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Let \(\left(X_{n}: n \geq 1\right)\) be independent random variables with continuous common distribution function \(F\). We call \(X_{k}\) a record value for the sequence if \(X_{k}>X_{r}\) for \(1 \leq r

Short Answer

Expert verified
(a) The indicators \( I_k \) are independent. (b) \( \frac{R_m}{\log m} \rightarrow 1 \) as \( m \to \infty \).

Step by step solution

01

Understanding the Event Indicator

The indicator function, \( I_k \), assumes the value 1 if \( X_k \) is a record value and 0 otherwise. A record value \( X_k \) exceeds all prior values in the sequence, i.e., \( X_k > X_r \) for all \( 1 \leq r < k \).
02

Proving Independence of Indicators

To show the independence of \( I_k \), consider the event \( X_k > X_r \) which depends only on the sequence up to \( k-1 \). The distribution function \( F \) governing each independent \( X_n \) ensures that the event \( X_k > X_r \) is independent of any events involving indices past \( k-1 \). Thus, the events forming \( I_k \) are independent. Hence, \( I_k \) are independent random variables.
03

Calculating Expected Value of Each Indicator

The probability that \( X_k \) is a record value is \( \frac{1}{k} \) because \( X_k \) must be the largest among \( k \) independently and identically distributed variables. Therefore, the expected value \( E[I_k] = P(I_k = 1) = \frac{1}{k} \).
04

Summing Expected Values for \( R_m \)

The sum \( R_m = \sum_{k=1}^{m} I_k \) counts the number of record values up to \( m \). By linearity of expectation, \( E[R_m] = \sum_{k=1}^{m} E[I_k] = \sum_{k=1}^{m} \frac{1}{k} \), which is the \( m \)-th harmonic number.
05

Using Asymptotic Behavior of Harmonic Numbers

The harmonic number \( \sum_{k=1}^{m} \frac{1}{k} \) asymptotically approaches \( \log m + \gamma \) as \( m \rightarrow \infty \), where \( \gamma \) is the Euler-Mascheroni constant. Thus, \( E[R_m] \approx \log m \).
06

Proving Convergence of \( \frac{R_m}{\log m} \)

Using the Strong Law of Large Numbers, the actual count \( R_m \) divided by its expected value \( \log m \) converges to 1 almost surely as \( m \rightarrow \infty \). Hence, \( \frac{R_m}{\log m} \rightarrow 1 \) as \( m \rightarrow \infty \).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Record Values
Record values in a sequence of independent random variables serve as benchmarks or milestones. A record value is identified when a particular variable exceeds all the previous variables in the sequence. This means that, for a variable \( X_k \) to be a record, it must be greater than each of the variables \( X_r \) where \( 1 \leq r < k \). Therefore, record values are significant in statistical analysis as they indicate new "records" in a data sequence. Understanding record values allows analysts to observe trends and fluctuations within datasets, noticing when exceptional values occur.
Independence of Events
In probability theory, the independence of events is a crucial concept, emphasizing that the occurrence of one event does not affect the probability of another. When dealing with independent random variables, such as in our exercise, this principle is fundamental. Here, determining that an indicator function of a record value, \( I_k \), is an independent event is pivotal to our solution.
For \( I_k \), the event \( X_k > X_r \) depends solely on the sequence until \( k-1 \) and disregards any succeeding events. This independence is rooted in each variable following a continuous common distribution function \( F \), which separates their occurrences from one another. Such independence facilitates simpler computations and predictions, simplifying probabilistic analyses.
Harmonic Numbers
Harmonic numbers are a series of mathematical values used to approximate and simplify expressions involving multiple terms. For any positive integer \( m \), the \( m \)-th harmonic number is expressed as:
  • \( H_m = 1 + \frac{1}{2} + \frac{1}{3} + \ldots + \frac{1}{m} \)
They simulate the summation of decreasing fractions, growing progressively smaller with \( m \).
The harmonic number's relevance is noted in its asymptotic approximation, \( H_m \approx \log m + \gamma \), where \( \gamma \) is the Euler-Mascheroni constant. This approximation helps simplify computations, especially in large sequences, essentially transitioning between discrete series and continuous logarithmic functions. Harmonic numbers thus provide insight and elegance into solving complex problems like determining the expectation of record counts as the size of datasets increases.
Strong Law of Large Numbers
The Strong Law of Large Numbers (SLLN) is a robust probabilistic theorem declaring that, as the size of a sample increases, the sample mean will converge almost surely to the expected mean of the population. This principle is fundamental in statistical inference, providing guarantees on predictability and reliability with large data sets.
In the context of our problem, as \( m \) grows toward infinity, the ratio \( \frac{R_m}{\log m} \) converges to 1 with almost certainty. This is due to the SLLN, which reassures us that random fluctuations in small samples diminish as samples grow, aligning with expected values. Thus, this legally solidifies the pattern of record values, ensuring they stabilize around predictable expectations as data series extend indefinitely.

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Most popular questions from this chapter

Hewitt-Savage zero-one law. Let \(X_{1}, X_{2} \ldots\) be independent identically distributed random variables. The event \(A\), defined in terms of the \(X_{n}\), is called exchangeable if \(A\) is invariant under finite permutations of the coordinates, which is to say that its indicator function \(I_{A}\) satisfies \(I_{A}\left(X_{1}, X_{2}, \ldots, X_{n} \ldots\right)=I_{A}\left(X_{i_{1}}, X_{h_{2}}, \ldots ., X_{i_{n}}, X_{n+1} \ldots\right)\) for all \(n \geq 1\) and all permutations \(\left(i_{1}, i_{2}, \ldots, i_{n}\right)\) of \((1,2, \ldots, n)\). Show that all exchangeable events \(A\) are such that cither \(P(A)=0\) or \(\mathbb{P}(A)=1\).

Suppose that the sequences \(\left\\{X_{n}: n \geq 1 \mid\right.\) and \(\left[Y_{n}: n \geq 1\right]\) are tail equivalent, which is to say that \(\sum_{n=1}^{\infty} P\left(X_{n} \not Y_{n}\right)<\infty\). Show that: (a) \(\sum_{n=1}^{\infty} X_{n}\) and \(\sum_{n=1}^{\infty} Y_{n}\) converge or diverge together, (b) \(\sum_{n=1}^{\infty}\left(X_{n}-Y_{n}\right)\) converges almost surely, (c) if there exist a random variable \(X\) and a sequence \(a_{n}\) such that \(a_{n} \uparrow \infty\) and \(a_{n}^{-1} \sum_{r=1}^{n} X_{r} \stackrel{\text { a.s }}{\longrightarrow} X\), then $$ \frac{1}{a_{n}} \sum_{r=1}^{n} Y_{r} \stackrel{a s}{\longrightarrow} X $$.

Let \(X_{1}, X_{2}, \ldots\) be independent identically distributed random variables with the common distribution function \(F\), and suppose that \(F(x)<1\) for all \(x\). Let \(\left.M_{n}=\max \mid X_{1}, X_{2}, \ldots, X_{n}\right\\}\) and suppose that there exists a strictly increasing unbounded positive sequence \(a_{1}, a_{2}, \ldots\) such that \(\mathrm{P}\left(M_{n} / a_{n} \leq x\right) \rightarrow H(x)\) for some distribution function \(H\). Let us assume that \(H\) is continuous with \(00 $$ (b) Deduce that if \(x>0\) $$ \frac{1-F(t x)}{1-F(t)} \rightarrow \frac{\log H(x)}{\log H(1)} \quad \text { as } t \rightarrow \infty $$ (c) Set \(x=x_{1} x_{2}\) and make the substitution $$ g(x)=\frac{\log H\left(e^{x}\right)}{\log H(1)} $$ to find that \(g(x+y)=g(x) g(y)\), and deduce that $$ H(x)= \begin{cases}\exp \left(-\alpha x^{-\beta}\right) & \text { if } x \geq 0 \\ 0 & \text { if } x<0\end{cases} $$ for some non-negative constants \(\alpha\) and \(\beta\). You have shown that \(H\) is the distribution function of \(Y^{-1}\), where \(Y\) has a Weibull distribution.

Anscombe's theorem. Let \(\left\\{X_{i}: i \geq 1\right\\}\) be independent identically distributed random variables with zero mean and finite positive variance \(\sigma^{2}\), and let \(S_{n}=\sum_{1}^{n} X_{i}\). Suppose that the integer-valued random process \(M(t)\) satisfies \(t^{-1} M(t) \stackrel{P}{\rightarrow} \theta\) as \(t \rightarrow \infty\), where \(\theta\) is a positive constant. Show that $$ \frac{S_{M(t)}}{\sigma \sqrt{\theta t}} \stackrel{\mathrm{D}}{\rightarrow} N(0,1) \text { and } \frac{S_{M(t)}}{\sigma \sqrt{M(t)}} \stackrel{\mathrm{D}}{\rightarrow} N(0,1) \quad \text { as } t \rightarrow \infty $$ You should not assume that the process \(M\) is independent of the \(X_{i}\).

Let \(\left\\{X_{r}: r \geq 1\right)\) be independent, non-negative and identically distributed with infinite mean. Show that \(\lim \sup _{r \rightarrow \infty} X_{r} / r=\infty\) almost surely. (ii) Let \(\left(X_{r}\right]\) be a stationary Markov chain on the positive integers with transition probabilities, $$ p_{j k}=\left\\{\begin{array}{l} \frac{j}{j+2} & \text { if } k=j+1 \\ \frac{2}{j+2} & \text { if } k=1 \end{array}\right. $$ (a) Find the stationary distribution of the chain, and show that it has infinite mean. (b) Show that lim sup \(_{r \rightarrow \infty} X_{r} / r \leq 1\) almost surely.

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