A Geometric Algorithm for Solving Linear Systems

TL;DR

This paper introduces a geometric iterative algorithm based on the Triangle Algorithm for efficiently testing the solvability of linear systems and approximating solutions, with competitive performance and robustness.

Contribution

It presents a novel geometric algorithm for solving linear systems that is simple, robust, and competitive with existing methods, applicable to real and complex inputs.

Findings

Algorithm computes approximate solutions or certificates of unsolvability.

Operates with $O(mn)$ complexity per iteration and logarithmic iteration count.

Proven to be effective and competitive compared to BiCGSTAB in computational tests.

Abstract

Based on the geometric {\it Triangle Algorithm} for testing membership of a point in a convex set, we present a novel iterative algorithm for testing the solvability of a real linear system $Ax=b$, where $A$ is an $m \times n$ matrix of arbitrary rank. Let $C_{A,r}$ be the ellipsoid determined as the image of the Euclidean ball of radius $r$ under the linear map $A$. The basic procedure in our algorithm computes a point in $C_{A,r}$ that is either within $\varepsilon$ distance to $b$, or acts as a certificate proving $b \not \in C_{A,r}$. Each iteration takes $O(mn)$ operations and when $b$ is well-situated in $C_{A,r}$, the number of iterations is proportional to $\log{(1/\varepsilon)}$. If $Ax=b$ is solvable the algorithm computes an approximate solution or the minimum-norm solution. Otherwise, it computes a certificate to unsolvability, or the minimum-norm least-squares solution. It is also applicable to complex input. In a computational comparison with the state-of-the-art algorithm BiCGSTAB ({\it Bi-conjugate gradient method stabilized}), the Triangle Algorithm is very competitive. In fact, when the iterates of BiCGSTAB do not converge, our algorithm can verify $Ax=b$ is unsolvable and approximate the minimum-norm least-squares solution. The Triangle Algorithm is robust, simple to implement, and requires no preconditioner, making it attractive to practitioners, as well as researchers and educators.