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Probability:Theory and Examples.pdf
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详细说明:概率论非常经典的书籍,包括大数定律、中心极限定理、随机游动、鞅、马尔可夫链、遍历定理和布朗运动。它是一种综合性的处理方法,集中在对应用最有用的结果上。它的哲学是,学习概率的最好方法是看到它的实际行动,所以有200个例子和450个问题。Preface
Some times the lights are shining on me. Other times I can barely see
Lately it occurs to me what a long strange trip its been
ful dead
In 1989 when the first, edition of the book was completed, my sons David and greg
were 3 and 1, and the cover picture showed the Dow Jones at 2650. The last twenty
years have brought many changes but the song remains the same. The title of the
book indicates that as we develop the theory, we will focus our attention on examples
Hoping that the book would be a useful reference for people who apply probability in
their work, we have tried to emphasize the results that are important for applications,
and illustrated their use with roughly 200 examples. Probability is not a spectator
sport. so the book contains almost 450 exercises to challenge the reader and to deepen
their understanding
The fourth edition has two major changes(in addition to a new publisher
(i) The book has been converted from TeX to LaTeX. The systematic use of labels
should eventually climinate problcms with rcfcrcnccs to other points in the text. In
from the third edition to be reintroduced, and a number of new ones to be a dded 2
additiOnl, the picture environent and graphicx package has allowed for the figures lo
(ii) Four sections of the old appendix have been combined with the first three section
of Chapter 1 to Iake a llew first chapter on Measure theory, which should allow the
Dook to be used by people who do not have this background without making the text.
tedious for those who have
AcknlowledgeInents. I aIn always grateful to the Illally people who sent ine
comments and typos. Helping to correct the first edition were David Aldous, Ker
Alcxandcr, Daren Clinc, Ted Cox, Robort Dalang, Joc Glover, David Griffcath, Phil
Ken ross, Byron Schmuland, Steve Samuels, Jon Wellner, and Ruth Williams eres
The third edition benefitted from input from Manel Baucells, Eric blair, Zhen
Qing Chen, Ted Cox, Bradford Crain, Winston Crandall, Finn Christensen. Amir
Dembo, Neil Falkner, Changyong Feng, Brighten Godfrey, Boris Granovsky, Jan
Hannig, Andrew Hayen, Martin Hildebrand, Kyoungmun Jang: Anatole Joffe, Daniel
Kifor, Stevc Kronc, Greg Lawlcr, T.Y. Lcc, Shlomo Levcntal, Torgny Lindvall, Arif
Mardin, Carl Mueller, Robin Pemantle, Yuval Peres, Mark Pinsky, Ross Pinsky, Boris
Pittel, David Pokorny, Vinayak Prabhu, Brett Presnell, Jim Propp, Yossi Schwarz
fuchs, Rami Shakarchi, Lian Shen, Marc Shivers, Rich Sowers, Bob Strain, Tsachy
Weissman, and llao zhan
New helpers for the fourth edition include John Angus, Phillipe Harmony, Adam
Cruz, Ricky Der, Justin Dyer, Piet Groeneboom, Vlad Island, Elena Kosygina, Richard
Laugesen, Sungchul Lee, Shlomo Levental, Ping Li, Fredddy lopez, Piotr Milos,
Davey Owen, Brett Presnell, Alex Smith, Harsha Wabgaonkar, John Walsh, Tsachy
Weissman, Neil Wu, Ofer Zeitouni, Martin Zerner, Andrei Zherebtsov. I apologize to
those whose names have been omitted or are new typos
Family Update. David graduated from Ithaca College in May 2009 with a degree
in print journalism and like many of his peers is struggling to find work. Greg has
one semester to go at mit and is applying to graduate schools in computer science
He says he wants to do research in"machine learning, so perhaps he can wrote a
program to find and correct the typos in my books
After 25 years in Ithaca, my wife Susan and I are moving to Durham next sum
Iner,so I can take a position in the lllath departIment at Duke. Everyone seellIs to
focus on the fact that we are trading very cold winters for hotter summers and a
much longer growing season. but the real attraction is the excellent opportunities for
interdisciplinary research in the Research Triangle
The more things change the more they stay the same: inevitably there will be
typos in the new version. My new coordinates are not yet set but I am sure google
can find me
Rick Durrett, January 2010
Contents
1 Measure Theory
1. 1 Probability spaces
1.2 Distributions
1.3 Random variables
12
1.4 Integration
15
1.5P
ties of the integra. I
21
1.6E
d valu
24
1.6.1 Inequalities
24
1. 6.2 Integration to the limit
1.6.3 Computing Expected values
2
1.7 Product Measures. Fubini's Theorem
31
2 Laws of Large Numbers
37
2.1 Independence
....37
2.1.1 Sufficient Conditions for Independence
2.1.2 Independence, Distribution, and Expect ation
41
2.1.3 Sums of Independent random Variables
2.1.4 Constructing Independent Random Variables
45
2.2 Weak laws of large nuimbers
2.2.2 Triangular Arrays
2.2.3 Truncation
2.3 Borcl-Cantclli
camas
6
2578 ence of Random Series·,…
2.4 Strong
dw c
urge Nuinbers
71
2.5.2 Infinite Meal
73
2.6 Large Deviations*
3 Central limit Theorems
81
3. 1 The De Moivre-Laplace TheoreM
81
3.2 Weak Convergence
3.2.1 Examples
3.2.2 Theory
86
3.3 Characteristic Functions
91
3.3.1 Definition. Inversion formula.
91
3.3.2 Weak Convergence
9
3.3.3 Moments and derivatives
3.3.4 Polya,'s Criterion*k
101
CONTENTS
3.3.5 The moment problem*
103
3.1 Central Limit Theorems
106
3.4.1 i.i.d. Sequences
106
3.4.2 Triangular Arrays
110
3.4.3 Prime di
Erdos-Kac)
3.4.4 Rates of Convergence(Berry-Esseen)*
118
3.5 Local limit theorems
121
3.6 Poisson Convergence
3.6.1 The basic Limit Theorem
126
3.6.2 Two Examples with Dependence
130
3.6.3 Poisson Processes
132
3. 7 Stable Law
135
3.8 Infinitely Divisible Distributions
3.9 Limit Theorems in rd
4 Randoin walk
153
4.1 Stopping times
4.2Re
162
4.3 Visits to 0. Arcsine laws
4.4 Renewal Theory*
5 Martingales
189
5.1 Conditional E
189
Examples
191
5.1.2Pro
roperties
193
51.3R
197
5.2 Martingales, Almost Sure Convergence
198
5.3 Example
204
5.3.1 Bounded increments
204
5.3.2 Polva's Urn Scheme
205
5.3.3 Radon-Nikodym Derivatives
5.3.4 Branching Processes
209
5. 4 Dooh's Inequa lity, Converge
P
5.4.1 Square Integrable Martingales
....216
5.5 Uniform Integrability, Convergence in LI
220
5.6 Backwards Martingales
225
5.7 Optional Stopping theorems
229
6 Markov Chains
233
6.1 Definitions
233
6.2 Example
6.3 Extensions of the Markov Property
6.4 Recurrence and Transience
245
6.5 Stationary Measures
51
6.6 Asymptotic Behavior
260
6.7 Periodicity, Tail o-field
266
6.8 General
270
6.8.1 Recurrence and Transience
6.8.2 Static
274
6.8.3 Convergence Theorem
275
6.8.4 GI/G/1 queue
75
CONTENTS
T Ergodic Theorems
279
7. 1 Definitions and Examples
279
7.2 Birkhoff's Ergodic Theorem
283
7.4 A Subadditive Ergodic Theorems
7. 3 Recurrence
287
290
7.5 Applications
8 Brownian motion
301
8.1 Definition and construction
301
8.2 Markov Property, Blumenthal's 0-1Law
307
8.3 Stopping Times, Strong Markov Property
312
8.4 Path Properites
8. 4.1 Zeros of Brownian motion
316
8.4.2 Hitting times
316
8.4.3 Levy s Modulus of Continuity
319
8.5 Matinga.les
320
8.5.1 Multidimensional brownian motion
324
8.6 Donsker’ s Theoren
326
8.7 Empirical Distributions, Brownian Bridge
333
8.8 Laws of the Iterated Logarithm*
338
a Mcasurc Thcory details
343
A 1 Caratheeodory's Extension Theorem
.343
A2 which sets are measurable?
348
A 3 Kolmogorov' s Extension Theorem
350
A 4 Radon-Nikodym Theorem
352
A.5 Differentiating Under the Integral
355
CONTENTS
Chapter 1
Measure theory
In this chapter, we will recall some definitions and results from measure theory. Our
purpose here is to provide an introduction for readers who have not seen these concept
before and to review that material for those who have. Harder proofs, especially those
that do not contribute much to one's intuition, are hidden away in the appendix
Readers with a solid background in mcasurc thcory can skip Scctions 1.4, 1.5, and
1.7, which were previously part of the appendix
1.1 Probability spaces
Here and throughout the book terms being defined are set in boldface. We begin
with the most basic quantity. A probability space is a triple (Q2, F, P)where Q
is a set of“ outcomes,” f is a set of events,”andP:F→[0,1] is a function that
assigns probabilities to events. We assume that F is a a-field(or a-algebra),i.e,a
(nonempty) collection of subsets of 2 that satisfy
ifA∈then4c∈,and
(i)ifA;∈ F is a countable sequence of sets then U:A∈万.
Here and in what follows, count able means finite or countably infinite. Since (i A
(UiAn, it follows that a o-field is closed under countable intersections. We omit the
last property from the definition to make it easier to check
Without P,(Q2, F)is called a measurable space, i.e., it is a space on which we
can put a measure. A measure is a nonnegative countably additive set function; that
s, a function:→ R with
(i)(A)≥p(0)=0 for all a∈,and
(ii) if Ai E F is a countable sequence of disjoint sets, then
A)=∑(A)
If u(Q2)=l, we call u a probability measure. In this book, probability measures
are usually denoted by P
The next result gives some consequences of the definition of a measure that we
will need later. In all cases we assume that the sets we mention are in F
CHIAPTER 1. MEASURE TIIEORY
Theorem 1.1.1. Let u be a measure on( Q2, F)
(i) monotonicity. If A CB then u(A)1,Bn≡A=Um=1(A1m). Since
the Bn are disjoint and have union A we have using (1) of the definition of measure,
Bm C Am. and (i) of this theorem
1(A)=∑(Bn)∑(An)
m-1
(iii) Let Bn An An-I. Then the Bn arc disjoint and havc Umm== 4,
m=iBm= An
(4)=∑(B
∑风B
u(An)
(iv)A1-An↑A1-Aso(i) impliesμ(41-An)↑p(A1=A). Since Ab b we have
A(Al B)=u(A1 u(B) and it follows that u(An)L u(A)
The simplest setting, which should be familiar from undergraduate probability, is
Example 1.1.1. Discrete probability spaces. Let Q= a countable set, i.e., finite
or countably infinite. Let F= the set of all subsets of Q2. Let
∑p(u)whcp(a)≥0and∑p(a)=1
∈A
A little thought reveals that t his is the most genera. I probability measure on this space.
In many cases when Q2 is a finite set, we have p(w)=1/Q2 where 9= the number
of points in
g
For a simple concrete example that requires this level of generality consider the
astragali, dice used in ancient Egypt made from the ankle bones of sheep. This die
ould comc to rest on the top sidc of the bonc for four points or on the bottom for
three points. The side of the bone was slightly rounded. The die could cone
on a fat and narrow piece for six points or somewhere on the rest of the side for one
point. There is no reason to think that all four outcomes are equally likely so we need
probabilities p1, p3, p4, and p6 to describe P
To prepare for our next definition, we need
Exercise11.1.(i)If丌,i∈Iareσ- fields then∩∈nJis. HIere l≠ is an arbitrary
index set (i. e, possibly uncountable). (ii) Use the result in(i) to show if we are given
a set S2 and a collection of subsets of a, then there is a smallest o-field containing
A. We will call this the a-field generated by A and denote it by a(A)
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