概率论与数理统计 (Spring 2023)/Problem Set 2

目前作业非最终版本！

Problem 1 (Warm-up problems)

• [Function of random variable (I)] Let [math]X[/math] be a random variable and [math]g:\mathbb{R} \to \mathbb{R}[/math] be a continuous and strictly increasing function. Show that [math]Y = g(X)[/math] is a random variable.
• [Function of random variable (II)] Let [math]X[/math] be a random variable with distribution function [math]\max(0,\min(1,x))[/math]. Let [math]F[/math] be a distribution function which is continuous and strictly increasing. Show that [math]Y=F^{-1}(X)[/math] be a random variable with distribution function [math]F[/math].
• [Marginal distribution] Let [math](X_1, X_2)[/math] be a random vector satisfying [math]\mathbf{Pr}[(X_1,X_2) = (0,0)] = \mathbf{Pr}[(X_1,X_2) = (1,0)] = \mathbf{Pr}[(X_1,X_2)=(0,1)]=\frac{1}{3}[/math]. Find out the marginal distribution of [math]X_1[/math].
• [Independence] Show that discrete random variables [math]X[/math] and [math]Y[/math] are independent if and only if [math]f_{X,Y}(x,y) = f_X(x) f_Y(y)[/math], where [math]f_{X,Y}[/math] is the joint mass function of [math](X,Y)[/math], and [math]f_X[/math] (respectively, [math]f_Y[/math]) is the mass function of [math]X[/math] (respectively, [math]Y[/math]).
• [Entropy of discrete random variable] Let [math]X[/math] be a discrete random variable with range of values [math]\mathbb{N}_+[/math] and probability mass function [math]p[/math]. Define [math]H(X) = -\sum_{n \ge 1} p(n) \log p(n)[/math] with convention [math]0\log 0 = 0[/math]. Prove that [math]H(X) \ge 0[/math] using Jensen's inequality.
• [Law of total expectation] Let [math]X \sim \mathrm{Geom}(p)[/math] for some parameter [math]p \in (0,1)[/math]. Calculate [math]\mathbf{E}[X][/math] using the law of total expectation.

Problem 2 (Distribution of random variable)

• [CDF] Let [math]\displaystyle{ X }[/math] be a random variable with cumulative distribution function [math]\displaystyle{ F }[/math].
  1. Show that [math]\displaystyle{ Y = aX+b }[/math] is a random variable where [math]\displaystyle{ a }[/math] and [math]\displaystyle{ b }[/math] are real constants, and express the CDF of [math]\displaystyle{ Y }[/math] by [math]\displaystyle{ F }[/math].
  2. Let [math]\displaystyle{ G }[/math] be the CDF of random variable [math]\displaystyle{ Z:\Omega\rightarrow \mathbb{R} }[/math] and [math]\displaystyle{ 0\leq \lambda \leq 1 }[/math], show that
     • [math]\displaystyle{ \lambda F + (1-\lambda)G }[/math] is a CDF function.
     • The product [math]\displaystyle{ FG }[/math] is a CDF function, and if [math]\displaystyle{ Z }[/math] and [math]\displaystyle{ X }[/math] are independent, then [math]\displaystyle{ FG }[/math] is the CDF of [math]\displaystyle{ \max\{X,Z\} }[/math].
• [PMF] We toss [math]\displaystyle{ n }[/math] coins, and each one shows heads with probability [math]\displaystyle{ p }[/math], independently of each of the others. Each coin which shows head is tossed again. (If the coin shows tail, it won't be tossed again.) Let [math]\displaystyle{ X }[/math] be the number of heads resulting from the second round of tosses, and [math]\displaystyle{ Y }[/math] be the number of heads resulting from all tosses, which includes the first and (possible) second round of each toss.
  1. Find the PMF of [math]\displaystyle{ X }[/math] and [math]\displaystyle{ Y }[/math].
  2. Find [math]\displaystyle{ \mathbb{E}[X] }[/math] and [math]\displaystyle{ \mathbb{E}[Y] }[/math].
  3. Let [math]\displaystyle{ f }[/math] be the PMF of [math]\displaystyle{ X }[/math], show that [math]f(k-1)f(k+1)\leq f(k)^2[/math] for [math]\displaystyle{ 1\leq k \leq n-1 }[/math].
• [PDF] Let [math]p(x) = C e^{-x-e^{-x}}[/math]. Find the value of [math]C[/math] so that [math]p[/math] is a probability density function.

Problem 3 (Discrete random variable, 20 points)

[Geometric distribution] Every package of some intrinsically dull commodity includes a small and exciting plastic object. There are [math]c[/math] different types of object, and each package is equally likely to contain any given type. You buy one package each day.

1. Find the expected number of days which elapse between the acquisitions of the [math]j[/math]-th new type of object and the [math](j + 1)[/math]-th new type.
2. Find the expected number of days which elapse before you have a full set of objects.