# CUR LRA at Sublinear Cost Based on Volume Maximization

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## Abstract

A matrix algorithm runs at *sublinear cost* if it uses much fewer memory cells and arithmetic operations than the input matrix has entries. Such algorithms are indispensable for Big Data Mining and Analysis, where input matrices are so immense that one can only access a small fraction of all their entries. Typically, however, such matrices admit their Low Rank Approximation *(LRA)*, which one can access and process at sublinear cost. Can, however, we compute LRA at sublinear cost? Adversary argument shows that no algorithm running at sublinear cost can output accurate LRA of worst case input matrices or even of the matrices of small families of our Appendix A, but we prove that some sublinear cost algorithms output a reasonably close LRA of a matrix *W* if (i) this matrix is sufficiently close to a low rank matrix or (ii) it is a Symmetric Positive Semidefinite *(SPSD)* matrix that admits LRA. In both cases supporting algorithms are deterministic and output LRA in its special form of CUR LRA, particularly memory efficient. The design of our algorithms and the proof of their correctness rely on the results of extensive previous study of CUR LRA in Numerical Linear Algebra using volume maximization. In case (i) we apply Cross-Approximation *(C-A)* iterations, running at sublinear cost and computing accurate LRA worldwide for more than a decade. We provide the first formal support for this long-known empirical efficiency assuming non-degeneracy of the initial submatrix of at least one C-A iteration. We cannot ensure non-degeneracy at sublinear cost for a worst case input but prove that it holds with a high probability *(whp)* for any initialization in the case of a random or randomized input. Empirically we can replace randomization with sparse multiplicative preprocessing of an input matrix, performed at sublinear cost. In case (ii) we make no additional assumptions about the input class of SPSD matrices admitting LRA or about initialization of our sublinear cost algorithms for CUR LRA, which promise to be practically valuable. We hope that proper combination of our deterministic techniques with randomized LRA methods, popular among Computer Science researchers, will lead them to further progress in LRA.

## Keywords

Low Rank Approximation (LRA) CUR LRA Sublinear cost Symmetric Positive Semidefinite (SPSD) matrices Cross-Approximation (C-A) Maximal volume## 2000 Math. Subject Classification:

65Y20 65F30 68Q25 15A52## Notes

### Acknowledgements

Our research has been supported by NSF Grants CCF-1116736, CCF-1563942, and CCF-133834 and PSC CUNY Award 69813 00 48. We also thank A. Cortinovis, A. Osinsky, N. L. Zamarashkin for pointers to their papers [CKM19] and [OZ18], S. A. Goreinov for reprints, of his papers, and E. E. Tyrtyshnikov for pointers to the bibliography and the challenge of formally supporting empirical power of C-A algorithms.

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