## Preliminary list of abstracts

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New cross interpolation algorithm and Krylov/Wedderburn scheme for TT format
Keywords: TT format; cross algorithm; Krylov subspaces
Wed, 14:50--15:15
• Savostyanov, Dmitry (Institute of Numerical Mathematics, Russian Academy of Sciences, Russia)
• Oseledets, Ivan (Institute of Numerical Mathematics, Russian Academy of Sciences, Russia)

Tensor train (TT) format has been recently proposed in [7] for data-sparse representation of the high-dimensional tensors based on separation of variables. TT format has the advances of both canonical and Tucker formats: the number of parameters does not grow exponentially with dimension, decomposition/approximation problem is stable and can be computed by SVD-based methods.

To break the curse of dimensionality, approximation methods that do not require all elements of initial tensor are crutial. For matrices such an algorithm is well known and used, for example in mosaic/$\mathcal{H}$–matrices formats [9]. It interpolates the given matrix on the positions of several columns and rows and then checks the approximation accuracy over the random set of elements. The choice of the proper positions for the interpolation depends on matrix and is very important. In [2] it is shown that good positions can be selected using the maximum-volume principle, in [3] properties of maximum-volume and closely related dominating submatrices are studied in more details. For three-dimensional tensors, cross interpolation algorithms in Tucker format [6] and also are based on the maximum volume principle.

For arbitrary dimension the interpolation method for the TT format is proposed in [8]. However, sometimes its convergence is not satisfactory even for data with "good" structure. To improve this, we propose a new method combining the maximum-volume interpolation idea with the DMRG approximation scheme, popular among the quantum chemistry community. We apply the resulted algorithm for the fast compression of high-dimensional data and high-scale data using the quantization idea proposed in [5,4].

Also, the DMRG scheme can be improved using the Krylov/Wedderburn framework. Krylov methods are methods of choice to approximate large matrices that allow fast matrix-by-vector multiplication. In [1] we generalize them to three-dimensional tensors. In this talk we discuss the generalization to larger dimensions.

This work is supported by RFBR grants 09-01-91332 (with DFG), 09-01-00565, 10-01-00757.

[1] S. A. Goreinov, I. V. Oseledets, and D. V. Savostyanov. Wedderburn rank reduction and Krylov subspace method for tensor approximation. Part 1: Tucker case. Preprint 2010-01, INM RAS, Moscow, 2010.
[2] S. A. Goreinov and E. E. Tyrtyshnikov. The maximal-volume concept in approximation by low-rank matrices. Contemporary Mathematics, 208:47–51, 2001.
[3] S.A. Goreinov, I.V. Oseledets, D.V. Savostyanov, E.E. Tyrtyshnikov, and N.L. Zamarashkin. How to find a good submatrix. In V. Olshevsky and E. Tyrtyshnikov, editors, Matrix Methods: Theory, Algorithms, Applications, pages 247–256. World Scientific Publishing, 2010.
[4] B. N. Khoromskij and I. V. Oseledets. Quantics-TT approximation of elliptic solution operators in high dimensions. J. Numer. Math., (2011).
[5] I. V. Oseledets. Approximation of $2^d \times 2^d$ matrices using tensor decomposition. SIAM J. Matrix Anal. Appl., 31(4):2130–2145, 2010.
[6] I. V. Oseledets, D. V. Savostianov, and E. E. Tyrtyshnikov. Tucker dimensionality reduction of three-dimensional arrays in linear time. SIAM J. Matrix Anal. Appl., 30(3):939–956, 2008.
[7] I. V. Oseledets and E. E. Tyrtyshnikov. Breaking the curse of dimensionality, or how to use svd in many dimensions. SIAM J. Sci. Comp., 31(5):3744–3759, 2009.
[8] I. V. Oseledets and E. E. Tyrtyshnikov. TT-cross algorithm for the approximation of multidimensional arrays. Linear Algebra Appl., 432(1):70–88, 2010.
[9] E. E. Tyrtyshnikov. Incomplete cross approximation in the mosaic–skeleton method. Computing, 64(4):367–380, 2000.