Local Optima in Diversity Optimization: Non-trivial Offspring Population is Essential

TL;DR

This paper demonstrates that non-trivial offspring populations are crucial in diversity optimization, revealing limitations of simple algorithms and proposing new methods to effectively find diverse solutions in complex problems.

Contribution

It introduces the importance of non-trivial offspring populations in diversity optimization and proposes the $(1_ty + 1_ty)$ EA$_D$ algorithm for improved diversity in complex problems.

Findings

Non-trivial offspring populations can escape local optima in diversity optimization.

The $(ty + ty)$ EA with a specialized mutation effectively finds diverse solutions.

The $(1_ty + 1_ty)$ EA$_D$ efficiently optimizes diversity in $k$-vertex cover.

Abstract

The main goal of diversity optimization is to find a diverse set of solutions which satisfy some lower bound on their fitness. Evolutionary algorithms (EAs) are often used for such tasks, since they are naturally designed to optimize populations of solutions. This approach to diversity optimization, called EDO, has been previously studied from theoretical perspective, but most studies considered only EAs with a trivial offspring population such as the $(\mu + 1)$ EA. In this paper we give an example instance of a $k$-vertex cover problem, which highlights a critical difference of the diversity optimization from the regular single-objective optimization, namely that there might be a locally optimal population from which we can escape only by replacing at least two individuals at once, which the $(\mu + 1)$ algorithms cannot do. We also show that the $(\mu + \lambda)$ EA with $\lambda \ge \mu$ can effectively find a diverse population on $k$-vertex cover, if using a mutation operator inspired by Branson and Sutton (TCS 2023). To avoid the problem of subset selection which arises in the $(\mu + \lambda)$ EA when it optimizes diversity, we also propose the $(1_\mu + 1_\mu)$ EA$_D$, which is an analogue of the $(1 + 1)$ EA for populations, and which is also efficient at optimizing diversity on the $k$-vertex cover problem.