FOCS Conference 2023 Conference Paper
Query lower bounds for log-concave sampling
- Sinho Chewi
- Jaume de Dios Pont
- Jerry Li 0001
- Chen Lu 0002
- Shyam Narayanan
Log-concave sampling has witnessed remarkable algorithmic advances in recent years, but the corresponding problem of proving $\lt$bold$\gt$lower bounds$\lt$/bold$\gt$ for this task has remained elusive, with lower bounds previously known only in dimension one. In this work, we establish the following query lower bounds: (1) sampling from strongly log-concave and log-smooth distributions in dimension $d \geq 2$ requires $\Omega(\log \kappa)$ queries, which is sharp in any constant dimension, and (2) sampling from Gaussians in dimension d (hence also from general logconcave and log-smooth distributions in dimension d) requires $\widetilde{\Omega}(\min (\sqrt{\kappa} \log d, d))$ queries, which is nearly sharp for the class of Gaussians. Here $\kappa$ denotes the condition number of the target distribution. Our proofs rely upon (1) a multiscale construction inspired by work on the Kakeya conjecture in geometric measure theory, and (2) a novel reduction that demonstrates that block Krylov algorithms are optimal for this problem, as well as connections to lower bound techniques based on Wishart matrices developed in the matrix-vector query literature.