2Cournot’s Model of Oligopoly Single good produced by n firmsCost to firm i of producing qi units: Ci(qi), where Ci is nonnegative and increasingIf firms’ total output is Q then market price is P(Q), where P is nonincreasingProfit of firm i, as a function of all the firms’ outputs:
3Cournot’s Model of Oligopoly Strategic game:players: firmseach firm’s set of actions: set of all possible outputseach firm’s preferences are represented by its profit
4Example: Duopoly two firms Inverse demand: constant unit cost: Ci(qi) = cqi, where c < a
10q1 = ( a - c - q2)/2 and q2 = (a - c - q1)/2 Example: DuopolyNash equilibrium:Pair (q*1, q*2) of outputs such that each firm’s action is a best response to the other firm’s actionorq*1 = b1(q*2) and q*2 = b2(q*1)Solution:q1 = ( a - c - q2)/2 and q2 = (a - c - q1)/2q*1 = q*2 = (a - c)/3
12(q*1, q*2) = (( a - c)/3, (a - c)/3) Example: DuopolyConclusion:Game has unique Nash equilibrium:(q*1, q*2) = (( a - c)/3, (a - c)/3)At equilibrium, each firm’s profit isp = ((a - c)2)/9Total output 2/3(a- c) lies between monopoly output (a - c)/2 and competitive output a - c.
13Cournot’s Model of Oligopoly Comparison of Nash equilibrium with collusive outcomes. Can both firms do better than the Cournot Nash? Yes! (if they can collude)Exercise 60.2 in OsborneDependence of Nash equilibrium on number of firms. What happens to the Nash quantity and price as the number of firms grows?Exercise 61.1 in Osborne
14Bertrand’s Model of Oligopoly Strategic variable price rather than output.Single good produced by n firmsCost to firm i of producing qi units: Ci(qi), where Ci is nonnegative and increasingIf price is p, demand is D(p)Consumers buy from firm with lowest priceFirms produce what is demanded
15Bertrand’s Model of Oligopoly Strategic game:players: firmseach firm’s set of actions: set of all possible priceseach firm’s preferences are represented by its profit
16Example: Duopoly2 firmsCi(qi) = cqi for i = 1, 2D(p) = a - p
17Nash Equilibrium (p1, p2) = (c, c) Example: DuopolyNash Equilibrium (p1, p2) = (c, c)If each firm charges a price of c then the other firm can do no better than charge a price of c also (if it raises its price it sells no output, while if it lowers its price it makes a loss), so (c, c) is a Nash equilibrium.
18Example: Duopoly No other pair (p1, p2) is a Nash equilibrium since If pi < c then the firm whose price is lowest (or either firm, if the prices are the same) can increase its profit (to zero) by raising its price to cIf pi = c and pj > c then firm i is better off increasing its price slightlyif pi ≥ pj > c then firm i can increase its profit by lowering pi to some price between c and pj (e.g. to slightly below pj if D(pj) > 0 or to pm if D(pj) = 0).
20Hotelling’s Model of Electoral Competition Several candidates run for political officeEach candidate chooses a policy positionEach citizen, who has preferences over policy positions, votes for one of the candidatesCandidate who obtains the most votes wins.
21Hotelling’s Model of Electoral Competition Strategic game:Players: candidatesSet of actions of each candidate: set of possible positionsEach candidate gets the votes of all citizens who prefer her position to the other candidates’ positions; each candidate prefers to win than to tie than to lose.Note: Citizens are not players in this game.
22Example Two candidates Set of possible positions is a (one-dimensional) interval.Each voter has a single favorite position, on each side of which her distaste for other positions increases equally.Unique median favorite position m among the voters: the favorite positions of half of the voters are at most m, and the favorite positions of the other half of the voters are at least m.
23Example Direct argument for Nash equilibrium (m, m) is an equilibrium: if either candidate chooses a different position she loses.No other pair of positions is a Nash equilibrium:If one candidate loses then she can do better by moving to m (where she either wins or ties for first place)If the candidates tie (because their positions are either the same or symmetric about m), then either candidate can do better by moving to m, where she wins.
24The War of Attrition Two parties involved in a costly dispute E.g. two animals fighting over preyEach animal chooses time at which it intends to give upOnce an animal has given up, the other obtains all the preyIf both animals give up at the same time then they split the prey equally.Fighting is costly: each animal prefers as short a fight as possible.Also a model of bargaining between humans.
25The War of Attrition Strategic game players: the two parties each player’s set of actions is [0,∞) (set of possible concession times)player i’s preferences are represented by payoff function
26The War of AttritionIf t1 = t2 then either player can increase her payoff by conceding slightly later (in which case she obtains the object for sure, rather than getting it with probability 1/2 ).If 0 < ti < tj then player i can increase her payoff by conceding at 0.If 0 = ti < tj < vi then player i can increase her payoff (from 0 to almost vi - tj > 0) by conceding slightly after tj.
27The War of AttritionThus there is no Nash equilibrium in which t1 = t2, 0 < ti < tj, or 0 = ti < tj < vi (for i = 1 and j = 2, or i = 2 and j = 1).The remaining possibility is that 0 = ti < tj and tj ≥ vi for i = 1 and j = 2, or i = 2 and j = 1. In this case player i’s payoff is 0, while if she concedes later her payoff is negative; player j’s payoff is vj, her highest possible payoff in the game.
28The War of Attrition In no equilibrium is there any fight There is an equilibrium in which either player concedes first, regardless of the sizes of the valuations.Equilibria are asymmetric, even when v1 = v2, in which case the game is symmetric.
30Second-Price Sealed-Bid Auction Assume every bidder knows her own valuation and every other bidder’s valuation for the good being soldModeleach person decides, before auction begins, maximum amount she is willing to bidperson who bids most winsperson who wins pays the second highest bid.
31Second-Price Sealed-Bid Auction Strategic game:players: biddersset of actions of each player: set of possible bids (nonnegative numbers)preferences of player i: represented by a payoff function that gives player i vi - p if she wins (where vi is her valuation and p is the second-highest bid) and 0 otherwise.
32Second-Price Sealed-Bid Auction Simple (but arbitrary) tie-breaking rule: number players 1, , n and make the winner the player with the lowest number among those that submit the highest bid.Assume that v1 > v2 > · · · > vn.
33Nash equilibria of second-price sealed-bid auction One Nash equilibrium(b1, , bn) = (v1, , vn)Outcome: player 1 obtains the object at price v2; her payoff is v1 - v2 and every other player’s payoff is zero.
34Nash equilibria of second-price sealed-bid auction Reason:Player 1:If she changes her bid to some x ≥ b2 the outcome does not change (remember she pays the second highest bid)If she lowers her bid below b2 she loses and gets a payoff of 0 (instead of v1 - b2 > 0).
35Nash equilibria of second-price sealed-bid auction Players 2, , n:If she lowers her bid she still losesIf she raises her bid to x ≤ b1 she still losesIf she raises her bid above b1 she wins, but gets a payoff vi - v1 < 0.
36Nash equilibria of second-price sealed-bid auction Another Nash equilibrium(v1, 0, , 0) is also a Nash equilibriumOutcome: player 1 obtains the object at price 0; her payoff is v1 and every other player’s payoff is zero.
37Nash equilibria of second-price sealed-bid auction Reason:Player 1:any change in her bid has no effect on the outcomePlayers 2, , n:if she raises her bid to x v1 she still losesif she raises her bid above v1 she wins, but gets a negative payoff vi - v1.
38Nash equilibria of second-price sealed-bid auction For each player i the action vi weakly dominates all her other actionsThat is: player i can do no better than bid vi no matter what the other players bid.
40First-Price Sealed-Bid Auction Strategic game:players: biddersactions of each player: set of possible bids (nonnegative numbers)preferences of player i: represented by a payoff function that gives player i vi - p if she wins (where vi is her valuation and p is her bid) and 0 otherwise.
41Nash Equilibria of First-Price Sealed-Bid Auction (b1, , bn) = (v2, v2, v3, , vn) is a Nash equilibriumReason:If player 1 raises her bid she still wins, but pays a higher price and hence obtains a lower payoff. If player 1 lowers her bid then she loses, and obtains the payoff of 0.If any other player changes her bid to any price at most equal to v2 the outcome does not change. If she raises her bid above v2 she wins, but obtains a negative payoff.
42Nash Equilibria of First-Price Sealed-Bid Auction Property of all equilibriaIn all equilibria the object is obtained by the player who values it most highly (player 1)Argument:If player i ≠ 1 obtains the object then we must have bi > b1.But there is no equilibrium in which bi > b1 :If bi > v2 then i’s payoff is negative, so she can do better by reducing her bid to 0If bi ≤ v2 then player 1 can increase her payoff from 0 to v1 - bi by bidding bi.
43First-Price Sealed-Bid Auction As in a second-price auction, any player i’s action of bidding bi > vi is weakly dominated by the action of bidding vi :If the other players’ bids are such that player i loses when she bids bi, then it makes no difference to her whether she bids bi or viIf the other players’ bids are such that player i wins when she bids bi, then she gets a negative payoff bidding bi and a payoff of 0 when she bids vi
44First-Price Sealed-Bid Auction Unlike a second-price auction, a bid bi < vi of player i is NOT weakly dominated by any bid.It is not weakly dominated by a bid bi’ < biNeither by a bid bi’ > bi
45Revenue EquivalenceThe price at which the object is sold, and hence the auctioneer’s revenue, is the same in the equilibrium (v1, v2, , vn) of the second-price auction as it is in the equilibrium (v2, v2, v3, , vn) of the first-price auction.