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On the minimum of several random variables
Authors:Y Gordon  A E Litvak  C Schü  tt  E Werner
Institution:Department of Mathematics, Technion, Haifa 32000, Israel ; Department of Mathematics and Statistics Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2G1 ; Mathematisches Seminar, Christian Albrechts Universität, 24098 Kiel, Germany ; Department of Mathematics, Case Western Reserve University, Cleveland, Ohio 44106 and Université de Lille 1, UFR de Mathématique, 59655 Villeneuve d'Ascq, France
Abstract:For a given sequence of real numbers $ a_{1}, \dots, a_{n}$, we denote the $ k$th smallest one by $ {k\mbox{-}\min} _{1\leq i\leq n}a_{i}$. Let $ \mathcal{A}$ be a class of random variables satisfying certain distribution conditions (the class contains $ N(0, 1)$ Gaussian random variables). We show that there exist two absolute positive constants $ c$ and $ C$ such that for every sequence of real numbers $ 0< x_{1}\leq \ldots \leq x_{n}$ and every $ k\leq n$, one has

$\displaystyle c \max_{1 \leq j \leq k} \frac {k+1-j}{\sum_{i=j}^n 1/x_i } \leq \mathbb{E} \, \, k$-$\displaystyle \min_{1\leq i\leq n} \vert x_{i} \xi_{i}\vert \leq C\, \ln(k+1)\, \max_{1 \leq j \leq k} \frac{k+1-j}{\sum_{i=j}^n 1/x_i}, $

where $ \xi_1, \dots, \xi_n$ are independent random variables from the class $ \mathcal{A}$. Moreover, if $ k=1$, then the left-hand side estimate does not require independence of the $ \xi_i$'s. We provide similar estimates for the moments of $ {k\mbox{-}\min}_{1\leq i\leq n} \vert x_{i} \xi_{i}\vert$ as well.

Keywords:Order statistics  expectations  moments  normal distribution  exponential distribution  
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