Title: On Ridge Estimation in High-dimensional Rotationally Sparse Linear Regression

Abstract: Recently, deep neural networks have been found to nearly interpolate training data but still generalize well in various applications. To help understand such a phenomenon, it has been of interest to analyze the ridge estimator and its interpolation limit in high-dimensional regression models. For this motivation, we study the ridge estimator in a rotationally sparse setting of high-dimensional linear regression, where the signal of a response is aligned with a small number, $d$, of covariates with large or spiked variances, compared with the remaining covariates with small or tail variances, \textit{after} an orthogonal transformation of the covariate vector. We establish high-probability upper and lower bounds on the out-sample and in-sample prediction errors in two distinct regimes depending on the ratio of the effective rank of tail variances over the sample size $n$. The separation of the two regimes enables us to exploit relevant concentration inequalities and derive concrete error bounds without making any oracle assumption or independent components assumption on covariate vectors. Moreover, we derive sufficient and necessary conditions which indicate that the prediction errors of ridge estimation can be of the order $O(\frac{d}{n})$ if and only if the gap between the spiked and tail variances are sufficiently large. We also compare the orders of optimal out-sample and in-sample prediction errors and find that, remarkably, the optimal out-sample prediction error may be significantly smaller than the optimal in-sample one. Finally, we present numerical experiments which empirically confirm our theoretical findings.