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Limit Theorem
极限定理
Probability and Mathematical Statistics
(概率与数理统计)
Xi ZHANG
In many cases, we don’t need to calculate exactly the probability but roughly know it
? Especially when the probability is very large or very small
? e.g P{haze tomorrow in Lhasa} = ?
? For example, suppose tossing a coin 1000 times, we would like to know if the probability of consecutive 17
appearance of heads
? Let ??be the number of occurrences of 17 consecutive heads in 1000 coin flips.
N = I + … + I
1 984
E [I ] = P(I = 1) = 1/217
i i
E [N ] = 984 ?1/217=0.007507
Outlines
? Chebyshev’s Inequality and the Weak Law of Large Numbers (切比雪夫不
等式及弱大数定律)
? The Central Limit Theorem( 中心极限定理)
? The Strong Law of Large Numbers (强大数定律)
? Summary
Markov’s Inequality (马尔可夫不等式)
Proposition: Markov’s Inequality
If ???is a random variable that takes only nonnegative values, then, for any
value ??0
E X
? ?
P?X ? a??
a
Hence, P [N ≥ 1] ≤ E [N ] / 1 ≤ 0.75%.
E [X ] = E [X | X ≥ a ] P(X ≥ a) + E [X | X a ] P(X a)
≥ a ≥ 0 ≥ 0
E [X ] ≥ a P(X ≥ a) + 0.
Chebyshev’s Equality (切比雪夫不等式)
Proposition: Chebyshev’s Inequality
2
???is a random variable with finite mean ???and variance ??? , the
n, for any value ?? 0,
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