Quasar-Convex Optimization: Fundamental Properties and High-Order Proximal-Point Methods

TL;DR

This paper explores the properties of quasar-convex functions and introduces high-order proximal-point algorithms with convergence guarantees and complexity bounds, demonstrating their effectiveness through numerical experiments.

Contribution

It develops fundamental properties of quasar-convex functions and proposes high-order proximal algorithms with a detailed convergence analysis and complexity results.

Findings

High-order proximal methods achieve different convergence rates depending on the order p.

For p in (1,2), local linear convergence with logarithmic complexity.

For p=2, global linear convergence with the same complexity.

Abstract

We study the optimization of (strongly) quasar-convex functions, a class that arises naturally in many machine learning and data science applications due to its favorable properties. The fundamental properties of this class are first developed, including its stability under standard calculus operations, growth conditions, and the absence of spurious critical points, which together imply a benign global geometry with no saddle points. Motivated by these properties, a class of proximal-point algorithms (HiPPA) with high-order regularization of order $p>1$ is introduced. Conditions are identified under which the iterates converge globally to minimizers, and a unified convergence analysis is provided with explicit rates and iteration complexity bounds under appropriate regularity assumptions. The results reveal a sharp transition in behavior with respect to the order $p$: for $p\in(1,2)$, the method achieves local linear convergence with complexity $\mathcal{O}(\log(\varepsilon^{-1}))$ when initialized sufficiently close to a minimizer; for $p=2$, it attains global linear convergence with the same complexity; and for $p>2$, it exhibits superlinear convergence with complexity $\mathcal{O}(\log\log(\varepsilon^{-1}))$, where $\varepsilon>0$ denotes the target accuracy. The theory is complemented with preliminary numerical experiments on selected machine learning problems, which illustrate the effectiveness of the proposed methods and are consistent with the theoretical findings.