Nash Equilibrium - Thinking Like Computers - Lecture Slides, Slides of Artificial Intelligence

During the course work of Thinking Like Computers, we study the key concept of artificial intelligence. The main points in these lecture slides are:Nash Equilibrium, Payoff Matrix, Symmetric Interaction, Game of Chicken, Multi-Agent Interactions, Design Protocols, Mechanism Design, Pirates’ Democracy, Auctioneer and Bidders, Vickrey Auctions, Lies and Collusion

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2012/2013

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CSCI 100
Think Like Computers
Lecture 26
Fall 2008
Last Time …
Nash Equilibrium
Prisoner’s Dilemma
Other 2x2 symmetric game
Stag hunt
Identify a Nash Equilibrium
in the Payoff Matrix
Y9
X9
Y8
X8
Y7
X7
C
Y5
X5
Y2
X2
V
Y6
X6
Y4
X4
B
Y3
X3
Y1
X1
A
WU
Each Nash Equilibrium: max X for its column AND max Y for its row
2x2 Symmetric Interaction
W
W
Y
Z
C
Z
Y
X
X
D
CD
Prisoner’s dilemma: Y > W > X > Z
One Nash equilibrium: (D, D)
3
3
5
0
C
0
5
2
2
D
CD
The Game of Chicken
To establish who is bravest out of two
young thugs.
Both drive their cars at high speed towards
a cliff.
The least brave of the two (the ‘chicken’)
will be the first to drop out of the game by
steering away from the cliff.
Chicken
2
2
3
1
C (steer away)
1
3
0
0
D
C (steer away)D
Nash equilibrium?
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CSCI 100

Think Like Computers

Lecture 26

Fall 2008

Last Time …

  • Nash Equilibrium
  • Prisoner’s Dilemma
  • Other 2x2 symmetric game Š Stag hunt

Identify a Nash Equilibrium

in the Payoff Matrix

Y

X

Y

X

Y

X

C

Y

X

Y

X

V

Y

X

Y

X

B

Y

X

Y

X

A

U W

Each Nash Equilibrium: max X for its column AND max Y for its row

2x2 Symmetric Interaction

W

W

Y

Z

C

Z

Y

X

X

D

D C

Prisoner’s dilemma: Y > W > X > Z One Nash equilibrium: (D, D) 3 3

5 0

C

0 5

2 2

D

D C

The Game of Chicken

  • To establish who is bravest out of two

young thugs.

  • Both drive their cars at high speed towards

a cliff.

  • The least brave of the two (the ‘chicken’)

will be the first to drop out of the game by

steering away from the cliff.

Chicken

2 2

3 1

C (steer away)

1 3

0 0

D

D C (steer away)

Nash equilibrium?

Multi-Agent Interactions

  • The key questions: how do we cooperate?
  • An obvious problem: reaching agreements in a society of self-interested agents.
  • Some interactions are zero-sum. Š What does this mean?
  • However, most other scenarios are not so extreme: Š To reach mutually beneficial agreement on matters of common interest. (Win-win!)

Multi-agent Interactions

  • Negotiation
  • Argumentation
  • They do not occur in a vacuum – they will

be governed by a particular mechanism, or

protocol.

Š “rules of encounter” Š Social norms

Two issues

    1. Design protocols so that the interactions

would have desired outcomes

Š This is known as “mechanism design”

    1. Given a particular protocol, how can a

particular strategy be developed for an

agent so that it can maximize its own

welfare

JFK Quote

  • Ask not what your country can do for you – ask what you can do for your country Š Some retorts: ask not what we can do for our country, because we have done enough. Let's ask our country what they can do for us.
  • Philosophically: Š Self-interested vs Self-less
  • Both are not quite proper things to ask … Š How to make the system better so that every one can do his/her best

Why Mechanism Design?

  • Desired properties of good mechanisms: Š Guaranteed success Š Maximizing social welfare Š Pareto efficiency – can’t happier without hurting anyone else Š Individual rationality (to play by rules) Š Stability (Nash equilibrium for example) Š Simplicity Š Distribution (no single point of failure)

Some Interesting Problems

  • How to divide a cake? Š 2 people? Š N people?

Lies and Collusion

  • These protocols are not immune to lying

and collusion by both bidders and

auctioneers.

  • Examples: Š Grand coalition of bidders Š Auctioneer lying in Vickery auctions Š Shills