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Expectation

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Definition
ProbabilityStochasticProcesses

The expectation is the “weighted sum” of the “possible values” that XX can take on. Discrete The expectation or expected value of a discrete RV XX with range X\mathscr{X} and pmf pp is E[X]=xXx p(x)=xXx P(X=x)E[X]=\sum_{x\in\mathscr{X}}x\ p(x)=\sum_{x\in\mathscr{X}}x\ P(X=x) Continuous The expectation or expected value of a continuous RV XX with range X\mathscr{X} and pdf ff is E[X]=ΩXdP=Xxf(x) dxE[X]=\int\limits _{\Omega}X \, dP =\int_{\mathscr{X}}xf(x) \ dxonly if:E[X]=xf(x) dx<E[|X|]=\int|x|f(x) \ dx<\infty i.e. XX is integrable or XL1(Ω,F,P)X\in\mathscr{L}^{1}(\Omega,\mathcal{F},P).

Let (Ω,F,P)(\Omega,\mathcal{F},P) be a Probability Space. 1. Linearity: X,YL1(Ω,F,P)\forall X,Y\in\mathscr{L}^{1}(\Omega,\mathcal{F},P)E[αX+βy]=αE[X]+βE[Y]E[\alpha X+\beta y]=\alpha E[X]+\beta E[Y] 2. Monotonicity: XL1(Ω,F,P)\forall X\in\mathscr{L}^{1}(\Omega,\mathcal{F},P) X0 a.s.    E[X]0 a.s.X\ge0\text{ a.s.}\implies E[X]\ge0\text{ a.s.} 3. MCT: Let (Xn)nN(X_{n})_{n\in\mathbb{N}} be a sequence of rvs where Xn0,nNX_{n}\ge0,\forall n\in\mathbb{N} and XnXX_{n}\uparrow X as nn\to\infty then E[Xn]E[X] as nE[X_{n}]\uparrow E[X]\text{ as }n\to\infty 4. Fatou’s Lemma: Let (Xn)nN(X_{n})_{n\in\mathbb{N}} be rv where Xn0,nNX_{n}\ge0,\forall n\in\mathbb{N} then E[lim infnXn]lim infnE[Xn]E\left[\liminf_{n \to \infty }X_{n} \right]\le \liminf_{ n \to \infty } E[X_{n}] 5. Dominated Convergence Theorem: Let (Xn)nNL1(Ω,F,P)(X_{n})_{n\in\mathbb{N}}\subset \mathscr{L}^{1}(\Omega,\mathcal{F},P), let YL1(Ω,F,P)Y\in\mathscr{L}^{1}(\Omega,\mathcal{F},P) and assume XnY, nN|X_{n}|\le Y, \ \forall n\in\mathbb{N}. Let X:ΩRX:\Omega\to \mathbb{R} s.t. XnXX_{n}\to X. Let GF\mathcal{G}\subset \mathcal{F} sub-σ-algebra. Then XL1X\in \mathscr{L}^{1}and E[XnG]L1E[XG]E[X_{n}|\mathcal{G}]\xrightarrow{L^{1}} E[X|\mathcal{G}]

Let f:XRf:\mathscr{X}\to\mathbb{R}, g:YRg:\mathscr{Y}\to\mathbb{R}. If XX and YY are independent, E[f(X)g(Y)]=E[f(X)]E[g(Y)]E[f(X)g(Y)]=E[f(X)]E[g(Y)]

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