? Even for fixed , this is an interesting problem but we want to know what happens for Lebesgue generic . My conjecture is that for the Sobolev norm is bounded in time. This is true for a.e. . The partial results were recently obtained here but more analysis is needed. For example, for the transport equation the solution can be explicitly written and the Carleson theorem on the Fourier series yields the sharp result. Here is Harmonic analysis in action. For the general case the combination of soft-analysis methods and the Christ-Kiselev type perturbation arguments is needed. The quantity most relevant to multidimensional case is

One wants to control it by norm of . The small values of correspond to the small gaps in the spectrum of the diagonal operator and this regime is the most difficult for analysis.

The difficulty of the problem is this– if one takes complex-valued then the norm of the solution is not preserved in time anymore. So, whatever analytical machinery one applies, it must be sensitive enough to account for that fact. A good classical example is function . Its Taylor expansion is hardly useful for the analysis of this function for large real .

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**June 2013.** I recently posted the new preprint on my webpage in which, in particular, the following statement is proved:

Consider in which is only bounded in both and . Then, for a.e. the solution has Sobolev norm , uniformly bounded for all .

This result is based on a recent generalization of Carleson theorem on Fourier maximal function (through the variational norm). The statement is rather striking as it gives more regularity for the solution than one can guess as long as the generic coupling constant is taken.

**January 2012.** Let denote the unit circle and let be a probability measure on . Take the sequence and orthonormalize in the Hilbert space to produce . We have

by definition. The question is what is the size of if and we assume some additional information on the measure ? This is a classical question in approximation theory. The Steklov’s conjecture was: assume that is purely a.c. and has the weight uniformly bounded away from zero, i.e.

Is it true that uniformly in and ? The negative answer to this question was given by Rakhmanov. He proved that the possible growth is essentially up to an inverse logarithmic factor. Can one drop it? This is a nice problem.** !!! SOLVED, see below.** Another important class of measures extensively studied in the literature is the so-called Szego class. It is defined as follows: where the singular component is arbitrary and for one has

How large the polynomials can be in this case? Well, for a while the conjecture was that the polynomial entropy is bounded, i.e.

uniformly in . That, however, again tuned out to be wrong with the possible growth like . This result is sharp. Then, what restriction on the size of do we get from the orthogonality? Not clear to me. _______________________________________________________________________ **August 2013.** In the joint paper with A. Aptekarev and D. Tulyakov which you can find on my webpage, we proved the sharp estimates for the uniform norms of the orthogonal polynomials in the Steklov class. This class is defined by the condition that

for a.e. point on the circle where is some small positive constant. The world record so far was made by Rakhmanov about 30 years ago. The method we used turned out to be powerful enough to also give the sharp bounds for the polynomial entropy in this class. We hope to iterate the construction to prove the lower bounds over the subsequence. If we succeed, this will give the full solution to the famous Steklov’s conjecture. ______________________________________________________________________________ ** February 2014.** Building on the previous paper (discussed above), we were able to settle a problem by Steklov. That involved a lot of technical work. Here is the main result: suppose is a probability measure on the circle which satisfies

and . That condition defines the Steklov’s class. If are the corresponding orthonormal polynomials, then the following upper bound is well-known and easy to prove:

In the paper, we prove **Theorem.** For every sequence , there is an absolutely continuous probability measure from the Steklov class such that

for some sequence . The proof is constructive and can be used when the measure satisfies different lower bounds. There are other interesting questions that naturally follow from the paper, some of them do not seem to be so hard anymore. The project on getting the uniform bound on the polynomials is mainly finished. What is left wide open is the problem on controlling the for a.e. provided the constructive information on measure is known. This is the so-called nonlinear Luzin’s conjecture and we know very little about it. ____________________________________________________________ **November 6, 2014 ** Fix , large and such that . Consider the class of probability measures defined as where , are arbitrary points on , and . Let . In the recent paper, I proved the following estimate It implies, for example, that the maximizer in the Steklov problem has point masses. In the same paper, I also proved the following inequality improving the bound by Kos: if and , then . That was done by using the duality between two extremal problems for one of which the bound is easy to obtain if the generalization of the Halasz’ result is used. ___________________________________________________________________________________________________________________ **May 19, 2015** The solution to a problem by Steklov discussed above allows to study other variational problems. For example, Then, the old idea by S.Bernstein and our method used for getting the lower bounds can be combined to get when is fixed and . In the case when the deviation of the weight is large, we have two results: 1. For every large , there is such that for every weight . 2. For every small , there is large and a weight such that . One can see that the growth can not be achieved by weights uniformly bounded in .

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**July 2015. ** We significantly simplified several proofs in the paper “On a problem by Steklov” and submitted a new version to arXiv. We also fixed some minor mistakes and typos.

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**September 2015. **In his original paper on the Steklov’s conjecture, Rakhmanov used a formula for orthogonal polynomial obtained by adding mass points to the measure of orthogonality. Although its uniform norm can grow not faster than (where is a degree), it is interesting to deduce this polynomial by method used for getting the sharp lower bound in the problem by Steklov. This was carried out in my recent preprint. The interesting feature is a different kind of cancellation. It is less effective than the one giving the bound but it still might be interesting.

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**November 2015. **The method used to solve the problem of Steklov proves the possibility of growth of polynomials with weights that satisfy and with arbitrary . If then the upper bound gives an exponent smaller than : . In a joint project with my student Keith Rush, we proved that is sufficient to have . The proof gives more that that. Under these assumptions, we can show that the polynomial entropies are uniformly bounded. In terms of the regularity of weight, this results is rather sharp since the entropies are known to grow logarithmically for with any .

**January 2012. ** The 2D Euler equation can be written in the following form

where is — periodic in both and and it denotes the vorticity of the velocity field. The symbol stands for . To define the Cauchy problem, one needs to specify

This is a transport equation with the divergence-free vector field and so all norms of are preserved in time. The global regularity is known for many functional spaces, e.g. or with large will do. The quantitative version is a bound

Is this estimate sharp? I do not know but one can have double exponential growth for arbitrarily large but finite time. The lower bound valid for all time is only superlinear.

Another interesting problem is dynamics of patches. Assume that is the characteristic function of a domain and the problem is considered on the whole plane, not on the 2d torus. Then, and the boundary of is smooth provided that the boundary of was smooth. However, one can study the growth of curvature and the rate of merging (if there are several patches).

The merging mechanism was studied recently and the INFINITE double exponential rate is possible at least if the regular strain is present. This result is sharp.

If one modifies equation for vorticity by writing instead of , then the resulting equation is called SQG (surface quasi-geostrophic). The outstanding conjecture in the field is that the singularity forms in finite time. No proof so far.

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**May 2013.** I have finished one more revision of the paper on the corner formation and you can check either my webpage or arxiv. The estimates are now sharp and the regularity of the strain is as optimal as the method can give. I think I approach the stage when the paper can finally be read by the students.

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**October 2013.** In the recent preprint, Kiselev and Sverak considered the 2d Euler equation on the disc and proved the infinite in time double-exponential growth of the Lipschitz norm for the vorticity. The singularity forms on the boundary and happens as two identical vortices of different sign slide along the boundary towards each other. This interesting result might indicate that singularity formation is generic when the equation is considered on the domain with the boundary. The initial data is smooth but is not zero near the boundary (this is important for the construction). So, if one writes the equation on the Fourier side, the decay of

in is very weak. The stable mechanism for the singularity formation in the bulk is still missing.

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**October 2013.** Another interesting result obtained for 2d Euler. My colleague Andrej Zlatos proved the possibility of infinite exponential growth of the Hessian of vorticity for smooth initial data. This was done for the 2d torus and no boundary effects were used. He also showed that the Lipschitz norm can grow exponentially for initial data. In this work the hyperbolic scenario is exploited. I think the interesting question is the possibility of the exponential growth of the Lipschitz norm for smooth initial data (e.g., trig polynomial). So far we have only superlinear estimate. Even better problem is to show that the sharp front can form with width decaying exponentially. This would prove that the results of Fefferman and D. Cordoba on the fronts are essentially sharp. There are plausible mechanisms but the justification for Euler equation is hard.