论文标题
在几乎matrix乘法时间中稳健的高斯协方差估计
Robust Gaussian Covariance Estimation in Nearly-Matrix Multiplication Time
论文作者
论文摘要
强大的协方差估计是高维统计中的以下良好问题:给定$ n $样品中的$ d $二维高斯$ \ Mathcal {n}(\ boldsymbol {0},σ)$,但是在$ \ varepsilon $ fraction $ procutioned $ the proultional prounted的情况下$ \ MATHCAL {n}(\ BoldSymbol {0},σ)$和$ \ Mathcal {n}(\ BoldSymbol {0},\wideHatς)$。这对应于Frobenius Norm的天然仿射不变变体中的$σ$,称为\ emph {Mahalanobis Norm}。 Cheng等人的先前工作证明了一种算法,即给定$ n =ω(d^2 / \ varepsilon^2)$样品,达到了$ O(\ varepsilon \ log 1 / \ log 1 / \ varepsilon)$的几乎最佳误差,此外,他们的algorithm在algorithm中$ \ wideTiLdem / dim n om \ widetiilde} c(O \ \ \ \ o \ o \ o \ o {O {O {O {O {O {O {O {O {O {O {O. \ Mathrm {poly}(\ varepsilon))$,其中$ t(n,d)$是将$ d \ times n $矩阵乘以transpose所需的时间,而$κ$是$σ$的条件数量。当$ \ varepsilon $相对较小时,其多项式依赖性对$ 1/\ varepsilon $在运行时非常大。在本文中,我们演示了一种具有相同统计保证的新颖算法,但在时间上运行$ \ widetilde {o}(t(n,d)\ log -κ)$。特别是,我们的运行时不依赖$ \ varepsilon $。当$σ$合理地条件时,我们的运行时间与无异常值的协方差估算算法的算法相匹配,直到多种含量因素,这表明我们可以免费获得稳健性。
Robust covariance estimation is the following, well-studied problem in high dimensional statistics: given $N$ samples from a $d$-dimensional Gaussian $\mathcal{N}(\boldsymbol{0}, Σ)$, but where an $\varepsilon$-fraction of the samples have been arbitrarily corrupted, output $\widehatΣ$ minimizing the total variation distance between $\mathcal{N}(\boldsymbol{0}, Σ)$ and $\mathcal{N}(\boldsymbol{0}, \widehatΣ)$. This corresponds to learning $Σ$ in a natural affine-invariant variant of the Frobenius norm known as the \emph{Mahalanobis norm}. Previous work of Cheng et al demonstrated an algorithm that, given $N = Ω(d^2 / \varepsilon^2)$ samples, achieved a near-optimal error of $O(\varepsilon \log 1 / \varepsilon)$, and moreover, their algorithm ran in time $\widetilde{O}(T(N, d) \log κ/ \mathrm{poly} (\varepsilon))$, where $T(N, d)$ is the time it takes to multiply a $d \times N$ matrix by its transpose, and $κ$ is the condition number of $Σ$. When $\varepsilon$ is relatively small, their polynomial dependence on $1/\varepsilon$ in the runtime is prohibitively large. In this paper, we demonstrate a novel algorithm that achieves the same statistical guarantees, but which runs in time $\widetilde{O} (T(N, d) \log κ)$. In particular, our runtime has no dependence on $\varepsilon$. When $Σ$ is reasonably conditioned, our runtime matches that of the fastest algorithm for covariance estimation without outliers, up to poly-logarithmic factors, showing that we can get robustness essentially "for free."