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## QTT-Finite-Element Approximation For Multiscale Problems

### by V. Kazeev and I. Oseledets and M. Rakhuba and Ch. Schwab

(Report number 2016-06)

**Abstract**

Tensor-compressed numerical solution
of elliptic multiscale-diffusion and high frequency scattering problems is considered.
For either problem class, solutions exhibit multiple length scales
governed by the corresponding scale parameter:
the scale of oscillations of the diffusion coefficient
or smallest wavelength, respectively.
As is well-known, this imposes a scale-resolution requirement on
the number of degrees of freedom required to accurately
represent the solutions in standard finite-element (FE) discretizations.
Low-order FE methods are by now generally perceived unsuitable for
high-frequency diffusion coefficients and high wavenumbers,
and special techniques have been proposed instead
(such as numerical homogenization, heterogeneous multiscale method, oversampling, etc.).
They require, in some form, a-priori information on the microstructure of the solution.
We propose the use of tensor-structured compressed
first-order FE methods for scale resolution without a-priori information.
The FE methods are based on principal components
dynamically extracted from the FE solution by non-linear,
quantized tensor train (QTT) decomposition
of the system matrix, load and solution vectors.
For prototypical model problems, we prove that this approach
identifies effective degrees of freedom
from a uniform ``virtual'' (i.e. never directly accessed) mesh and its
corresponding degrees of freedom (whose number may be prohibitively large).
Precisely,
solutions of model elliptic homogenization and high frequency acoustic scattering problems
are proved to admit QTT-formatted approximations whose number of effective
degrees of freedom is robust in the scale parameter
and polylogarithmic with respect to the reciprocal of the
target Sobolev-norm accuracy.
No a-priori information on the nature of the problems and
intrinsic length scales of the solution is required
in the proposed approach.
As a corollary of our analysis,
we prove that the Kolmogorov \(n\)-widths of solutions sets are
exponentially small for analytic data, independent of the problems'
scale parameters. That implies, in particular,
robust exponential convergence of reduced basis and MOR techniques.
Detailed numerical experiments confirm the theoretical bounds.

**Keywords:** multiscale problems, Helmholtz equation, homogenization,
scale resolution, exponential convergence, tensor decompositions,
quantized tensor trains

**BibTeX**

@Techreport{KORS16_643, author = {V. Kazeev and I. Oseledets and M. Rakhuba and Ch. Schwab}, title = {QTT-Finite-Element Approximation For Multiscale Problems}, institution = {Seminar for Applied Mathematics, ETH Z{\"u}rich}, number = {2016-06}, address = {Switzerland}, url = {https://www.sam.math.ethz.ch/sam_reports/reports_final/reports2016/2016-06.pdf }, year = {2016} }

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