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22
problem set 4from M. Osborne’s
An Introduction to Game theory
To view the problem set
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3
Some (classical) examples of simultaneous games
CCooperate
D defect
C cooperate
3 , 3 0 , 6
D defect 6 , 0 1 , 1
Prisoners’ Dilemma
The ‘D strategy strictly dominates the C strategy
4
Strategy s1 strictly dominates strategy s2 if for all strategies t of the other player
G1(s1,t) > G1(s2,t)
Strategy s1 weakly dominates strategy s2 if for all strategies t of the other player
G1(s1,t) ≥ G1(s2,t)and for some t
G1(s1,t) > G1(s2,t)
weakly
5
1 , 5 2 , 3 7 , 4
3 , 3 4 , 7 5 , 2
X
X
Nash Equilibrium
Successive deletion of dominated strategies
example of strict dominance
6
1 , 0 1 , 4 1 , 0 1 , 4
1 , 2 1 , 2 0 , 3 0 , 3
2 , 3 1 , 4 2 , 3 1 , 4
1 , 2 1 , 2 0 , 3 0 , 3
weak dominance
X
? ?
???an example
7
Successive deletion of
dominated strategies
And Sub-game Perfectness
2
1
2
10 , 3 1 , 41 , 2
1 , 02 , 3
Examplerl
1
2
12
8
2
1
2
10 , 3 1 , 41 , 2
1 , 02 , 3
rl1
2
12
rr lr rl ll
rr
lr
rl
ll
rr lr rl ll
rr 1 , 0 1 , 4 1 , 0 1 , 4
lr 1 , 2 1 , 2 0 , 3 0 , 3
rl 2 , 3 1 , 4 2 , 3 1 , 4
ll 1 , 2 1 , 2 0 , 3 0 , 3
9
2
1
2
10 , 3 1 , 41 , 2
1 , 02 , 3
rl1
2
12
rr lr rl ll
rr 1 , 0 1 , 4 1 , 0 1 , 4
lr 1 , 2 1 , 2 0 , 3 0 , 3
rl 2 , 3 1 , 4 2 , 3 1 , 4
ll 1 , 2 1 , 2 0 , 3 0 , 3
Sub-game perfect equiibrium
( r,l ) ( l,l )
rr lr rl ll
rr 1 , 0 1 , 4 1 , 0 1 , 4
lr 1 , 2 1 , 2 0 , 3 0 , 3
rl 2 , 3 1 , 4 2 , 3 1 , 4
ll 1 , 2 1 , 2 0 , 3 0 , 3
10
rr lr rl ll
rr 1 , 0 1 , 4 1 , 0 1 , 4
lr 1 , 2 1 , 2 0 , 3 0 , 3
rl 2 , 3 1 , 4 2 , 3 1 , 4
ll 1 , 2 1 , 2 0 , 3 0 , 3
2
1
2
10 , 3 1 , 41 , 2
1 , 02 , 3
rl1
2
12
( x , l )
X
X
rr lr rl ll
rr 1 , 0 1 , 4 1 , 0 1 , 4
lr 1 , 2 1 , 2 0 , 3 0 , 3
rl 2 , 3 1 , 4 2 , 3 1 , 4
ll 1 , 2 1 , 2 0 , 3 0 , 3
( x , l )
X X
rr lr rl ll
rr 1 , 0 1 , 4 1 , 0 1 , 4
lr 1 , 2 1 , 2 0 , 3 0 , 3
rl 2 , 3 1 , 4 2 , 3 1 , 4
ll 1 , 2 1 , 2 0 , 3 0 , 3
delete ( x , r ) delete ( x , r )
weakly weakly dominating
X
rr lr rl ll
rr 1 , 0 1 , 4 1 , 0 1 , 4
lr 1 , 2 1 , 2 0 , 3 0 , 3
rl 2 , 3 1 , 4 2 , 3 1 , 4
ll 1 , 2 1 , 2 0 , 3 0 , 3
rr lr rl ll
rr 1 , 0 1 , 4 1 , 0 1 , 4
lr 1 , 2 1 , 2 0 , 3 0 , 3
rl 2 , 3 1 , 4 2 , 3 1 , 4
ll 1 , 2 1 , 2 0 , 3 0 , 3
11
Another example of a simultaneous game
The Stag Hunt
Stag Hare
Stag 2 , 2 0 , 1
Hare 1 , 0 1 , 1
A generalization to n person game:
There are n types of stocks. Stock of type k yields payoff k if at least k individuals chose it, otherwise it yields 0.
12
Another example of a simultaneous game
The Stag Hunt
Stag Hare
Stag 2 , 2 0 , 1
Hare 1 , 0 1 , 1
Stag Hare
Stag 2 , 2 0 , 1
Hare 1 , 0 1 , 1Equilibria
payoff dominant equilibrium
risk dominant equilibrium
14
Yet another example of a simultaneous game
Battle of the sexes
Ballet Boxing
Ballet 2 , 1 0 , 0
Boxing 0 , 0 1 , 2
man
woman
Bach or Stravinsky (BOS)
Ballet Boxing
Ballet 2 , 1 0 , 0
Boxing 0 , 0 1 , 2
Equilibria
15
Yet another example of a simultaneous game
Battle of the sexes
man
womanBallet Boxing
Ballet 2 , 1 0 , 0
Boxing 0 , 0 1 , 2
A generalization to a bargaining situation
16
Nash Demand Game
Two players divide a Dollar.Each demands an amount ≥ 0.Each receives his demand if the total amount demanded is ≤ 1.Otherwise they both get 0.
Demand of player 1Dem
and
of
pla
yer
2
1
18
last example of a simultaneous game
Matching Pennies
head tails
head 1 , -1 -1 , 1
tails -1 , 1 1 , -1
no purepure strategies equilibrium exists
19
last example of a simultaneous game
Matching Pennies
head tails
head 1 , -1 -1 , 1
tails -1 , 1 1 , -1
no purepure strategies equilibrium exists
20
last example of a simultaneous game
Matching Pennies
head tails
head 1 , -1 -1 , 1
tails -1 , 1 1 , -1
no purepure strategies equilibrium exists
Mixed strategies
A player may choose
head with probability
and tails with probability 1-
21
last example of a simultaneous game
Matching Pennies Mixed strategies
head tails
head 1 , -1 -1 , 1
tails -1 , 1 1 , -1
1- head
tails
head 1 , -1 -1 , 1
tails -1 , 1 1 , -1
player 2 mixes:
if player 1 plays ‘head’ his payoff is the lottery:
1 - β β
1 -1
if the payoffs are in terms of his vN-M utility then his utility from the lottery is
β -1 + 1 - β 1 = 1 - 2β
22
last example of a simultaneous game
Matching Pennies Mixed strategies
head tails
head 1 , -1 -1 , 1
tails -1 , 1 1 , -1
1- head
tails
head 1 , -1 -1 , 1
tails -1 , 1 1 , -1
player 2 mixes:
1 - 2β
Similarly, if player 1 plays ‘tails’ his payoff is …….
2β - 1
1 - β β
-1 1
23
last example of a simultaneous game
Matching Pennies Mixed strategies
head tails
head 1 , -1 -1 , 1
tails -1 , 1 1 , -1
1- head tails
head 1 , -1 -1 , 1
tails -1 , 1 1 , -1
player 2 mixes:
1 - 2β
2β - 1
He prefers to play ‘head’ if:
1 - 2β > 2β - 10.5 > β
He prefers to play ‘tails’ if:
2β - 1 > 1 - 2ββ > 0.5
When β = 0.5 player 1 is indifferentbetween the two strategies
and any mix of the two
24player 1’s mix
player 2’s mix
α
(1-α , α)
β
1
1
head tails
Player 1 prefers to play ‘head’ if: 0.5 > ββ > 0.5Player 1 prefers to play ‘tails’ if:
Player 1 is indifferent when β = 0.5
Player 1’sBest Response
function
Player 2’sBest Response
function ??
25player 1’s mix
player 2’s mix
α
β
1
1
Player 1’sBest Response
function
Player 2’sBest Response
function ??
When player 1 plays ‘head’ oftenPlayer 2 prefers to play ‘tails’
Player 2’sBest Response
function
Nash equilibiumα = β= 0
27
Exercises from M. Osborne’s
An Introduction to Game Theory
EXERCISE 30.1 (Variants of the Stag Hunt) Consider variants of the n-hunter Stag Hunt in which only m hunters, with 2 ≤ m < n, need to pursue the stag in order to catch it. (Continue to assume that there is a single stag.) Assume that a captured stag is shared only by the hunters who catch it. Under each of the following assumptions on the hunters’ preferences, find the Nash equilibria of the strategic game that models the situation.
a. As before, each hunter prefers the fraction 1 / n of the stag to a hare
b. Each hunter prefers the fraction 1 / k of the stag to a hare, but prefers a hare to any smaller fraction of the stag, where k is an integer with m ≤ k ≤ n.
The following more difficult exercise enriches the hunters’ choices in the Stag Hunt. This extended game has been proposed as a model that captures Keynes’ basic insight about the possibility of multiple economic equilibria, some of which are undesirable (Bryant 1983, 1994).
Next exercise
28Next exercise
EXERCISE 31.1 (Extension of the Stag Hunt) Extend the n-hunter Stag Hunt by giving each hunter K (a positive integer) units of effort, which she can allocate between pursuing the stag and catching hares. Denote the effort hunter i devotes to pursuing the stag by ei , a nonnegative integer equal to at
most K. The chance that the stag is caught depends on the smallest of all the hunters’ efforts, denoted minj ej. (“A chain is as strong as its weakest link.”)
Hunter i’s payoff to the action profile (e1 . . ., en ) is 2minjej -ei . (She is better
off the more likely the stag is caught, and worse off the more effort she devotes to pursuing the stag, which means the catches fewer hares.) Is the action profile (e, . . . e), in which every hunter devotes the same effort to pursuing the stag, a Nash equilibrium for any value of e? (What is a player’s payoff to this profile? What is her payoff if she deviates to a lower or higher effort level?) Is any action profile in which not all the players’ effort levels are the same a Nash equilibrium? (Consider a player whose effort exceeds the minimum effort level of all players. What happens to her payoff if the reduces her effort level to the minimum?)
29
2.7.5 Hawk-Dove
The Game in the next exercise captures a basic feature of animal conflict.
EXERCISE 31.2 (Hawk-Dove) Two animals are fighting over some prey. Each can be passive or aggressive. Each prefers to be aggressive if its opponent is passive, and passive if its opponent is aggressive; given its own stance, it prefers the outcome in which its opponent is passive to that in which its opponent is aggressive. Formulate this situation as a strategic game and find its Nash equilibria.
Next exercise
30
EXERCISE 34.1 (Guessing two-thirds of the average) Each of three people announces an integer from 1 to K. If the three integers are different, the person whose integer is closest to 2/3 of the average of the three integers wins $1. If two or more integers are the same , $1 is split equally between the people whose integer is closest to 2/3 of the average integer. Is there any integer k such that the action profile (k,k,k), in which every person announces the same integer k, is a Nash equilibrium? (If k ≥ 2, what happens if a person announces a smaller number?) Is any other action profile a Nash equilibrium? (What is the payoff of a person whose number is the highest of the three? Can she increase this payoff by announcing a different number?)
Last excercise
31
Game theory is used widely in political science, especially in the study of elections. The game in the following exercise explores citizens’ costly decisions to vote.
EXERCISE 34.2 (Voter participation) Two candidates , A and B, compete in an election. Of the n citizen, k support candidate A and m (= n - k) support candidate B. Each citizen decides whether to vote, at a cost, for the candidate she supports, or to abstain. A citizen who abstains receives the payoff of 2 if the candidate she supports wins, 1 if this candidate ties for first place , and 0 if this candidate loses. A citizen who votes receives the payoffs 2 - c, 1 - c, and -c in these three cases, where 0 < c < 1.
a. For k = m = 1, is the game the same (except for the names of the actions) as any considered so far in this chapter?
b. For k = m, find the set of Nash equilibria. (Is the action profile in which everyone votes a Nash equilibrium? Is there any Nash equilibrium in which one of the candidates wins by one vote? Is there any Nash equilibrium in which one of the candidates wins by two or more votes?)
c. What is the set of Nash equilibria for k < m?
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