Question

In: Statistics and Probability

1. Let K be the index of the first successful trial that is immediately followed by...

1. Let K be the index of the first successful trial that is immediately followed by a failure. In symbols, (3.53) K = inf{n ∈ Z>0 : Xn = 1, Xn+1 = 0}. Find the probability mass function of K. Check that your answer is a legitimate probability mass function. Hint. Decompose the event {K = m} into disjoint components expressed in terms of the trial outcome variables {Xi}. Note that a success before m cannot be immediately followed by a failure.

2. Let N be the index of the first success, as defined in (3.18). Let K be the index of the first success that is immediately followed by a failure, as defined in (3.53). (a) Find the probability P(XN+1 = 0, XN+2 = 1, XN+3 = 0). (b) Find the probability P(XK+1 = 0, XK+2 = 1, XK+3 = 0). The task is to find the probability that the three trials immediately following the random index K yield a failure, a success, and a failure, in that order. Explain why your answer makes intuitive sense.

3. Assume that 0 < p < 1. (a) Let Sn ∼ Bin(n, p) count the number of successes in the first n trials. Fix a positive integer k. Show that lim n→∞ P(Sn ≤ k) = 0. (b) Show that in infinitely many trials there are infinitely many successes with probability one.

Solutions

Expert Solution

Derivation of the PMF of K

Derivation of the PMF of K (Continued) + Proof that it is a valid PMF
The PMF of K was derived as :

Proof that it is a valid PMF (Continued) + Solution to 2.(a)

Solution to 2.(a) (Continued) + Solution to 2.(b) + Solution to 3. (a)

Solution to 3.(a) (Continued) using Hoeffding's inequality + Solution to 3.(b)

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