Codes and Expansions (CodEx) Seminar

Hannah Lawrence (MIT):
Learning Polynomial Problems with \(SL(2,\mathbb{R})\)-Equivariance

Optimizing and certifying the positivity of polynomials are fundamental primitives across mathematics and engineering applications, from dynamical systems to operations research. However, solving these problems in practice requires large semidefinite programs, with poor scaling in dimension and degree. In this work, we demonstrate for the first time that neural networks can effectively solve such problems in a data-driven fashion, achieving tenfold speedups while retaining high accuracy. Moreover, we observe that these polynomial learning problems are equivariant to the non-compact group \(SL(2,\mathbb{R})\), which consists of area-preserving linear transformations. We therefore adapt our learning pipelines to accommodate this structure, including data augmentation, a new \(SL(2,\mathbb{R})\)-equivariant architecture, and an architecture equivariant with respect to its maximal compact subgroup, \(SO(2,\mathbb{R})\). Surprisingly, the most successful approaches in practice do not enforce equivariance to the entire group, which we prove arises from an unusual lack of architecture universality for \(SL(2,\mathbb{R})\) in particular. A consequence of this result, which is of independent interest, is that there exists an equivariant function for which there is no sequence of equivariant polynomials, multiplied by arbitrary invariants, that approximates the original function. This is a rare example of a symmetric problem where data augmentation outperforms a fully equivariant architecture, and provides interesting lessons in both theory and practice for other problems with non-compact symmetries.