Abstract
The randomized version of the Kaczmarz method for the solution of consistent linear systems is known to converge linearly in expectation. And even in the possibly inconsistent case, when only noisy data is given, the iterates are expected to reach an error threshold in the order of the noise-level with the same rate as in the noiseless case. In this work we show that the same also holds for the iterates of the recently proposed randomized sparse Kaczmarz method for recovery of sparse solutions. Furthermore we consider the more general setting of convex feasibility problems and their solution by the method of randomized Bregman projections. This is motivated by the observation that, similarly to the Kaczmarz method, the Sparse Kaczmarz method can also be interpreted as an iterative Bregman projection method to solve a convex feasibility problem. We obtain expected sublinear rates for Bregman projections with respect to a general strongly convex function. Moreover, even linear rates are expected for Bregman projections with respect to smooth or piecewise linear-quadratic functions, and also the regularized nuclear norm, which is used in the area of low rank matrix problems.
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Notes
Because very general control sequences besides simple cyclic control fulfill this requirement, the corresponding method was also called method of random Bregman projections in [4]. But such control sequences are not necessarily stochastic objects, in contrast to the situation in the present work. Hence we use the word randomized in Algorithm 3 instead of random to distinguish between the cases.
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The work of D.L. was partially supported by the National Science Foundation under Grant DMS-1127914 to the Statistical and Applied Mathematical Sciences Institute. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
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Schöpfer, F., Lorenz, D.A. Linear convergence of the randomized sparse Kaczmarz method. Math. Program. 173, 509–536 (2019). https://doi.org/10.1007/s10107-017-1229-1
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DOI: https://doi.org/10.1007/s10107-017-1229-1
Keywords
- Randomized Kaczmarz method
- Linear convergence
- Bregman projections
- Sparse solutions
- Split feasibility problem
- Error bounds