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Natalia Kopteva

Publications and source records attributed to Natalia Kopteva.

29 records · Page 2Linked to original sources
arXiv2018

Error analysis of the L1 method on graded and uniform meshes for a fractional-derivative problem in two and three dimensions

An initial-boundary value problem with a Caputo time derivative of fractional order $α\in(0,1)$ is considered, solutions of which typically exhibit a singular behaviour at an initial time. For this problem, we give a simple framework for the analysis of the error of L1-type discretizations on graded and uniform temporal meshes in the $L_\infty$ and $L_2$ norms. This framework is employed in the analysis of both finite difference and finite element spatial discretiztions. Our theoretical findings are illustrated by numerical experiments.

math.NA↗
arXiv2017

Fully computable a posteriori error estimator using anisotropic flux equilibration on anisotropic meshes

Fully computable a posteriori error estimates in the energy norm are given for singularly perturbed semilinear reaction-diffusion equations posed in polygonal domains. Linear finite elements are considered on anisotropic triangulations. To deal with the latter, we employ anisotropic quadrature and explicit anisotropic flux reconstruction. Prior to the flux equilibration, divergence-free corrections are introduced for pairs of anisotropic triangles sharing a short edge. We also give an upper bound for the resulting estimator, in which the error constants are independent of the diameters and the aspect ratios of mesh elements, and of the small perturbation parameter.

math.NA↗
arXiv2013

Perturbed asymptotic expansions for interior-layer solutions of a semilinear reaction-diffusion problem with small diffusion

A semilinear reaction-diffusion two-point boundary value problem, whose second-order derivative is multiplied by a small positive parameter $\eps^2$, is considered. It can have multiple solutions. An asymptotic expansion is constructed for a solution that has an interior layer. Further properties are then established for a perturbation of this expansion. These are used in\cite{KoStMain} to obtain discrete sub-solutions and super-solutions for certain finite difference methods described there, and in this way yield convergence results for those methods.

math.NA↗
arXiv2011

On the sharpness of Green's function estimates for a convection-diffusion problem

Linear singularly perturbed convection-diffusion problems with characteristic layers are considered in three dimensions. We demonstrate the sharpness of our recently obtained upper bounds for the associated Green's function and its derivatives in the $L_1$ norm. For this, in this paper we establish the corresponding lower bounds. Both upper and lower bounds explicitly show any dependence on the singular perturbation parameter.

math.NA↗
arXiv2009

Some asymptotic expansions for a semilinear reaction-diffusion problem in a sector

A semilinear singularly perturbed reaction-diffusion equation with Dirichlet boundary conditions is considered in a convex unbounded sector. The singular perturbation parameter is arbitrarily small, and the "reduced equation" may have multiple solutions. A formal asymptotic expansion for a possible solution is constructed that involves boundary and corner layer functions. For this asymptotic expansion, we establish certain inequalities that are used in a subsequent paper to construct sharp sub- and super-solutions and then establish the existence of a solution to a similar nonlinear elliptic problem in a convex polygon.

math.AP↗
arXiv2007

The Tits alternative for non-spherical Pride groups

Pride groups, or ``groups given by presentations in which each defining relator involves at most two types of generators'', include Coxeter groups, Artin groups, triangles of groups, and Vinberg's groups defined by periodic paired relations. We show that every non-spherical Pride group that is not a triangle of groups satisfies the Tits alternative.

math.GR↗
arXiv2006

Two-generator Kleinian orbifolds

We give a complete list of orbifolds uniformised by discrete non-elementary two-generator subgroups of PSL(2,C) without invariant plane whose generators and their commutator have real traces.

math.GT↗
arXiv2005

A two-dimensional slice through the parameter space of two-generator Kleinian groups

We describe all real points of the parameter space of two-generator Kleinian groups with a parabolic generator, that is, we describe a certain two-dimensional slice through this space. In order to do this we gather together known discreteness criteria for two-generator groups and present them in the form of conditions on parameters. We complete the description by giving discreteness criteria for groups generated by a parabolic and a $π$-loxodromic elements whose commutator has real trace and present all orbifolds uniformized by such groups.

math.GR↗
arXiv2005

Blaschke addition and convex polyhedra

This is an extended version of a talk on October 4, 2004 at the research seminar ``Differential geometry and applications'' (headed by Academician A. T. Fomenko) at Moscow State University. The paper contains an overview of available (but far from well-known) results about the Blaschke addition of convex bodies, some new theorems on the monotonicity of the volume of convex bodies (in particular, convex polyhedra with parallel faces) as well as description of a software for visualization of polyhedra with prescribed outward normals and face areas.

math.MG↗