Minimal length in an orbit closure as a semiclassical limit

Abstract

Consider the action of a connected complex reductive group on a finite-dimensional vector space. A fundamental result in invariant theory states that the orbit closure of a vector v is separated from the origin if and only if some homogeneous invariant polynomial is nonzero on v, i.e. v is not in the null cone. Thus, efficiently finding the minimum distance between the orbit closure and the origin can lead to deterministic algorithms for null cone membership, an important polynomial identity testing problem including the non-commutative Edmonds problem. This connection to optimization has recently led to efficient algorithms for many problems in invariant theory. Here we explore a refinement of the famous duality between orbit closures and invariant polynomials, which holds that the following two quantities coincide: (1) the logarithm of the Euclidean distance between the orbit closure and the origin and (2) the rate of exponential growth of the 'invariant part' of $v^{\otimes k}$ in the semiclassical limit as k tends to infinity. This result can be deduced from work of S. Zhang (Geometric reductivity at Archimedean places, 1994), which uses sophisticated tools in arithmetic geometry. We provide a new and independent elementary proof inspired by the Fourier-analytic proof of the local central limit theorem. We generalize the result to projections onto highest weight vectors and isotypical components, and explore connections between such semiclassical limits and the asymptotic behavior of multiplicities in representation theory, large deviations theory in classical and quantum statistics, and the Jacobian conjecture as reformulated by Mathieu. Our formulas imply that they can be computed, in many cases efficiently, to arbitrary precision.