# Nash Equilibrium. Ichiro Obara. January 11, 2012 UCLA. Obara (UCLA) Nash Equilibrium January 11, / 31

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1 Nash Equilibrium Ichiro Obara UCLA January 11, 2012 Obara (UCLA) Nash Equilibrium January 11, / 31

2 Best Response and Nash Equilibrium In many games, there is no obvious choice (i.e. dominant action). In such games, players are in strategic situation: each player s optimal choice depends on the other players choices. Obara (UCLA) Nash Equilibrium January 11, / 31

3 Best Response and Nash Equilibrium Coordination Game B B 1, 1 C 0, 0 C 0, 0 1, 1 Obara (UCLA) Nash Equilibrium January 11, / 31

4 Best Response and Nash Equilibrium Chicken Game A A 0, 0 B 4, 1 B 1, 4 3, 3 Obara (UCLA) Nash Equilibrium January 11, / 31

5 Best Response and Nash Equilibrium Matching Penny One more. H H 1, -1 T -1, 1 T -1, 1 1, -1 Obara (UCLA) Nash Equilibrium January 11, / 31

6 Best Response and Nash Equilibrium We need a solution concept to make a prediction in such situations. The one that is most common one in Economics is Nash Equilibrium In Nash equilibrium, every player plays a best response against the other players simultaneously. Obara (UCLA) Nash Equilibrium January 11, / 31

7 Best Response and Nash Equilibrium Best Response Best Response a i A i is a best response to a i A i if u i (a i, a i ) u i (a i, a i ) for all a i A i For example, B is player 1 s best response to A by player 2 in Chicken game. Obara (UCLA) Nash Equilibrium January 11, / 31

8 Best Response and Nash Equilibrium Nash Equilibrium Nash equilibrium for a strategic game is a profile of actions such that each action is a best response to the other actions. Let B i (a i ) A i be the set of player i s best response actions against a i A i. Here is the formal definition of Nash equilibrium. Nash Equilibrium a = (a1,, a n) A is a Nash Equilibrium if ai B i (a i ) for every i N. Obara (UCLA) Nash Equilibrium January 11, / 31

9 Best Response and Nash Equilibrium Nash Equilibrium How to find a Nash equilibrium? It s easy for games with two players and finite actions. Chicken Game A A 0, 0 B 4, 1 B 1, 4 3, 3 Obara (UCLA) Nash Equilibrium January 11, / 31

10 Best Response and Nash Equilibrium Nash Equilibrium How to find a Nash equilibrium? It s easy for games with two players and finite actions. Player 1 s best response against each of player 2 s strategy = red circle. Chicken Game A A 0, 0 B 4, 1 B 1, 4 3, 3 Obara (UCLA) Nash Equilibrium January 11, / 31

11 Best Response and Nash Equilibrium Nash Equilibrium How to find a Nash equilibrium? It s easy for games with two players and finite actions. Player 1 s best response against each of player 2 s strategy = red circle. Player 2 s best response against each of player 1 s strategy = blue circle. Chicken Game A A 0, 0 B 4, 1 B 1, 4 3, 3 Obara (UCLA) Nash Equilibrium January 11, / 31

12 Best Response and Nash Equilibrium Nash Equilibrium How to find a Nash equilibrium? It s easy for games with two players and finite actions. Player 1 s best response against each of player 2 s strategy = red circle. Player 2 s best response against each of player 1 s strategy = blue circle. NE = (A,B),(B,A). Chicken Game A A 0, 0 B 4, 1 B 1, 4 3, 3 NE Obara (UCLA) Nash Equilibrium January 11, / 31

13 Best Response and Nash Equilibrium Nash Equilibrium (B, B) and (C, C) are NE for Coordination game. There is no NE for MP. A few observations regarding NE and DSE: A dominant strategy equilibrium is a Nash equilibrium. If every player has a strictly dominant action (as in PD), then a profile of strictly dominant actions is the only Nash equilibrium. Obara (UCLA) Nash Equilibrium January 11, / 31

14 Best Response and Nash Equilibrium Is Nash equilibrium reasonable? 1 Nash Equilibrium as Self-Enforcing Behavior: If every player believes that a particular Nash equilibrium is played, then there is no incentive to deviate from it for any player. 2 Nash Equilibrium as a Steady State of Learning/Evolution: Suppose that a player plays the same game repeatedly with different players in a large population. If players behavior converges to a particular action profile, then it is reasonable to think that it must be a Nash equilibrium. 1 1 This may be true even if a player plays the same game with the same player repeatedly. Obara (UCLA) Nash Equilibrium January 11, / 31

15 Best Response and Nash Equilibrium Is Nash equilibrium reasonable? There are some issues, which we will deal with in more detail later. Coordination: There are often many NE. Why should we expect that the expectations of the players are consistent? Equilibrium Selection: Suppose that players play a NE. Even so, as an outsider, we are not sure about which NE is played. Some NE seems to be more reasonable than others. So we need refinement of NE. Non-Nash Behavior: Nash equilibrium is sometimes too strong. In some experiments, people don t play any Nash equilibrium, especially in the short run. We may want to relax the notion of Nash equilibrium or come up with a new notion of equilibrium to capture actual behavior in such games. Obara (UCLA) Nash Equilibrium January 11, / 31

16 Applications Example 1: First Price Auction We discuss a few examples to illustrate the idea of NE. Let s start with FPA. Suppose that n bidders with values v 1 > v 2 >... > v n > 0 submit bids b = (b 1,..., b n ) R n simultaneously for some good. The highest bidder wins and pays his or her own bid. Bidder i s payoff is v i b i when winning the good by bidding b i R +. The winner is determined randomly when tied. The bidders maximize their expected payoffs. Can you find any Nash equilibrium? Who wins? Change the tie-breaking rule: the bidder with the smallest index would win when tied. Can you find any Nash equilibrium? Who wins? Obara (UCLA) Nash Equilibrium January 11, / 31

17 Applications Example 2: Cournot Model of Duopoly Firm 1 and firm 2 produce a homogeneous good. The firms choose their output q = (q 1, q 2 ) R 2 + simultaneously. The market price is determined by the inverse demand function: P(q 1 + q 2 ). Firm i s profit is π i (q) = P(q 1 + q 2 )q i C i (q i ).C i (q i ) is firm i s cost function. Obara (UCLA) Nash Equilibrium January 11, / 31

18 Applications Example 2: Cournot Model of Duopoly Suppose that P(q) = 12 q 1 q 2 (or 0 if negative) and C i (q i ) = 3q i. Find a Nash equilibrium. Consider a simple adaptive process (q 1 (0), q 2 (1), q 1 (2),...) where q i (t) is a best response to q i (t 1). Verify that it converges to a Nash equilibrium given any starting point (q 1 (0)). Consider n firms with the same identical linear cost function and the same inverse demand function. Can you find a Nash equilibrium? Is there any relationship between the market price and the number of firms in equilibrium? Obara (UCLA) Nash Equilibrium January 11, / 31

19 Applications Example 3: Bertrand Model of Duopoly There are two firms 1 and 2 that produce similar, but different products (differentiated products). The firms choose their prices p = (p 1, p 2 ) R + simultaneously. Each firm s demand q i depends on both prices (their products are imperfect substitutes, ex. coffee and tea). Firm i s profit is π i (p) = p i q i (p) C i (q i (p)). Obara (UCLA) Nash Equilibrium January 11, / 31

20 Applications Example 3: Bertrand Model of Duopoly Suppose that q i (p) = a p i + bp j and each firm s marginal cost is c. Assume that b < 2. Find a Nash equilibrium. Obara (UCLA) Nash Equilibrium January 11, / 31

21 Applications Example 4: Voluntary Contribution of Public Good There is a village with n residents. Each resident s wealth is W. Assume that W is large enough. The residents decide simultaneously how much they donate their money, which will be used to provide public goods in the village. Resident i s payoff is W x i + y where x i is player i s private donation and y = n j=1 x j is the amount of public goods. Find a Nash equilibrium. Obara (UCLA) Nash Equilibrium January 11, / 31

22 Applications Example 5: Rent Seeking Game There are n lobbying groups. Only one of them wins and gains V > 0. Group i chooses its effort level x i R + at the cost of cx i 0. The probability that group i wins is Group i s expected payoff is Find a Nash equilibrium. x i n j=1 x j. x i n j=1 x j V cx i. Obara (UCLA) Nash Equilibrium January 11, / 31

23 Applications Example 6: AdAuction M links on a webpage are auctioned among n > M bidders The top link gets the largest number of clicks, then the second top link and so on. Suppose that bidders need only one link and their values per click are v 1 >... > v n. Bidders bid how much they would pay per click. The highest bidder wins the top link and pays the second highest bid. The second highest bidder wins the second link and pays the third highest bid... This ad auction or Pay-per-Click auction is a generalized second price auction (it reduces to SPA when M = 1). Obara (UCLA) Nash Equilibrium January 11, / 31

24 Applications Example 6: AdAuction Bidder i s payoff given b = (b 1,..., b n ) is u i (b) = M k=1 ( 1(b i = b (k) ) v i b (k+1)) x k where 1(b i = b (k) ) = 1 if and only if bidder i s bid is the kth highest bid (otherwise 0) and x k is the number of clicks at the kth link (ignoring any tie). Obara (UCLA) Nash Equilibrium January 11, / 31

25 Applications Example 6: AdAuction Suppose that M = 2 and n = 3. The first link generates 100 click/day. The second link generates 60/day. The value of one click is v1 = \$20, v 2 = \$10, and v 3 = \$5 for bidder 1,2, and 3 respectively. Is it a dominant action to bid v i for bidder i? Find a Nash equilibrium. Obara (UCLA) Nash Equilibrium January 11, / 31

26 Existence Existence of NE There is sometimes no Nash equilibrium (ex. MP, complete information FPA with random tie-breaking). When does a Nash equilibrium exist? Obara (UCLA) Nash Equilibrium January 11, / 31

27 Existence Existence of NE Nash Existence Theorem Suppose that A i is a nonempty compact convex subset of R k and u i is continuous in A and quasi-concave in A i given any a i A i for every i N for strategic game (N, (A i ), (u i )). Then there exists a Nash equilibrium. Obara (UCLA) Nash Equilibrium January 11, / 31

28 Existence Kakutani s Fixed Point Theorem We use Kakutani s fixed point theorem Kakutani s Fixed Point Theorem Let X be a nonempty compact and convex subset of R k and f : X X be a correspondence. Suppose that f is convex-valued and upper hemicontinuous (has a closed graph). Then there exists x such that x f (x ). Note: We always assume that a correspondence s value (f (x) in this case) is not empty. Obara (UCLA) Nash Equilibrium January 11, / 31

29 Existence Proof For any a i, B i (a i ) is nonempty (by continuity and compactness), convex (by quasi-concavity) and upper hemicontinuous (by Maximum theorem). The product B = (B 1,..., B n ) is a correspondence from A to A and is nonempty, convex-valued and upper hemicontinuous. By Kakutani s fixed point theorem there is a fixed point a where ai B i (a i ) for every i N. Clearly a is a NE. Obara (UCLA) Nash Equilibrium January 11, / 31

30 Zero-Sum Game Zero-Sum game A two-player strategic game G is a zero-sum game (a strictly competitive game) if u 1 (a) + u 2 (a) = 0 for every a A. Note: If u1 (a) + u 2 (a) = K for every a A for some constant K (constant-sum game), then we can subtract K from one player s payoff without loss of generality to turn it into a zero-sum game. For this class of games, the set of NE has a simple structure. Obara (UCLA) Nash Equilibrium January 11, / 31

31 Zero-Sum Game Maxmin Maxmin a i A i is a maxminimizer for player i if min u i (ai, a i ) min u i (a i, a i ) a i A i. a i A i a i A i That is, a maximizer is a (part of) solution for max ai min a i A i u i (a i, a i ). A maximizer maximizes the payoff in the worst case scenario. So it may be a reasonable choice when someone makes a very conservative choice. Obara (UCLA) Nash Equilibrium January 11, / 31

32 Zero-Sum Game Minimax Theorem This is a version of Minimax theorem. Minimax Theorem Let G = ({1, 2}, (A i ), (u i )) be a zero-sum game where A i is compact and u i is continuous for every i N. Then a A is a Nash equilibrium if and only if, for i = 1, 2, 1 ai is a maxminimizer. 2 u i (a ) = max ai A i min a i A i u i (a) = min a i A i max ai A i u i (a). Obara (UCLA) Nash Equilibrium January 11, / 31

33 Zero-Sum Game Proof. Suppose that a A is a NE. It s easy to show that u1 (a ) min a2 max a1 u 1 (a) max a1 min a2 u 1 (a). Since u 2 (a 1, a 2 ) u 2(a 1, a 2) for any a 2, u 1 (a 1, a 2) u 1 (a ) for any a 2. Then max a1 min a2 u 1 (a) u 1 (a ) min a2. Hence u 1 (a ) = max a1 min a2 u 1 (a) = min a2 max a1 u 1 (a), which also implies that a1 is a maxminimizer for player 1 (the same for i = 2). Suppose that ai solves max ai min a i u i (a) and u i (a ) = max ai min a i u i (a) = min a i max ai u i (a) for i = 1, 2. The assumption implies u 2 (a 1, a 2 ) max a 2 min a1 u 2 (a) for any a 1. Since u2 (a 1, a 2 ) = u 1(a 1, a 2 ) and max a 2 min a1 u 2 (a) = min a1 max a2 u 2 (a) = max a1 min a2 u 1 (a) u 1 (a1, a 2 ), we have u 1 (a 1, a 2 ) u 1(a 1, a 2 ) for any a 1. The same proof applies to player 2. Hence a is a NE. Obara (UCLA) Nash Equilibrium January 11, / 31

34 Zero-Sum Game Comments. Every NE generates exactly the same payoff for both players in zero-sum games. When there exists a NE for a zero-sum game, we can find it by solving for a maxminimizer for both players. If â and ã are a NE for a zero-sum game, then (â 1, ã 2 ) and (ã 1, â 2 ) are also NE (NE are interchangeable). For a zero-sum game, we cannot distinguish standard utility maximizing players playing a NE and maxminimizing players (if there exists a NE). Obara (UCLA) Nash Equilibrium January 11, / 31

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