Learning Intersections of Two Margin Halfspaces under Factorizable Distributions

Abstract

Learning intersections of halfspaces is a central problem in Computational Learning Theory. Even for just two halfspaces, it remains a major open question whether learning is possible in polynomial time with respect to the margin γ of the data points and their dimensionality d. The best-known algorithms run in quasi-polynomial time d^O( log1/γ ), and it has been shown that this complexity is unavoidable for any algorithm relying solely on correlational statistical queries (CSQ). In this work, we introduce a novel algorithm that provably circumvents the CSQ hardness barrier. Our approach applies to a broad class of distributions satisfying a natural, previously studied, factorizability assumption. Factorizable distributions lie between the distribution-specific and distribution-free settings, and significantly extend previously known tractable cases. For these distributions, we show that CSQ-based methods still require quasipolynomial time even for weak learning. Our main result is a learning algorithm for intersections of two margin halfspaces under factorizable distributions that achieves poly(d,1/γ) time by leveraging more general statistical queries (SQ). As a corollary, we establish a strong separation between CSQ and SQ for this fundamental PAC learning problem. Our main result is grounded in a rigorous analysis utilizing a novel duality framework that characterizes the moment tensor structure induced by the marginal distributions. Building on these structural insights, our learning algorithm combines a refined variant of Jennrich’s Algorithm with PCA over random projections of the moment tensor, along with a gradient-descent-based non-convex optimization framework.