Faginteresser:

Real Analysis
PDE
Quasiconformal and quasiregular mappings
Non-linear potential theory
Fluid mechanics
Sub-Riemannin geometry

Hovedresultater:

1) Based on the description of point multipliers given by Maz'ya and Shaposhnikova, we define analytic properties of a mapping F, inducing an isomorphism F* of spaces L of  differentiable functions acting according to the change of variable rule  F*f=f o F. As the space L,  we  considered the Sobolev space, the Besov space and the spaces of Bessel and Riesz potentials.

2) We obtained some results related to the nonlinear potential theory associated with solutions of the second order subelliptic equation
-div* A(x, Lu)=0.                    (1)
where  Lu =(X1u,..., Xku) is a subgradient defined by smooth vector fields (X1u,..., Xku) satisfying the Hörmander hypoellipticity condition, and the mapping A satisfies some additional conditions.

Regularity of supersolutions of equation (1), as well as mutual relations between capacity in Sobolev spaces and the geometry of vector fields, are studied. In addition, metric and analytic conditions are obtained for removable singularities of bounded solutions of general equations, in particular, of the equations of the form (1).  A-superharmonic functions possess the following basic properties: the comparison principle, the Harnack inequality for A-harmonic  functions, the convergence theorems for monotone sequences, etc. These facts allow us to develop a theory that simila to the linear potential theory. Particular attention is focused on the questions connected with the geometry of vector fields.

3) Due to  L. Ahlfors  and L. Sario  there is a classification of Riemannian surfaces depending on  properties of the set of harmonic functions on a Riemannian surface. Some years ago I.Holopainen  and S.Rickman  have shown the way of generalization the above-mentioned classification to the case of Riemannian manifolds of arbitrary dimension. We used their idea to obtain the classification of subriemannian manifolds (M,g,D) based on characteristics of solutions of equation (1). Here M is an n-dimensional, non-compact, connected, orientable smooth manifold, g is a Riemannian tensor and  D is a bracket generating  k-dimensional tangent subbundle. We showed that there are inclusions between the subriemannian manifolds in accordance with the existence of a Green function to equation (1), the existence of non-constant positive (bounded) solutions to (1) or the presence of a non-constant solution with bounded Dirichlet's integral. The Carnot group is an example of subriemannian manifolds. As a consequence of the classification we deduce that nonhomeomorphic quasiconformal mappings of Carnot groups of different dimensions are constant.

4) We consider a bounded domain on a Carnot group with two disjoint compact sets in the closure of this domain. For this geometrical configuration we proved that the p-module of a family of curves connecting the compacts coincides with the p-capacity of this condenser. Generalizing concepts of the p-capacity and the p-module of a family of curves, we consider a p-module of family of vector measures on a Carnot groups (in Euclidean space firstly introduced by B. Fuglede) and capacities associated with linear sub-elliptic equations. We proved continuity properties of p-module of vector measure, obtained some equalities and reciprocal relations between the above mentioned notions, that in particular case in Euclidean space reduce to classical relations, established by F. W. Gehring (p=n) and W. P. Zeimer (p<n, p>n).

5) We are interested also in studying quasiregular mappings (non-homeomorphic quasiconformal mapping) and their generalization on Carnot groups. Some estimates of the coefficient of distortion of quasiregular mapping, of a capacity of a condenser, and other  characteristic were obtained. We showed that a quasiregular mapping with the coefficient of distortion close to 1 is a local homeomorphism  under the condition that on the Carnot group any non-constant quasiregular mapping with the coefficient of distortion 1 is a global homeomorphism. We proved a lemma of type of Poletskii on an arbitrary Carnot group and gave some applications in the particular case of the Heisenberg group. The theory of quasiregular mappings (or in another terminology mapping with bounded distortion) in Euclidean space was founded and developed by Yu. G. Reshetnyak, O. Martio, S. Rickman.

6) We study the Hele-Shaw problem for the plane dynamics of incompressible fluids describing the two-dimensional flow in bounded or unbounded simply connected plane domains. We obtained the differential equation of Polubarinova-Galin type of free boundary of above-mentioned moving flows for the case of non-zero surface tension. The methods of the geometric function theory are applied to prove that the property of the free boundary to be starlike is preserved during the time of the existence of  solutions of the Polubarinova-Galin equation. We constructed some new explicit solutions describing finite and infinite bubbles in a two-dimensional corner flow.

7) We study the H-type homogeneous groups related to the division algebras. The parametric equations of sub-Riemannian geodesics constructed making use of Hamiltonian method. The Lagrangian formalism for quaternion and octonion H-type groups developed. The sub-Riemannian invariants such like complex action function and the volume element are found. The fundamental solutions to the Laplace and Heat equations are presented. We also studied the geodesics with additional constraint on velocity on the unit sphere in 4-dimentional Euclidean space and pseudo-hyperboloid in 4- dimensional space with pseudo-metric with signature (--++). 
 
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