Superiorization: The asymmetric roles of feasibility-seeking and objective function reduction

TL;DR

Superiorization is a methodology that seeks feasible solutions with improved objective function values by leveraging feasibility-seeking algorithms, emphasizing the primary importance of constraints over objective reduction.

Contribution

The paper provides a rigorous formulation of the superiorization methodology and discusses its current state and unresolved guarantee problems.

Findings

Introduces the concept of superiorization as an intermediate approach

Highlights the asymmetric roles of feasibility and objective reduction

Proposes a formal framework and identifies open problems

Abstract

The superiorization methodology can be thought of as lying conceptually between feasibility-seeking and constrained minimization. It is not trying to solve the full-fledged constrained minimization problem composed from the modeling constraints and the chosen objective function. Rather, the task is to find a feasible point which is "superior" (in a well-defined manner) with respect to the objective function, to one returned by a feasibility-seeking only algorithm. We telegraphically review the superiorization methodology and where it stands today and propose a rigorous formulation of its, yet only partially resolved, guarantee problem. The real-world situation in an application field is commonly represented by constraints defined by the modeling process and the data, obtained from measurements or otherwise dictated by the model-user. The feasibility-seeking problem requires to find a point in the intersection of all constraints without using any objective function to aim at any specific feasible point. At the heart of the superiorization methodology lies the modeler desire to use an objective function, that is exogenous to the constraints, in order to seek a feasible solution that will have lower (not necessarily minimal) objective function value. This aim is less demanding than full-fledged constrained minimization but more demanding than plain feasibility-seeking. Putting emphasis on the need to satisfy the constraints, because they represent the real-world situation, one recognizes the "asymmetric roles of feasibility-seeking and objective function reduction", namely, that fulfilling the constraints is the main task while reduction of the exogenous objective function plays only a secondary role. There are two research directions in the superiorization methodology: Weak superiorization and strong superiorization.