Sách GAME THEORY Thomas S. Ferguson

GAME THEORY
Thomas S. Ferguson
University of California at Los Angeles
Contents
Introduction.
References.
Part I. Impartial Combinatorial Games.
1.1 Take-Away Games.
1.2 The Game of Nim.
1.3 Graph Games.
1.4 Sums of Combinatorial Games.
1.5 Coin Turning Games.
1.6 Green Hackenbush.
References.
Part II. Two-Person Zero-Sum Games.
2.1 The Strategic Form of a Game.
2.2 Matrix Games. Domination.
2.3 The Principle of Indifference.
2.4 Solving Finite Games.
2.5 The Extensive Form of a Game.
1
2.6 Recursive and Stochastic Games.
2.7 Continuous Poker Models.
Part III. Two-Person General-Sum Games.
3.1 Bimatrix Games — Safety Levels.
3.2 Noncooperative Games — Equilibria.
3.3 Models of Duopoly.
3.4 Cooperative Games.
Part IV. Games in Coalitional Form.
4.1 Many-Person TU Games.
4.2 Imputations and the Core.
4.3 The Shapley Value.
4.4 The Nucleolus.
Appendixes.
A.1 Utility Theory.
A.2 Contraction Maps and Fixed Points.
A.3 Existence of Equilibria in Finite Games.
2
INTRODUCTION.
Game theory is a fascinating subject. We all know many entertaining games, such
as chess, poker, tic-tac-toe, bridge, baseball, computer games — the list is quite varied
and almost endless. In addition, there is a vast area of economic games, discussed in
Myerson (1991) and Kreps (1990), and the related political games, Ordeshook (1986),
Shubik (1982), and Taylor (1995). The competition between firms, the conflict between
management and labor, the fight to get bills through congress, the power of the judiciary,
war and peace negotiations between countries, and so on, all provide examples of games in
action. There are also psychological games played on a personal level, where the weapons
are words, and the payoffs are good or bad feelings, Berne (1964). There are biological
games, the competition between species, where natural selection can be modeled as a game
played between genes, Smith (1982). There is a connection between game theory and the
mathematical areas of logic and computer science. One may view theoretical statistics as
a two person game in which nature takes the role of one of the players, as in Blackwell and
Girshick (1954) and Ferguson (1968).
Games are characterized by a number of players or decision makers who interact,
possibly threaten each other and form coalitions, take actions under uncertain conditions,
and finally receive some benefit or reward or possibly some punishment or monetary loss.
In this text, we study various models of games and create a theory or a structure of the
phenomena that arise. In some cases, we will be able to suggest what courses of action
should be taken by the players. In others, we hope to be able to understand what is
happening in order to make better predictions about the future.
As we outline the contents of this text, we introduce some of the key words and
terminology used in game theory. First there is the number of players which will be
denoted by n. Let us label the players with the integers 1 to n, and denote the set of
players by N = {1, 2, . , n}. We will study mostly two person games, n = 2, where the
concepts are clearer and the conclusions are more definite. When specialized to one-player,
the theory is simply called decision theory. Games of solitaire and puzzles are examples
of one-person games as are various sequential optimization problems found in operations
research, and optimization, (see Papadimitriou and Steiglitz (1982) for example), or linear
programming, (see Chv´atal (1983)), or gambling (see Dubins and Savage(1965)). There
are even things called “zero-person games”, such as the “game of life” of Conway (see
Berlekamp et al. (1982) Chap. 25); once an automaton gets set in motion, it keeps going
without any person making decisions. We will assume throughout that there are at least
two players, that is, n ≥ 2. In macroeconomic models, the number of players can be very
large, ranging into the millions. In such models it is often preferable to assume that there
are an infinite number of players. In fact it has been found useful in many situations to
assume there are a continuum of players, with each player having an infinitesimal influence
on the outcome as in Aumann and Shapley (1974). In this course, we always take n to be
finite.
There are three main mathematical models or forms used in the study of games, the
extensive form, the strategic form and the coalitional form. These differ in the
3
amount of detail on the play of the game built into the model. The most detail is given
in the extensive form, where the structure closely follows the actual rules of the game. In
the extensive form of a game, we are able to speak of a position in the game, and of a
move of the game as moving from one position to another. The set of possible moves
from a position may depend on the player whose turn it is to move from that position.
In the extensive form of a game, some of the moves may be random moves, such as the
dealing of cards or the rolling of dice. The rules of the game specify the probabilities of
the outcomes of the random moves. One may also speak of the information players have
when they move. Do they know all past moves in the game by the other players? Do they
know the outcomes of the random moves?
When the players know all past moves by all the players and the outcomes of all past
random moves, the game is said to be of perfect information. Two-person games of
perfect information with win or lose outcome and no chance moves are known as combinatorial
games. There is a beautiful and deep mathematical theory of such games. You
may find an exposition of it in Conway (1976) and in Berlekamp et al. (1982). Such a
game is said to be impartial if the two players have the same set of legal moves from each
position, and it is said to be partizan otherwise. Part I of this text contains an introduction
to the theory of impartial combinatorial games. For another elementary treatment of
impartial games see the book by Guy (1989).
We begin Part II by describing the strategic form or normal form of a game. In the
strategic form, many of the details of the game such as position and move are lost; the main
concepts are those of a strategy and a payoff. In the strategic form, each player chooses a
strategy from a set of possible strategies. We denote the strategy set or action space
of player i by Ai, for i = 1, 2, . , n. Each player considers all the other players and their
possible strategies, and then chooses a specific strategy from his strategy set. All players
make such a choice simultaneously, the choices are revealed and the game ends with each
player receiving some payoff. Each player’s choice may influence the final outcome for all
the players.
We model the payoffs as taking on numerical values. In general the payoffs may
be quite complex entities, such as “you receive a ticket to a baseball game tomorrow
when there is a good chance of rain, and your raincoat is torn”. The mathematical and
philosophical justification behind the assumption that each player can replace such payoffs
with numerical values is discussed in the Appendix under the title, Utility Theory. This
theory is treated in detail in the books of Savage (1954) and of Fishburn (1988). We
therefore assume that each player receives a numerical payoff that depends on the actions
chosen by all the players. Suppose player 1 chooses a1 ∈ Ai, player 2 chooses a2 ∈ A2, etc.
and player n chooses an ∈ An. Then we denote the payoff to player j, for j = 1, 2, . , n,
by fj (a1, a2, . , an), and call it the payoff function for player j.
The strategic form of a game is defined then by the three objects:
(1) the set, N = {1, 2, . , n}, of players,
(2) the sequence, A1, . , An, of strategy sets of the players, and
4
(3) the sequence, f1(a1, . , an), . , fn(a1, . , an), of real-valued payoff functions of
the players.
A game in strategic form is said to be zero-sum if the sum of the payoffs to the
players is zero no matter what actions are chosen by the players. That is, the game is
zero-sum if
n


i=1
fi(a1, a2, . , an) = 0
for all a1 ∈ A1, a2 ∈ A2, . , an ∈ An. In the first four chapters of Part II, we restrict
attention to the strategic form of two-person, zero-sum games. Theoretically, such games
have clear-cut solutions, thanks to a fundamental mathematical result known as the minimax
theorem. Each such game has a value, and both players have optimal strategies
that guarantee the value.
In the last three chapters of Part II, we treat two-person zero-sum games in extensive
form, and show the connection between the strategic and extensive forms of games. In
particular, one of the methods of solving extensive form games is to solve the equivalent
strategic form. Here, we give an introduction to Recursive Games and Stochastic Games,
an area of intense contemporary development (see Filar and Vrieze (1997), Maitra and
Sudderth (1996) and Sorin (2002)).
In Part III, the theory is extended to two-person non-zero-sum games. Here the
situation is more nebulous. In general, such games do not have values and players do not
have optimal optimal strategies. The theory breaks naturally into two parts. There is the
noncooperative theory in which the players, if they may communicate, may not form
binding agreements. This is the area of most interest to economists, see Gibbons (1992),
and Bierman and Fernandez (1993), for example. In 1994, John Nash, John Harsanyi
and Reinhard Selten received the Nobel Prize in Economics for work in this area. Such
a theory is natural in negotiations between nations when there is no overseeing body
to enforce agreements, and in business dealings where companies are forbidden to enter
into agreements by laws concerning constraint of trade. The main concept, replacing
value and optimal strategy is the notion of a strategic equilibrium, also called a Nash
equilibrium. This theory is treated in the first three chapters of Part III.
On the other hand, in the cooperative theory the players are allowed to form binding
agreements, and so there is strong incentive to work together to receive the largest total
payoff. The problem then is how to split the total payoff between or among the players.
This theory also splits into two parts. If the players measure utility of the payoff in the
same units and there is a means of exchange of utility such as side payments, we say the
game has transferable utility; otherwise non-transferable utility. The last chapter
of Part III treat these topics.
When the number of players grows large, even the strategic form of a game, though less
detailed than the extensive form, becomes too complex for analysis. In the coalitional
form of a game, the notion of a strategy disappears; the main features are those of a
coalition and the value or worth of the coalition. In many-player games, there is a
tendency for the players to form coalitions to favor common interests. It is assumed each
5
coalition can guarantee its members a certain amount, called the value of the coalition.
The coalitional form of a game is a part of cooperative game theory with transferable
utility, so it is natural to assume that the grand coalition, consisting of all the players,
will form, and it is a question of how the payoff received by the grand coalition should be
shared among the players. We will treat the coalitional form of games in Part IV. There we
introduce the important concepts of the core of an economy. The core is a set of payoffs
to the players where each coalition receives at least its value. An important example is
two-sided matching treated in Roth and Sotomayor (1990). We will also look for principles
that lead to a unique way to split the payoff from the grand coalition, such as the Shapley
value and the nucleolus. This will allow us to speak of the power of various members
of legislatures. We will also examine cost allocation problems (how should the cost of a
project be shared by persons who benefit unequally from it).
Related Texts. There are many texts at the undergraduate level that treat various
aspects of game theory. Accessible texts that cover certain of the topics treated in this
text are the books of Straffin (1993), Morris (1994) and Tijs (2003). The book of Owen
(1982) is another undergraduate text, at a slightly more advanced mathematical level. The
economics perspective is presented in the entertaining book of Binmore (1992). The New
Palmgrave book on game theory, Eatwell et al. (1987), contains a collection of historical
sketches, essays and expositions on a wide variety of topics. Older texts by Luce and
Raiffa (1957) and Karlin (1959) were of such high quality and success that they have been
reprinted in inexpensive Dover Publications editions. The elementary and enjoyable book
by Williams (1966) treats the two-person zero-sum part of the theory. Also recommended
are the lectures on game theory by Robert Aumann (1989), one of the leading scholars of
the field. And last, but actually first, there is the book by von Neumann and Morgenstern
(1944), that started the whole field of game theory.
References.
Robert J. Aumann (1989) Lectures on Game Theory, Westview Press, Inc., Boulder, Colorado.
R. J. Aumann and L. S. Shapley (1974) Values of Non-atomic Games, Princeton University
Press.
E. R. Berlekamp, J. H. Conway and R. K. Guy (1982), Winning Ways for your Mathematical
Plays (two volumes), Academic Press, London.
Eric Berne (1964) Games People Play, Grove Press Inc., New York.
H. Scott Bierman and Luis Fernandez (1993) Game Theory with Economic Applications,
2nd ed. (1998), Addison-Wesley Publishing Co.
Ken Binmore (1992) Fun and Games —A Text on Game Theory, D.C. Heath, Lexington,
Mass.
D. Blackwell and M. A. Girshick (1954) Theory of Games and Statistical Decisions, John
Wiley & Sons, New York.
6
V. Chv´atal (1983) Linear Programming, W. H. Freeman, New York.
J. H. Conway (1976) On Numbers and Games, Academic Press, New York.
Lester E. Dubins amd Leonard J. Savage (1965) How to Gamble If You Must: Inequalities
for Stochastic Processes, McGraw-Hill, New York. 2nd edition (1976) Dover
Publications Inc., New York.
J. Eatwell, M. Milgate and P. Newman, Eds. (1987) The New Palmgrave: Game Theory,
W. W. Norton, New York.
Thomas S. Ferguson (1968) Mathematical Statistics – A decision-Theoretic Approach,
Academic Press, New York.
J. Filar and K. Vrieze (1997) Competitive Markov Decision Processes, Springer-Verlag,
New York.
Peter C. Fishburn (1988) Nonlinear Preference and Utility Theory, John Hopkins University
Press, Baltimore.
Robert Gibbons (1992) Game Theory for Applied Economists, Princeton University Press.
Richard K. Guy (1989) Fair Game, COMAP Mathematical Exploration Series.
Samuel Karlin (1959) Mathematical Methods and Theory in Games, Programming and
Economics, in two vols., Reprinted 1992, Dover Publications Inc., New York.
David M. Kreps (1990) Game Theory and Economic Modeling, Oxford University Press.
R. D. Luce and H. Raiffa (1957) Games and Decisions —I ntroduction and Critical Survey,
reprinted 1989, Dover Publications Inc., New York.
A. P. Maitra ans W. D. Sudderth (1996) Discrete Gambling and Stochastic Games, Applications
of Mathematics 32, S pringer.
Peter Morris (1994) Introduction to Game Theory, Springer-Verlag, New York.
Roger B. Myerson (1991) Game Theory —A nalysis of Conflict, Harvard University Press.
Peter C. Ordeshook (1986) Game Theory and Political Theory, Cambridge University
Press.
Guillermo Owen (1982) Game Theory 2nd Edition, Academic Press.
Christos H. Papadimitriou and Kenneth Steiglitz (1982) Combinatorial Optimization, reprinted
(1998), Dover Publications Inc., New York.
Alvin E. Roth and Marilda A. Oliveira Sotomayor (1990) Two-Sided Matching – A Study
in Game-Theoretic Modeling and Analysis, Cambridge University Press.
L. J. Savage (1954) The Foundations of Statistics, John Wiley & Sons, New York.
Martin Shubik (1982) Game Theory in the Social Sciences, The MIT Press.
John Maynard Smith (1982) Evolution and the Theory of Games, Cambridge University
Press.
7
Sylvain Sorin (2002) A First Course on Zero-Sum Repeated Games, Math´ematiques &
Applications 37, S pringer.
Philip D. Straffin (1993) Game Theory and Strategy, Mathematical Association of America.
Alan D. Taylor (1995) Mathematics and Politics —St rategy, Voting, Power and Proof,
Springer-Verlag, New York.
Stef Tijs (2003) Introduction to Game Theory, Hindustan Book Agency, India.
J. von Neumann and O.Morgenstern (1944) The Theory of Games and Economic Behavior,
Princeton University Press.
John D. Williams, (1966) The Compleat Strategyst, 2nd Edition, McGraw-Hill, New York.