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arXiv 0903.0614math.PRmath.CO
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随机矩阵:最小奇异值的分布

Random matrices: The distribution of the smallest singular values

Terence Tao, Van Vu

摘要:

设$\a$是均值为零、方差为1的实值随机变量。令$M_n(\a)$表示其条目是$\a$的iid副本的$n \times n$随机矩阵,并且$σ_n(M_n(\a))$表示$M_n(\a)$的最小奇异值。($σ_n(M_n(\a))^2$通常也被解释为Wishart矩阵$M_n M_n^{\ast}$的最小特征值。)我们表明,(在有限矩假设下)的概率分布$n σ_n(M_n(\a))^2$是{ \it通用}的意义上说,它不依赖于分布的$\a$。特别地,当$a$是实高斯时,它收敛到与特殊情况相同的极限分布。(The在这种情况下,Edelman明确计算了极限分布。)对于均值为零的复值随机变量,实部和虚部方差为1/2,协方差为零,我们也证明了类似的结果。对于任何固定的$k$(甚至对于$k$作为$n$的小幂增长)和矩形矩阵,对于$M_n(\a)$的底部$k$奇异值的联合分布也得到了类似的结果。我们的方法的动机是从组合学和理论计算机科学的一般思想的“属性测试”。这似乎是随机矩阵谱研究的一种新方法,并结合了数学各个领域的工具。

英文摘要:

Let $\a$ be a real-valued random variable of mean zero and variance 1. Let $M_n(\a)$ denote the $n \times n$ random matrix whose entries are iid copies of $\a$ and $σ_n(M_n(\a))$ denote the least singular value of $M_n(\a)$. ($σ_n(M_n(\a))^2$ is also usually interpreted as the least eigenvalue of the Wishart matrix $M_n M_n^{\ast}$.) We show that (under a finite moment assumption) the probability distribution $n σ_n(M_n(\a))^2$ is {\it universal} in the sense that it does not depend on the distribution of $\a$. In particular, it converges to the same limiting distribution as in the special case when $a$ is real gaussian. (The limiting distribution was computed explicitly in this case by Edelman.) We also proved a similar result for complex-valued random variables of mean zero, with real and imaginary parts having variance 1/2 and covariance zero. Similar results are also obtained for the joint distribution of the bottom $k$ singular values of $M_n(\a)$ for any fixed $k$ (or even for $k$ growing as a small power of $n$) and for rectangular matrices. Our approach is motivated by the general idea of "property testing" from combinatorics and theoretical computer science. This seems to be a new approach in the study of spectra of random matrices and combines tools from various areas of mathematics.

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