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410概率困难multi partmedium

Joint Density and Covariance of Two Uniform Order Statistics

题目

Let X1,,XnX_1, \ldots, X_n be iid Uniform(0,1)\operatorname{Uniform}(0,1). Consider the order statistics X(i)X_{(i)} and X(j)X_{(j)} with 1i<jn1 \le i < j \le n.

分步问题

分步 a

Derive the joint PDF fX(i),X(j)(x,y)f_{X_{(i)},X_{(j)}}(x,y) for 0<x<y<10 < x < y < 1.

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分步 b

Using the formula E[X(i)X(j)]=i(j+1)(n+1)(n+2)E[X_{(i)}X_{(j)}] = \frac{i(j+1)}{(n+1)(n+2)} for i<ji < j, derive Cov(X(i),X(j))\operatorname{Cov}(X_{(i)}, X_{(j)}).

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