arXiv subjects

Dmitri Zaitsev

Publications and source records attributed to Dmitri Zaitsev.

At least 19 recordsLinked to original sources

arXiv2025

Sphericity and Analyticity of a strictly pdeusoconvex hypersurface in low regularity I

In our earlier work \cite{KZ}, we introduced an analytic regularizability theory for smooth strictly pseudoconvex hypersurfaces in complex space. That is, we found a necessary and sufficient condition for a hypersurface to be CR-equivalent to an analytic target. The condition amount to the holomorphic extension property for a smooth function on a totally real submanifold, both the function and the submanifold being uniquely associated with the given hypersurface. In the present paper, we develop our method further. First, we extend the result in \cite{KZ} to hypersurfaces of finite (possibly low) smoothness. Second, we introduce a new tool for studying CR hypersurfaces in low regularity called {\em regularizing $(0,1)$ sections}. Using the latter key tool, we solve the open problem of checking the {\em sphericity} of a strictly pseudoconvex hypersurface in $\mathbb C^{2}$ in {low regularity}, precisely in the regularity $C^k,\,2\leq k< 7$ which is {\em not} covered by the classical Cartan-Tanaka-Chern-Moser theory. As an application of our theory, we deduce the sphericity of a strictly pseudoconvex hypersurface in $\mathbb C^{2}$ of regularity $C^6$ with vanishing Cartan-Chern CR-curvature.

math.CV↗

arXiv2024

Tower multitype and global regularity of the $\bar\partial$-Neumann operator

A new approach is given to property $(P_q)$ defined by Catlin for $q=1$ in a global and by Sibony in a local context, subsequently extended by Fu-Straube for $q>1$. This property is known to imply compactness and global regularity in the $\bar\partial$-Neumann problem by a result of Kohn-Nirenberg, as well as condition $R$ by a result of Bell-Ligocka. In particular, we provide a self-contained proof of property $(P_q)$ for pseudoconvex hypersurfaces of finite D'Angelo $q$-type, the case originally studied by Catlin. Moreover, our proof covers more general classes of hypersurfaces inspired by a recent work of Huang-Yin. Proofs are broken down into isolated steps, some of which do not require pseudoconvexity. Our tools include: a new multitype invariant based on distinguished nested sequences of $(1,0)$ subbundles, defined in terms of derivatives of the Levi form; real and complex formal orbits; $k$-jets of functions relative to pairs of formal submanifolds; relative contact orders generalizing the usual contact orders; a new notion of supertangent vector fields having higher than expected relative contact orders; and a formal variant of a result by Diederich-Fornæss arising as a key step in their proof of Kohn's ideal termination in the real-analytic case.

math.CV↗

arXiv2022

Formally-Reversible Maps of C^2

An element $g$ of a group is called {\em reversible} if it is conjugate in the group to its inverse. This paper is about reversibles in the group $G$ of formally-invertible pairs of formal power series in two variables, with complex coefficients. The main result is a description of the generic reversible elements of $G$. We list two explicit sequences of reversibles which between them represent all the conjugacy classes of such reversibles. We show that each such element is reversible by some element of finite order, and hence is the product of two elements of finite even order. Those elements that may be reversed by an involution are called {\em strongly reversible}. We also characterise these. We draw some conclusions about generic reversibles in the group of biholomorphic germs in two variables, and about the factorization of formal maps as products of reversibles. Specifically, each product of reversibles reduces to the product of five.

math.CV↗

arXiv2021

Q-effectiveness for holomorphic subelliptic multipliers

We provide a solution to the effectiveness problem in Kohn's algorithm for generating holomorphic subelliptic multipliers for $(0,q)$ forms for arbitrary $q$. As an application, we obtain subelliptic estimates for $(0,q)$ forms with effectively controlled order $ε>0$ (the Sobolev exponent) for domains given by sums of squares of holomorphic functions (J.J. Kohn called them "special domains"). These domains are of particular interest due to their relation with complex and algebraic geometry. Our methods include triangular resolutions introduced by the authors in their previous work.

math.CV↗

arXiv2020

Unique jet determination and extension of germs of CR maps into spheres

We provide a new way of simultaneously parametrizing arbitrary local CR maps from real-analytic generic manifolds $M\subset {\mathbb C}^N$ into spheres ${\mathbb S}^{2N'-1}\subset {\mathbb C}^{N'}$ of any dimension. The parametrization is obtained as a composition of universal rational maps with a holomorphic map depending only on $M$. As applications, we obtain rigidity results of different flavours such as unique jet determination and global extension of local CR maps.

math.CV↗

arXiv2020

Triangular resolutions and effectiveness for holomorphic subelliptic multipliers

A solution to the effectiveness problem in Kohn's algorithm for generating subelliptic multipliers is provided for domains that include those given by sums of squares of holomorphic functions (also including infinite sums). These domains are of particular interest due to their relation with complex and algebraic geometry and in particular, seem to include all previously known cases. Furthermore, combined with a recent result of M. Fassina, our effectiveness method allows establishing effective subelliptic estimates for more general classes of domains. Our main new tool, a triangular resolution, is the construction of subelliptic multipliers decomposable as $Q\circΓ$, where $Γ$ is constructed from pre-multipliers and $Q$ is part of a triangular system. The effectiveness is proved via a sequence of newly proposed procedures, called here meta-procedures, built on top of the Kohn's procedures, where the order of subellipticity can be effectively tracked. Important sources of inspiration are algebraic-geometric techniques by Y.-T. Siu and procedures for triangular systems by D.W. Catlin and J.P. D'Angelo. The proposed procedures are purely algebraic and as such can also be of interest for geometric and computational problems involving Jacobian determinants, such as resolving singularities of holomorphic maps.

math.CV↗

arXiv2019

Real-analytic coordinates for smooth strictly pseudoconvex CR-structures

For a smooth strictly pseudoconvex hypersurface in a complex manifold, we give a necessary and sufficient condition for being CR-diffeomorphic to a real-analytic CR manifold. Our condition amounts to a holomorphic extension property for the canonically associated function expressing $2$-jets of the formal Segre varieties in terms of their $1$-jets. We also express this condition in equivalent terms for a Fefferman type determinant

math.CV↗

arXiv2018

Jet vanishing orders and effectivity of Kohn's algorithm in dimension $3$

We propose a new class of geometric invariants called jet vanishing orders, and use them to establish a new selection algorithm in the Kohn's construction of subelliptic multipliers for special domains in dimension $3$, inspired by the work of Y.-T. Siu [S10]. In particular, we obtain effective termination of our selection algorithm with explicit bounds both for the steps of the algorithm and the order of subellipticity in the corresponding subelliptic estimates. Our procedure possesses additional features of certain stability under high order perturbations, due to deferring the step of taking radicals to the very end of the algorithm. We further illustrate by examples the sharpness in our technical results and demonstrate the complete procedure for arbitrary high order perturbations of the Catlin-D'Angelo example [CD10] in Section 5. Our techniques here may be of broader interest for more general PDE systems, in the light of the recent program initiated by the breakthrough paper of Y.-T. Siu [S17].

math.CV↗

arXiv2018

Sums of squares in pseudoconvex hypersurfaces and torsion phenomena for Catlin's boundary systems

Given a pseudoconvex hypersurface in C^n and an arbitrary weight, we show the existence of local coordinates in which the polynomial model contains a particularly simple sum of squares of monomials. Our second main result provides a normalization of a part of any Catlin boundary system. We illustrate by an example that this normalization cannot be extended to the rest of the boundary system due to the existence of what we refer to as torsion.

math.CV↗

arXiv2018

A geometric approach to Catlin's boundary systems

For a point $p$ in a smooth real hypersurface $M\subset\C^n$, where the Levi form has the nontrivial kernel $K^{10}_p$, we introduce an invariant cubic tensor $τ^3_p \colon \C T_p \times K^{10}_p \times \overline{K^{10}_p} \to \C\otimes (T_p/H_p)$, which together with Ebenfelt's tensor $ψ_3$, constitutes the full set of $3$rd order invariants of $M$ at $p$. Next, in addition, assume $M\subset\C^n$ to be {\em (weakly) pseudoconvex}. Then $τ^3_p$ must identically vanish. In this case we further define an invariant quartic tensor $τ^4_p \colon \C T_p \times \C T_p \times K^{10}_p\times \overline{K^{10}_p} \to \C\otimes (T_p/H_p)$, and for every $q=0, \ldots, n-1$, an invariant submodule sheaf of $(1,0)$ vector fields in terms of the Levi form, and an invariant ideal sheaf of complex functions generated by certain derivatives of the Levi form, such that the set of points of Levi rank $q$ is locally contained in certain real submanifolds defined by real parts of the functions in the ideal sheaf, whose tangent spaces have explicit algebraic description in terms of the quartic tensor $τ^4$. Finally, we relate the introduced invariants with D'Angelo's finite type, Catlin's mutlitype and Catlin's boundary systems.

math.CV↗

arXiv2017

Normal forms for almost non-integrable CR structures

We propose two constructions extending the Chern-Moser normal form to non-integrable Levi-nondegenerate (hypersurface type) almost CR structures. One of them translates the Chern-Moser normalization into pure intrinsic setting, whereas the other directly extends the (extrinsic) Chern-Moser normal form by allowing non-CR embeddings that are in some sense "maximally CR". One of the main differences with the classical integrable case is the presence of the non-integrability tensor at the same order as the Levi form, making impossible a good quadric approximation - a key tool in the Chern-Moser theory. Partial normal forms are obtained for general almost CR structures of any CR codimension, in particular, for almost-complex structures. Applications are given to the equivalence problem and the Lie group structure of the group of all CR-diffeomorphisms.

math.CV↗

arXiv2017

Normal form for second order differential equations

We solve the local equivalence problem for second order (smooth or analytic) ordinary differential equations. We do so by presenting a {\em complete convergent normal form} for this class of ODEs. The normal form is optimal in the sense that it is defined up to the automorphism group of the model (flat) ODE $y"=0$. For a generic ODE, we also provide a unique normal form. By doing so, we give a solution to a problem which remained unsolved since the work of Arnold. The method can be immediately applied to important classes of second order ODEs, in particular, the Painlevé equations. As another application of the convergent normal form, we discover distinguished curves associated with a differential equation that we call {\em chains}.

math.DS↗

arXiv2016

A family of compact strictly pseudoconvex hypersurfaces in $\mathbb C^2$ without umbilical points

We prove the following: For $ε>0$, let $D_ε$ be the bounded strictly pseudoconvex domain in $\mathbb C^2$ given by \begin{equation*} (\log|z|)^2+(\log|w|)^2<ε^2. \end{equation*} The boundary $M_ε:=\partial D_ε\subset \mathbb C^2$ is a compact strictly pseudoconvex CR manifold without umbilical points. This resolves a long-standing question in complex analysis that goes back to the work of S.-S. Chern and J. K. Moser in 1974.

math.CV↗

arXiv2016

Normal forms in Cauchy-Riemann Geometry: a survey

One of effective ways to solve the equivalence problem and describe moduli spaces for real submanifolds in complex space is the normal form approach. In this survey, we outline some normal form constructions in CR-geometry and formulate a number of open problems.

math.CV↗

arXiv2015

A normal form for 1-infinite type hypersurfaces in $\mathbb C^2$. I. Formal Theory

In this paper, we study the real hypersurfaces $M$ in $\mathbb C^2$ at points $p\in M$ of infinite type. The degeneracy of $M$ at $p$ is assumed to be the least possible, namely such that the Levi form vanishes to first order in the CR transversal direction. A new phenomenon, compared to known normal forms in other cases, is the presence of resonances as roots of an universal polynomial in the $7$-jet of the defining function of $M$. The main result is a complete (formal) normal form at points $p$ with no resonances. Remarkably, our normal form at such infinite type points resembles closely the Chern-Moser normal form at Levi-nondegenerate points. For a fixed hypersurface, its normal forms are parametrized by $S^1\times \mathbb R^*$, and as a corollary we find that the automorphisms in the stability group of $M$ at $p$ without resonances are determined by their $1$-jets at $p$. In the last section, as a contrast, we also give examples of hypersurfaces with arbitrarily high resonances that possess families of distinct automorphisms whose jets agree up to the resonant order.

math.CV↗

arXiv2015

A Compact Expression for Periodic Instantons

Instantons on various spaces can be constructed via a generalization of the Fourier transform called the ADHM-Nahm transform. An explicit use of this construction, however, involves rather tedious calculations. Here we derive a simple formula for instantons on a space with one periodic direction. It simplifies the ADHM-Nahm machinery and can be generalized to other spaces.

math.DG↗

arXiv2015

Convergent normal form and canonical connection for hypersurfaces of finite type in $\mathbb C^2$

We study the holomorphic equivalence problem for finite type hypersurfaces in $\mathbb C^2$. We discover a geometric condition, which is sufficient for the existence of a natural convergent normal form for a finite type hypersurface. We also provide an explicit construction of such a normal form. As an application, we construct a canonical connection for a large class of finite type hypersurfaces. To the best of our knowledge, this gives the first construction of an invariant connection for Levi-degenerate hypersurfaces in $\mathbb C^2$.

math.CV↗

arXiv2014

Convergent normal form for real hypersurfaces at generic Levi degeneracy

We construct a complete convergent normal form for a real hypersurface in $\CC{N},\,N\geq 2$ at generic Levi degeneracy. This seems to be the first convergent normal form for a Levi-degenerate hypersurface. In particular, we obtain, in the spirit of the work of Chern and Moser \cite{chern}, distinguished curves in the Levi degeneracy set, that we call \it degenerate chains.

math.CV↗