arXiv Machine Learning By Radmehr Karimian, Ali Mohades, Johannes Lederer

How Many Directions Must a Truncated Diffusion Sampler Retain? Matching Bounds Under Power-Law Spectra

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The paper investigates how many spectral directions a truncated diffusion sampler must retain when data exhibit power‑law covariance spectra. For Gaussian data compared to a smoothed target, it establishes matching upper and lower bounds on the necessary number of retained directions, assuming a sufficiently large ambient dimension. The analysis shows that truncation error depends on the combined Wiener gains of omitted directions, meaning that even weak individual directions can collectively contribute significant error, and it provides practical guidelines for selecting the retained subspace and diffusion noise level.

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arXiv Machine Learning
Oct 8

Lower Bounds for Parallel Diffusion Sampling

The paper establishes the first polynomial lower bounds on the number of parallel rounds required for diffusion sampling with approximate scores. It proves a “~Ω(d^{1/3})” lower bound for sampling smooth, near‑isotropic Gaussian mixtures in ℝ^d, and an Ω(d) lower bound for uniform sampling from anisotropic axis‑aligned boxes inside the unit ball. These bounds apply to any randomized algorithm that can make polynomially many queries per round at arbitrary locations and noise levels, while maintaining inverse‑polynomial score error and constant total variation accuracy.

By Yiwen Kou, Yimeng Wang