Derivative-free superiorization with component-wise perturbations

TL;DR

This paper introduces a generalized derivative-free superiorization framework using component-wise perturbations, enabling target function reduction without derivatives, demonstrated on CT image reconstruction.

Contribution

It refines superiorization methodology to incorporate component-wise perturbations, removing the need for derivatives and linking step-sizes to nonascent directions.

Findings

Effective in reducing target function in CT reconstruction

Perturbation steps are computationally efficient

Framework applicable to derivative-free optimization

Abstract

Superiorization reduces, not necessarily minimizes, the value of a target function while seeking constraints-compatibility. This is done by taking a solely feasibility-seeking algorithm, analyzing its perturbations resilience, and proactively perturbing its iterates accordingly to steer them toward a feasible point with reduced value of the target function. When the perturbation steps are computationally efficient, this enables generation of a superior result with essentially the same computational cost as that of the original feasibility-seeking algorithm. In this work, we refine previous formulations of the superiorization method to create a more general framework, enabling target function reduction steps that do not require partial derivatives of the target function. In perturbations that use partial derivatives the step-sizes in the perturbation phase of the superiorization method are chosen independently from the choice of the nonascent directions. This is no longer true when component-wise perturbations are employed. In that case, the step-sizes must be linked to the choice of the nonascent direction in every step. Besides presenting and validating these notions, we give a computational demonstration of superiorization with component-wise perturbations for a problem of computerized tomography image reconstruction.