Active Linear Regression for $\ell_p$ Norms and Beyond

We study active sampling algorithms for linear regression, which aim to query only a few entries of a target vector $b\in\mathbb R^n$ and output a near minimizer to $\min{x\in\mathbb R^d} |Ax-b|$, for a design matrix $A\in\mathbb R^{n \times d}$ and loss $|\cdot|$. For $p$ norm regression for any $0<p<\infty$, we give an algorithm based on Lewis weight sampling outputting a $(1+\epsilon)$-approximate solution using just $\tilde O(d/\epsilon^2)$ queries to $b$ for $p\in(0,1)$, $\tilde{O}(d/\epsilon)$ queries for $1<p<2$, and $\tilde{O}(d^{{p/2}/\epsilonp)$} queries for $2<p<\infty$. For $0<p<2$, our bounds are optimal up to log factors, settling the query complexity for this range. For $2<p<\infty$, our dependence on $d$ is optimal, while our dependence on $\epsilon$ is off by at most $\epsilon$, up to log factors. Our result resolves an open question of [CD21], who gave near optimal bounds for the $1$ norm, but required $d^2/\epsilon2$ samples for $\ellp$ regression with $1<p<2$, and gave no bounds for $2<p<\infty$ or $0<p<1$. We also give the first total sensitivity bound of $O(d^{{\max{1,p/2}}\log2n)$} for loss functions of degree $p$ polynomial growth, improving a result of [TMF20]. By combining this with our techniques for $\ell_p$ regression, we obtain an active regression algorithm making $\tilde O(d^{{1+\max{1,p/2}}/\mathrm{poly}(\epsilon))$} queries for such loss functions, including the Tukey and Huber losses, answering another question of [CD21]. For the Huber loss, we further improve our bound to $\tilde O(d^{{4-2\sqrt2}/\mathrm{poly}(\epsilon))$} samples. Our sensitivity bounds also have many applications, including Orlicz norm subspace embeddings, robust subspace approximation, and dimension reduction for smoothed $p$-norms. Finally, our active sampling results give the first sublinear time algorithms for Kronecker product regression under every $p$ norm.