Certifying solutions of degenerate semidefinite programs

TL;DR

This paper introduces a hybrid symbolic-numerical method for certifying the feasibility of semidefinite programs, especially when solutions are irrational, by constructing polynomial systems with guaranteed isolated solutions.

Contribution

It presents a novel approach that avoids the need for rational feasible solutions, using polynomial systems and algebraic geometry techniques to certify SDP feasibility.

Findings

Hybrid method successfully certifies feasibility where pure symbolic methods fail

Constructs polynomial systems with guaranteed isolated solutions for SDPs

Demonstrates potential for refining approximate solutions using algebraic geometry

Abstract

This paper deals with the algorithmic aspects of solving feasibility problems of semidefinite programming (SDP), aka linear matrix inequalities (LMI). Since in some SDP instances all feasible solutions have irrational entries, numerical solvers that work with rational numbers can only find an approximate solution. We study the following question: is it possible to certify feasibility of a given SDP using an approximate solution that is sufficiently close to some exact solution? Existing approaches make the assumption that there exist rational feasible solutions (and use techniques such as rounding and lattice reduction algorithms). We propose an alternative approach that does not need this assumption. More specifically, we show how to construct a system of polynomial equations whose set of real solutions is guaranteed to have an isolated correct solution (assuming that the target exact solution is maximum-rank). This allows, in particular, to use algorithms from real algebraic geometry for solving systems of polynomial equations, yielding a hybrid (or symbolic-numerical) method for SDPs. We experimentally compare it with a pure symbolic method; the hybrid method was able to certify feasibility of many SDP instances on which the exact method failed. Our approach may have further applications, such as refining an approximate solution using methods of numerical algebraic geometry for systems of polynomial equations.