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First page of Homogeneous polynomial approximation on symmetric convex bodies: A proof of Kroó's conjecture

Homogeneous polynomial approximation on symmetric convex bodies: A proof of Kroó's conjecture

Matthew J. Colbrook

math.NA Oct 28, 2026 · v1 math.CA math.FA
The paper's results, including the proof of Kroó's conjecture, are stated to be formally verified in Lean, with AI-assisted formalisation via Codex.
We resolve Kroó's 2004 conjecture on homogeneous polynomial approximation in every dimension, without smoothness or curvature assumptions. For every origin-symmetric convex body $K\subset\mathbb R^d$ with nonempty interior, every continuous real function on $\partial K$ is uniformly approximated by sums of homogeneous polynomials of degrees $n$ and $n+1$ as $n\to\infty$. Moreover, approximation of the constant function by homogeneous polynomials of degree $2n$ is uniform over all such bodies in each fixed dimension. Via homogenization, we also resolve Kroó's 2019 weighted approximation conjecture for every convex admissible weight on Euclidean space. The key construction produces homogeneous polynomials with a common Gaussian boundary profile and localization errors controlled by arbitrary polynomial weights. Its analytic core is a weighted Gaussian approximation theorem: for \(d\ge2\), sufficiently broad Gaussians on $\mathbb R^{d-1}$ can be approximated by $e^{-f}F$, where $f$ is nonnegative, convex and Lipschitz with $f(0)=0$, and $F$ is a Schwartz function with Fourier support in an arbitrarily small ball about the origin. Small Fourier support allows Taylor truncation to a lower degree, while the remainder enforces decay through homogenization. We then use positive spherical quadrature and degree raising to yield the full approximation theorem.

Kroó's 2004 conjecture asks whether every continuous function on the boundary of an origin-symmetric convex body can be uniformly approximated by sums of homogeneous polynomials of two consecutive degrees. A related 2019 conjecture concerns weighted approximation with convex admissible weights.

The proof builds homogeneous polynomials with a common Gaussian boundary profile, using a weighted Gaussian approximation theorem by functions e^{-f}F, where F has small Fourier support. Small Fourier support allows Taylor truncation to lower degree, and homogenization then supplies decay. Positive spherical quadrature and degree raising complete the argument. The results were also formally verified in Lean.

Kroó's conjecture is proved in every dimension, without smoothness or curvature assumptions. Approximation of the constant function by degree-2n homogeneous polynomials is uniform over all such bodies in a fixed dimension. The weighted conjecture is resolved for every convex admissible weight.