Abstract
The recently proposed iterative sparsification projection (ISP), a fast and robust sparse signal recovery algorithm framework, can be classified as smooth-ISP and nonsmooth-ISP. However, no convergence analysis has been established for the nonsmooth-ISP in the previous works. Motivated by this absence, the present paper provides a convergence analysis for ISP with soft-thresholding (ISP-soft) which is an instance of the nonsmooth-ISP. In our analysis, the composite operator of soft-thresholding and proximal projection is viewed as a fixed point mapping, whose nonexpansiveness plays a key role. Specifically, our convergence analysis for the sequence generated by ISP-soft can be summarized as follows: 1) For each inner loop, we prove that the sequence has a unique accumulation point which is a fixed point, and show that it is a Cauchy sequence; 2) for the last inner loop, we prove that the accumulation point of the sequence is a critical point of the objective function if the final value of the threshold satisfies a condition, and show that the corresponding objective values are monotonically nonincreasing. A numerical experiment is given to validate some of our results and intuitively present the convergence.
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Notes
Generally, under the case where \(J({\mathbf {x}})=\Vert {\mathbf {x}}\Vert _{1}\), problem (2) is equivalently reformulated as an unconstrained regularized optimization, termed basis pursuit denoising (BPDN).
The smooth version of (2) means that \(J(\cdot )\) is replaced by its smooth approximation.
In Algorithm 1, \({\mathbf {A}}^{\dagger }\) denotes the Moore–Penrose pseudo-inverse of \({\mathbf {A}}\), and other initial values for \({\mathbf {x}}\) can also be used.
We did multiple trials in our experiment, but only present the results of one trial, since in each trial we observed similar results.
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Funding
This work was supported in part by the Natural Science Foundation of Guangdong Province under Grants 2019A1515010861, in part by Guangzhou Technical Project under Grant 201902020008, and in part by NSFC under Grant 61471174.
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Zhu, T. A convergence analysis for iterative sparsification projection with soft-thresholding. SIViP 15, 1705–1712 (2021). https://doi.org/10.1007/s11760-021-01910-9
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DOI: https://doi.org/10.1007/s11760-021-01910-9