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随机变量的特征函数
大数定理与中心极限定理
数学期望
定义
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离散型随机变量 X X X的分布律为 P { X = x k } = p k , k = 1 , 2... P\{X=x_k\}=p_k,k=1,2... P{X=xk}=pk,k=1,2...,数学期望为
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连续型随机变量 X X X的概率密度为 f ( x ) f(x) f(x),数学期望为
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Y = g ( X ) Y=g(X) Y=g(X)
离散型:
连续型:
性质
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E(E(x))=E(x)
E(E(x))=E(x)
方差
定义
- 方差: D ( X ) = V a r ( X ) = E { [ X − E ( X ) ] 2 } D(X)=Var(X)=E\{[X-E(X)]^2\} D(X)=Var(X)=E{[X−E(X)]2}
- 均方差或标准差:
- 离散型
- 连续型
- D ( X ) = E ( X 2 ) − [ E ( X ) ] 2 D(X)=E(X^2)-[E(X)]^2 D(X)=E(X2)−[E(X)]2
- 切比雪夫不等式:
性质
- D ( C ) = 0 D(C)=0 D(C)=0
- D ( C X ) = C 2 D ( X ) , D ( X + C ) = D ( X ) D(CX)=C^2D(X), D(X+C)=D(X) D(CX)=C2D(X),D(X+C)=D(X)
- D ( X + Y ) = D ( X ) + D ( Y ) + 2 E { ( X − E ( X ) ) ( Y − E ( Y ) ) } D(X+Y)=D(X)+D(Y)+2E\{(X-E(X))(Y-E(Y))\} D(X+Y)=D(X)+D(Y)+2E{(X−E(X))(Y−E(Y))},若 X , Y X,Y X,Y相互独立,则 D ( X + Y ) = D ( X ) + D ( Y ) D(X+Y)=D(X)+D(Y) D(X+Y)=D(X)+D(Y)
- D ( X ) = 0 < = > P { X = E ( X ) } = 1 D(X)=0 <=> P\{X=E(X)\}=1 D(X)=0<=>P{X=E(X)}=1
协方差及相关系数
定义
- 协方差: C o v ( X , Y ) = E { ( X − E ( X ) ) ( Y − E ( Y ) ) } Cov(X,Y)=E\{(X-E(X))(Y-E(Y))\} Cov(X,Y)=E{(X−E(X))(Y−E(Y))}
- 相关系数:
性质
- C o v ( a X , b Y ) = a b C o v ( X , Y ) Cov(aX,bY)=abCov(X,Y) Cov(aX,bY)=abCov(X,Y)
- C o v ( X 1 + X 2 , Y ) = C o v ( X 1 , Y ) + C 0 v ( X 2 , Y ) Cov(X_1+X_2,Y)=Cov(X_1,Y)+C0v(X_2,Y) Cov(X1+X2,Y)=Cov(X1,Y)+C0v(X2,Y)
- C o v ( X , Y ) = C o v ( Y , X ) , C o v ( X , X ) = D ( X ) Cov(X,Y)=Cov(Y,X), Cov(X,X)=D(X) Cov(X,Y)=Cov(Y,X),Cov(X,X)=D(X)
矩、协方差矩阵
- k阶矩: E ( X k ) E(X^k) E(Xk)
- k阶中心矩: E { [ X − E ( X ) ] k } E\{[X-E(X)]^k\} E{[X−E(X)]k}
- k+l阶混合矩: E ( X k Y l ) E(X^kY^l) E(XkYl)
- k+l阶混合中心矩: E { [ X − E ( X ) ] k } { [ Y − E ( Y ) ] l } E\{[X-E(X)]^k\}\{[Y-E(Y)]^l\} E{[X−E(X)]k}{[Y−E(Y)]l}
- 协方差矩阵