Section 17.3

Bernoulli's Theorem. Chebyshev's Inequalities

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Problem 6222

Applying the Chebyshev--Markov lemma, indicate a lower bound for the probability P(P)\mathbb {P}\left(P\right) of the inequality

∣mn−12∣<1100,(*) \left|\frac{m}{n} - \frac{1}{2}\right| < \frac{1}{100}, \tag {*}

for nn independent trials, if in each trial the probability equals 12\frac{1}{2}, taking n=10 000n = 10\, 000; n=100 000n = 100\, 000.

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Problem 6223

Applying the Chebyshev--Markov lemma, determine how large nn must be so that the probability P(P)\mathbb {P}\left(P\right) of inequality (*) from problem 6222 (∣mn−12∣<1100\left|\frac{m}{n} - \frac{1}{2}\right| < \frac{1}{100}, for nn independent trials with probability 12\frac{1}{2} in each trial) is greater than 0.99?

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Problem 6224

Applying the Chebyshev--Markov lemma, for n=10 000n = 10\, 000, what upper bound for ∣mn−12∣\left|\frac{m}{n} - \frac{1}{2}\right| can be indicated with probability not less than 0.99?

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Problem 6225

If a player, in throwing a die, wins 4 rubles when 6 comes up and pays 1 ruble in all other cases, what is the probability, applying the Chebyshev--Markov lemma, that in 10000 throws he will lose at least 200 rubles?

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Problem 6226

Assuming that in nn successive independent trials the probabilities of event AA occurring are respectively pi=(12)ip_{i} = \left(\frac{1}{2}\right)^{i}, determine a lower bound for the probability that the number of its occurrences m⩽1m \leqslant 1.

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Problem 6227

Show that, whatever the random variable xx and the given positive number ε\varepsilon may be, the probability of the inequality

εx>t2+lg⁡I(A) \varepsilon x > t^{2} + \lg I \tag {A}

is less than e−t2e^{-t^{2}}, where I=E[eεx]I = \mathbb {E}\left[e^{\varepsilon x}\right].

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Problem 6228

Letting mm be the number of occurrences of event AA in a series of independent trials with respective probabilities pip_{i}, derive from (A) — inequality εx>t2+lg⁡I\varepsilon x > t^{2} + \lg I from problem 6227 — that the probability of the inequality

m−∑i=1npi⩾t2ε+ε∑i=1npiqi⩾2t∑i=1npiqi(B) m - \sum _{i=1}^{n} p_{i} \geqslant \frac{t^{2}}{\varepsilon } + \varepsilon \sum _{i=1}^{n} p_{i}q_{i} \geqslant 2t\sqrt{\sum _{i=1}^{n} p_{i}q_{i}} \tag {B}

is less than e−t2e^{-t^{2}} (if ε⩽1\varepsilon \leqslant 1, t2⩽∑n=1npiqit^{2} \leqslant \sum_{n=1}^{n} p_{i}q_{i}).

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Problem 6229

Show, by the same method as in problem 6228, that the probability of the inequality

∑i=1npi−m⩾2t∑i=1npiqi(C) \sum _{i=1}^{n} p_{i} - m \geqslant 2t\sqrt{\sum _{i=1}^{n} p_{i}q_{i}} \tag {C}

is less than e−t2e^{-t^{2}} (t2⩽∑i=1npiqit^{2} \leqslant \sum_{i=1}^{n} p_{i}q_{i}).

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Problem 6230

Using inequalities (B) and (C) from problems 6228 and 6229, solve problems 6223 and 6225.

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Problem 6231

Show that if p=12p = \frac{1}{2}, then the probability of the inequality

mn⩾t2n \frac{m}{n} \geqslant \frac{t}{\sqrt{2n}}

is less than e−t2e^{-t^{2}}. Apply this to the solution of problem 6224.

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Problem 6232

nn independent trials are performed, where the probability pip_{i} of event AA occurring in the ii-th trial equals (34)i\left(\frac{3}{4}\right)^{i}; determine an upper bound for the probability pp that the number mm of occurrences of AA will be more than 10.

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Problem 6233

The quantity xkx_{k} takes three values: +kβ+k^{\beta }, −kβ-k^{\beta } and 0 with probabilities equal respectively to 12kα\frac{1}{2k^{\alpha }}, 12kα\frac{1}{2k^{\alpha }} and 1−1kα1-\frac{1}{k^{\alpha }}. Prove that the law of large numbers applies to the sum of independent quantities ∑k=1nxk\sum_{k=1}^{n} x_{k}, if α>1\alpha > 1.

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Problem 6234

Generalize the result of the previous problem (problem 6233), by showing that the law of large numbers is applicable to the sum ∑k=1nxk\sum_{k=1}^{n} x_{k} of independent random variables xkx_{k}, defined as follows. The random variable xkx_{k} takes a series of values with probabilities given in the table:

xkx_{k}aka_{k}00−ak-a_{k}
pkp_{k}pkp_{k}1−2pk1-2p_{k}pkp_{k}

The series p1+p2+…+pk+…p_{1}+p_{2}+\ldots +p_{k}+\ldots is convergent.

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