El escenario de una nueva monumentalidad .1 Exilio

There is a large literature on repeated games with private monitoring and com-munication. Most papers in the literature focus on a folk theorem rather than robustness. Our approach is similar to Ben-Porath and Kahneman [6]. They prove a folk theorem when a player’s action is perfectly observed by at least two other players. For each individually rational and feasible payo¤ pro…le, they …x a strategy to support it with perfect monitoring, then augment it with a reporting strategy to support the same payo¤ pro…le with direct communication. Their strategies employ draconian punishments when a player’s announcement is inconsistent with others’

announcements (“shoot the deviator”). Our paper di¤ers from their paper in many respects. Firstly, our paper uses not only perfect monitoring but also imperfect public monitoring as a benchmark. Secondly, private signals can be noisy in our paper.

Compte [8] and Kandori and Matsushima [16] consider communication in re-peated games with private monitoring and provide su¢ cient conditions on the pri-vate monitoring structure to obtain a folk theorem. Our su¢ cient conditions are di¤erent from their conditions. Compte [8] assumes that players’ private signals are independent conditional on action pro…les.¹⁵ Example 1 does not satisfy this condition, but we can prove a folk theorem in such environments by combining Proposition 1 and Theorem 3. Kandori and Matsushima [16] assume that, among others, a deviation by one player and a deviation by another player can be statis-tically distinguished based on the private signals of the remaining players. Note that this condition is similar to, but di¤erent from our condition (2) and (3). Their condition and our condition impose the same restriction on the set of probability measures, but they impose it on the marginal distributions of private signals for each subset of n 2 players, whereas we impose it on the public signal distribution that is approximated by the private signal distribution via some public coordination device.

Mailath and Morris [18] study robustness of PPE when a public monitoring structure is perturbed slightly, but without any communication. One of the assump-tions they need for robustness is that private monitoring is almost public. Private monitoring is almost public if Si = Y for all i and jPr (s = (y; :::; y) ja) (yja)j

1 5Obara [23] extends Compte [8]’s result to the case where private signals are correlated.

is small for all a and y. In a subsequent work, Mailath and Morris [20] intro-duced a weaker notion of approximation called "-closeness, which does not as-sume Si = Y . Their de…nition of "-closeness (see De…nition 2 in [20]) implies that max_a;yjPr (8i; fi(s_i) = yja) (yja)j " for all a and y given some f_i : S_i ! Y f;g ; i 2 N; which map private signals to a public signal. While we allow com-munication to prove the robustness of PPE, our notion of closeness is weaker. In Example 2, p is a 0 approximation of and is 0 regular, but it is not close to in their sense. Furthermore, we can show that, when p is " close to in their sense, there exists a public coordination device such that p is "(jY j + 1) approximation of .¹⁶ They show that a certain class of PPE without bounded recall is not robust to small perturbations of public monitoring. Since there exist uniformly strict PPE within this class that are robust in our sense with respect to more general pertur-bations, our result suggests that communication is essential for the robustness of a certain class of PPE.

Fudenberg and Levine [11] prove a folk theorem for repeated games with private monitoring and communication when private monitoring is almost perfect messag-ing. Our folk theorem allows for more general perturbations because almost perfect messaging is similar to the above weak version of "-closeness in Mailath and Morris [18]. On the other hand, their result can be applied to two player games, whereas we need at least three players to guarantee every player is informationally small.

Aoyagi [4] proves a Nash-threat folk theorem in a setting similar to Ben-Porath and Kahneman [6], but with noisy private monitoring. In his paper, each player is monitored by a subset of players. Private signals are noisy and re‡ect the action

1 6To see this, let

of the monitored player very accurately when they are jointly evaluated. That is, private monitoring is jointly almost perfect. In his paper, players have access to a more general communication device than ours, namely, mediated communication.

Anderlini and Laguno¤ [3] consider dynastic repeated games with communica-tion where short-lived players care about their o¤springs. As in our paper, players may have an incentive to conceal bad information so that future generations do not su¤er from mutual punishments. Their model is based on perfect monitoring and their focus is on characterizing the equilibrium payo¤ set rather than establishing the robustness of equilibria or proving a folk theorem.

There is an extensive literature dealing with repeated games with private moni-toring and without communication, starting with Sekiguchi [27]. Many folk theorems have been obtained in this domain by imposing strong assumptions on the private monitoring structure. Bhaskar and Obara [7], Ely and Välimäki [9], Piccioni [24]

prove a folk theorem for a repeated prisoner’s dilemma game with private almost-perfect monitoring. Matsushima [19] proves a folk theorem for a repeated prisoner’s dilemma game with conditionally independent private monitoring. Mailath and Morris [18] prove a folk theorem for general repeated games with almost-perfect and almost-public private monitoring. Hörner and Olszewski [15] prove a folk the-orem for general repeated games with private almost-perfect monitoring.

7 Appendix

A. Preliminary Lemma

Here we prove several useful lemmas. First we derive a few upper bounds on player i’s ability to manipulate the distribution of a public coordinating signal.

Lemma 5 p ( ja; sⁱ) p ( ja; sⁱ; s⁰_i) 1 +p

2 v_i (si; a) for all s⁰_i; si 2 Sⁱ; a 2 A and i 2 N:

Proof.

p ( ja; si) p ja; si; s⁰_i E ( js) ( js⁰i; s _i) ja; si (Jensen’s inequality) 1 v_i (si; a) v_i (si; a) + v_i (si; a) max

c;d24(Y )kc dk 1 +p

2 v_i (s_i; a)

Lemma 6 p ( ja) p ( ja; i) 1 +p

2 v_i (si; a) for all _i; a 2 A and i 2 N:

Proof.

p ( ja) p ( ja; i) X

si2Si

p (s_ija) p ( ja; si) p ( ja; si; _i(s_i)) 1 +p

2 v_i (s_i; a)

The next lemma provides an upper bound on player i’s distributional variability.

Lemma 7

i (s_i; a) 2 1 p ( ja; si) p ( ja; s⁰i) kp ( ja; sⁱ)k kp ( ja; s⁰i)k for all si; s⁰_i 6= sⁱ; a 2 A and i 2 N:

Proof.

i (s_i; a) p ( ja; si) kp ( ja; sⁱ)k

p ( ja; s⁰i) kp ( ja; s⁰i)k

2

= 2 1 p ( ja; sⁱ) p ( ja; s⁰i) kp ( ja; si)k kp ( ja; s⁰i)k

B. Proof of Theorem 1

Proof. Fix a private monitoring game (G⁰; p) and 2 (0; 1) : Pick any payo¤

function w : Y ! Rⁿ and a 2 A: Without loss of generality, we will assume that min_y2Y wi(y) = 0 for all i 2 N: First we de…ne a public coordination device ⁰a;w: Next, for any pair of probability distributions _i; _i 2 4 (Y ) ; let

0i;a; _i;

Next let _i;w and _i;w be any pair of probability distributions on Y that satisfy

i;w 2 arg max

That is _i;w is a distribution on Y that maximizes player i’s expected value of w_i and _i;w is a distribution that minimizes player i’s expected value of wi. Finally, de…ne ⁰_a;w := ⁰_a;

w;

w and let

a;w := (1 ) + ⁰_a;w:

We prove the following claim. Note that this completes the proof of Theorem 1 because is chosen independent of w and a:

Claim: Suppose that

If p is regular, then truthful reporting is a Bayesian Nash equilibrium in the one-shot information revelation game G⁰; p; _a;w; w; a :

Proof of Claim: We will prove that, if satis…es the condition of the claim and if p is regular, then

X

We prove this claim in four steps. Fix player i and suppose that player i’s true signal is si; but she dishonestly reports s⁰_i:

Step 1: In this step, we show that X

where the …nal inequality follows from Lemmas 5 and 7 and the Cauchy-Schwartz inequality.

Step 2: We claim that where the …nal inequality follows from Lemma 5.

Step 3: We claim that, if j 6= i; then

where the …nal inequality follows from Lemma 5.

Step 4: Combining Steps 1-3, it follows that

if p is regular for any satisfying

0 < < 1 moni-toring game respectively. De…ne as follows

:= max

i2N;a2Ajgi(a) j:

The discount factor is …xed throughout the proof.

Step 1: Continuation payo¤s are close with truthful reporting when stage games are uniformly close.

Claim: Suppose that 2 is a public strategy in G¹( ) ; = f h^t : h^t 2 denotes player i’s continuation payo¤s after h^t in G¹( ) given and

w_i h^t = (1 )g⁰_i (h^t) + X denotes player i’s continuation payo¤s after h^t in G⁰¹( ; ) given .

Proof: Suppose that 2 is a public strategy in G¹( ) ; is a public

Computing the supremum of the left hand side, we obtain B (1 ) "+ p

jY j "+

B from which it follows that

B 1 +

pjY j 1

!

"

and the proof of the claim is complete.

Step 2: Constructing the public communication device

Claim: Choose any 2 (0; 1) : If satis…es (4) and if is a public coordinating device for which p is regular, then for any pure strategy PPE of G¹( );

there exists a collection of public coordinating devices f h^t : h^t2 Y^t; t 0g, a public communication device = f(1 ) + _h^t : h^t2 Y^t; t 0g and a truthful strategy pro…le = (( ₁; ₁); ::; ( _n; _n)) for G⁰¹_p ( ; ) such that, for each public history h^t; truthful reporting is a Bayesian Nash equilibrium in the one-shot information revelation game

G⁰; p; (1 ) + _ht; w(h^t; ); (h^t)

where fw(h^t) : h^t2 Y^t; t 0g is the collection of continuation payo¤s in G⁰¹p ( ; ) generated by the truthful strategy pro…le :

Proof: As in the proof of Theorem 1, let

i(a; s) := X

y2Y

p (yja; si)

kp ( ja; si)k (yjs) and for any pair of probability distributions _i; _i 2 4 (Y ) ; let

0i;a; _i;

i(yjs) := i(y) _i(a; s) + _i(y)(1 _i(a; s)) and ⁰_{a; ;} :=

Pn i=1 0

i;a; i; i

n :

Let K = (4 (Y ) 4 (Y ))ⁿ and let M denote the product of countably many copies of K indexed by the elements of H := [^{t 0}Y^t: Suppose that is a PPE in G¹( ): For each

= ( h^t _ht2H = ( ₁ h^t ; ₁ h^t ); ::; ( _n h^t ; _n h^t )

h^t2H 2 M where

h^t = ( ₁ h^t ; ₁ h^t ); ::; ( _n h^t ; _n h^t ) 2 R^{2njY j}; de…ne a public communication device as follows:

( ; ) = f(1 ) + _(h^t_{); (h}^t_{); (h}^t₎: h^t2 Y^t; t 0g:

For each i, choose a reporting strategy _i so that _i = ( _i; _i) is truthful. Then the strategy pro…le and the public communication device ( ; ) de…ne contin-uation payo¤s in the game G⁰¹_p ( ; ( ; )) at every public history. For each 2 M;

let w_i h^t; ; denote player i’s continuation payo¤ in G⁰¹_p ( ; ( ; )) at public

history h^t: Next, de…ne for each 2 M a subset ( ) M as follows: ⁰ 2 ( ) if has a …xed point by applying the Fan-Glicksberg …xed point theorem.

First, let X denote the Cartesian product of countably many copies of R^{2njY j} indexed by H := [t 0Y^t. Since R^{2njY j} is a locally convex topological vector space, it follows that X is a locally convex topological vector space with respect to the product topology. (Theorem 5.1 and Lemma 5.54 in Aliprantis and Border [2]).

Since K R^{2njY j} is nonempty, convex and compact, we conclude that M is a nonempty, convex, compact subset of X in the product topology.

Clearly is nonempty, convex valued, and compact valued. So we need only ver-ify that is upper hemicontinuous. Upper hemicontinuity will follow from Berge’s Theorem if we can establish that 7! wi (h^t; y); ; is a continuous real valued function on M for each t and h^t 2 Y^t since the product of upper hemicontinuous correspondences from M to K is upper hemicontinuous with respect to the product topology on X (to prove the last assertion, modify the argument of Theorem 16.28 in [2]). To see that 7! wi (h^t; y); ; is continuous, …rst de…ne

Therefore,

p ^ht( j (h^t)) p

0

ht( j (h^t)) n

pjY jd1( h^t ; ⁰ h^t ):

Next, note that the product topology on X is metrizable (recall that X is a countable product) with well known metric ^d as de…ned, for example, in Theorem 3.24 of [2]. For this metric ^d; the following condition holds: for each > 0 and each t, there exists a > 0 (depending on and t) such that for each s t and each h^s 2 Y^s;

d( ;^ ⁰) < ) d¹( (h^s) ; ⁰(h^s)) < :

That is, we can make (h^s) and ⁰(h^s) as close with respect to d₁ as we wish for all i 2 N and for any h^s with 0 s t by making and ⁰ close enough with respect to ^d. Since

sup

i;h^t; jwi h^t; ; j < 1

and we use a discounted average payo¤ criterion, we conclude that 7! wi (h^t; y); ; is a continuous real valued function on M for each h^t2 H: Therefore, we conclude that : M ! M is nonempty valued, convex valued, compact valued and up-per hemicontinuous. Applying the Fan-Glicksberg …xed point theorem, there exists

2 M such that 2 ( ):

Since

i h^t 2 arg max

q2 (Y )

X

y2Y

q(y)w_i (h^t; y); ; and

i h^t 2 arg min

q2 (Y )

X

y2Y

q(y)w_i (h^t; y); ; :

for every ht2 H and i 2 N; we can apply the claim in the proof of Theorem 1: if

0 < < 1

(1 )p

jY j + 1 +p 2 2p

jY jn

and if p is regular, then truthful reporting is a Bayesian Nash equilibrium in the one-shot information revelation game

G⁰; p; (1 ) + _(ht); (h^t); (h^t); w (h^t; ); ; ) ; (h^t) for every h_t2 H:

De…ning _h^t := _(h^t_{); (h}^t_{); (h}^t₎ and

:= f(1 ) + _(ht); (h^t); (h^t): h^t2 Y^t; t 0g completes the proof of the claim.

Step 3: Checking all one-period deviations.

Fix any > 0: In this step, we …nd " > 0 and > 0 that have the property stated in the theorem: if (G⁰; p; ) is " approximation of (G; ) and p is regular for ; then for any uniformly strict PPE of G¹( ); there exists a public communication device and a truthful reporting strategy such that = ( ; ) is a PPE of G⁰¹_p ( ; ). : Furthermore, suppose that is an uniformly strict PPE of G¹( ): Using (4) and applying step 2, there exists a collection f h^t : h^t 2 Y^t; t 0g and a public communication device = f h^t : h^t2 Y^t; t 0g; where h^t = (1 ) + _ht; such that truthful reporting is optimal in G⁰¹_p ( ; ) given : Next, de…ne a reporting strategy as follows:¹⁷ for h^t2 H; ai 2 Ai; s_i 2 Si;

sijh^t; ai = si if ai= _i(h^t)

any optimal report if a_i6= i(h^t) :

We will show that = ( ; ) is a truthful PPE of the public communica-tion extension G⁰¹_p ( ; ): It is clearly truthful by de…nition. To verify sequential rationality constraints, we apply the principle of optimality and check one-period deviations at every public history h^t2 H at the beginning of each period. We must check two types of deviations: those which involve a deviation at the action stage and those that do not. By construction of , honest reporting is optimal when the equilibrium action is played within the same period, i.e. when (h^t) is played given public history h^t: Consequently, the second type of deviation is not pro…table. To complete the argument, we must show that, for any h^t2 H and any i 2 N; player i cannot pro…tably deviate by …rst choosing an action a_i di¤erent from a_i h^t and then choosing any report including the optimal one .

Abusing notation, let

Since is uniformly strict, at every h^t and for every i 2 N; the following inequality must be satis…ed for every ai 6= i h^t

(1 ) g_i a_i; _i h^t g_i (h^t) + (6)

1 7We do not derive the optimal reporting strategy o¤ the equilibrium path explicitly, as it is not needed for our proof.

We compare the left hand side and the right hand of this inequality with the corre-sponding terms in G⁰¹_p ; : In particular, we will show that

g⁰_i ai; _i h^t g_i⁰ (h^t) gi ai; _i h^t gi (h^t) for any h^t and ai and that

E h

w_i h^t; j h^t i

E h

w_i (h^t; )jaⁱ; _i h^t i

E ^hth

w_i h^t; j h^t i

E ^hth

w_i (h^t; )jai; _i h^t ; _ii for any h^t; ai 6= i(h^t); and _i: Si ! Sⁱ:

To begin, note that (4) implies that 2" < ₃: Since (G⁰; p; ) is " approximation of (G; ); it follows that

g⁰_i a_i; _i h^t g_i⁰ (h^t) g_i a_i; _i h^t g_i (h^t) + 2" (7)

< gi ai; _i h^t gi (h^t) + 3: Next, we show that

E ^ht[w_i h^t; j h^t ] E ^ht[w_i h^t; jai; _i h^t ; _i]

> E[w_i h^t; j h^t ] E[w_i h^t; jai; _i h^t ] 3

To see this, note that player i’s expected loss at h^t in G⁰¹_p ( ; ) satis…es, for any ai6= i h^t and _i;

E ^ht[w_i h^t; j h^t ] E ^ht[w_i h^t; jaⁱ; _i h^t ; _i]

(1 ) E [w_i h^t; j h^t ] (1 ) E [w_i h^t; jaⁱ; _i h^t ; _i] 2 E [w_i h^t; j h^t ] E [w_i h^t; jai; _i h^t ; _i] 4

E [w_i h^t; j h^t ] E [w_i h^t; jai; _i h^t ; _i] 2 1 +

pjY j 1

!

" 4 (By Step 1)

= n

E [w_i h^t; j h^t ] E [w_i h^t; jai; _i h^t ]o +n

E [w_i h^t; jai; _i h^t ] E [w_i h^t; jai; _i h^t ; _i]o 2 1 +

pjY j 1

!

" 4

The …rst term of this expression can be bounded below as follows:

E [w_i h^t; j h^t ] E [w_i h^t; jaⁱ; _i h^t ]

= X

y2Y

w_i h^t; y p (yj h^t ) yj h^t

+X

y2Y

w_i h^t; y yj h^t yjai; _i h^t

+X

y2Y

w_i h^t; y yjaⁱ; _i h^t p yjaⁱ; _i h^t E[w_i h^t; j h^t ] E[w_i h^t; jaⁱ; _i h^t ]

w_i h^t; p ( j h^t ) j h^t

w_i^a h^t; jaⁱ; _i h^t p ( jaⁱ; _i h^t ) E[w_i h^t; j h^t ] E[w_i h^t; jai; _i h^t ] 2 p

Y For the second term, we have

E [w_i h^t; jaⁱ; _i h^t ] E [w_i h^t; jaⁱ; _i h^t ; _i]

= X

y2Y

w_i h^t; y p (yjai; _i h^t ) p yjai; _i h^t ; _i w_i h^t; p ( jaⁱ; _i h^t ) p ( jaⁱ; _i h^t ; _i) 1 +p

2 p

Y v_i (s_i; a) (by Lemma 6) 2 1 +p

2 p

Y (by -regularity and Lemma 7).

Therefore player i’s expected loss at h^t in G⁰¹_p ( ; ) is bounded from below by E[w_i h^t; j h^t ] E[w_i h^t; jai; _i h^t ] (8)

2p

Y + 1 +p

2 2 1 +

pjY j 1

!

" 4

> E[w_i h^t; j h^t ] E[w_i h^t; jai; _i h^t ]

3 (by (5)).

Combining (6), (7) and (8), we conclude that

(1 ) g_i⁰ a_i; _i h^t g_i⁰ (h^t) + 3

< E ^ht[w_i h^t; j h^t ] E ^ht[w_i h^t; jai; _i h^t ; _i]

for every _i; every h^t and every i 2 N. Let ⁰ = ₃: Then we can conclude that the suggested strategy = ( ; ) is a ⁰ uniformly strict truthful PPE of G⁰¹_p ( ; ).

This completes the proof of the theorem.

D. Proof of Theorem 3

Let Q = fq 2 Rⁿj kqk = 1g and eⁱ = (0; 0; ::; 1; :::; 0)^> 2 Q with the ith coordi-nate equal to 1. First we prove two lemmata to prove Lemma 4.

Lemma 8 Suppose that p is distinguishable for some public coordinating device : Then there exists > 0 such that, if p is regular, then for any q 2 Q and a 2 A, there exists : Y ! Rⁿ and another public coordinating device ⁰ that satisfy the following conditions:

(i)

E ⁰ _jja = 0 for j = 1; :::; n (9)

(ii) if 05 jqij < 1 for each i 2 N; then

E ⁰[ _jja] > E ⁰[ _jja⁰j; a j; _j] for all a⁰_j; _j with a⁰_j 6= a^j and for all j 2 N (10) E ⁰[ _jja] E [ _jja; j] for all _j and for all j 2 N (11)

q w (y) = 0 for all y 2 Y (12)

(iii) if jqⁱj = 1 for some i 2 N (hence q^j = 0 for every j 6= i); then

E ⁰[ _jja] > E ⁰[ _jja⁰j; a j; _j] for all a⁰_j; _j with a⁰_j 6= a^j and for all j 2 N (13) E ⁰[ _jja] E [ _jja; j] for all _j and for all j 2 N (14) Proof. Step 1: For each a 2 A and each pair (i; j) with i 6= j; there exist functions x^i;j;+a ; x^i;j;a : Y ! R satisfying the following conditions

E [zx^i;j;z_a ja] > E [zx^i;j;za ja⁰i; a i] for all a⁰_i 6= aⁱ for z = +;

E [x^i;j;z_a ja] > E [x^i;j;za ja⁰j; a j] for all a⁰_j 6= a^j for z = +;

and

jjx^i;j;+a jj = 1 = jjx^i;j;a jj:

Such functions x^i;j;+a and x^i;j;a exist as a consequence of (1)-(3) and an application of the separating hyperplane theorem.

Step 2: We …rst consider the case of (ii). Take any q 2 Q such that jqjj < 1 for any j. This q is …xed throughout step 1-4. Let i be a player such that jqⁱj jq^jj for all j. If q_i< 0, then de…ne x^(a;q): Y ! Rⁿ as follows: for each y 2 Y;

x^(a;q)_j (y) : = x^i;j;+_a (y) if qj 0 and j 6= i x^(a;q)_j (y) : = x^i;j;_a (y) if q_j < 0 and j 6= i x^(a;q)_i (y) : = X

j6=i

qj

q_ix^(a;q)_j (y) :

If q_i> 0, then de…ne x^(a;q): Y ! Rⁿ as follows: for each y 2 Y;

x^(a;q)_j (y) : = x^i;j;_a (y) if q_j 0 and j 6= i x^(a;q)_j (y) : = x^i;j;+_a (y) if q_j < 0 and j 6= i x^(a;q)_i (y) : = X

j6=i

q_j qi

x^(a;q)_j (y) :

From these de…nitions, it follows that q x^(a;q)(y) = 0 for all y 2 Y so that condition (12) is satis…ed:

Step 3: For each s 2 S and a 2 A; let

i(a; s) := X

y2Y

p (yja; si)

kp ( ja; si)k (yjs) as in the proof of Theorem 1. De…ne ⁰_a;x(a;q): S ! (Y ) as

0

a;x^(a;q)= Pn

j=1 0 j;a;x^(a;q)

n where

0

j;a;x^(a;q)(s) = _j(a; s) _j+ 1 _j(a; s) _j

and _j ( _j) is a probability measure on Y that assigns probability zero to any y not a member of arg max_y02Y x^(a;q)_j (y⁰) (arg min_y02Y x^(a;q)_j (y⁰)). Finally, let

a;x^(a;q):= (1 ) + ⁰_a;x(a;q)

for some 2 (0; 1) : Let

1 = minfE [zx^i;j;za ja] E [zx^i;j;z_a ja⁰i; a _i] : for all i; j; a; a⁰_i 6= ai and z = +; g;

2 = minfE [x^i;j;za ja] E [x^i;j;z_a ja⁰j; a _j] for all i; j; a; a⁰_j 6= aj and z = +; g and de…ne

:= minf 1; ₂g:

Note that > 0 and it is de…ned independent of a or q:

Step 4: In this step, we prove that condition (10) hold for x^(a;q) : Y ! Rⁿ if p is regular and the following condition is satis…ed for and :

2 1 +p

2 4 > 0 ( ) We need to show the following for all j :

E ^a;x[x^(a;q)_j ja] > E ^a;x[x^(a;q)_j ja⁰j; a j; ⁰_j] for all a⁰_j; ⁰_j with a⁰_j 6= a^j

To accomplish this, suppose that q_i < 0 and let x = x^(a;q) for notational ease. The case with qi > 0 is similar, thus omitted. First consider j 6= i for which q^j 0 so that xj := x^i;j;+a : For any a⁰_j 6= a^j and _j;

E ^a;x[x_jja] E ^a;x[x_jja⁰j; a _j; _j]

= (1 )

h

E [x^i;j;+_a ja] E [x^i;j;+_a ja⁰j; a j; _j] i

+ h

E ^0a;q[x^i;j;+_a ja] E ^0a;q[x^i;j;+_a ja⁰j; a _j; _j]i

E [x^i;j;+_a ja] E [x^i;j;+_a ja⁰j; a j; _j] 4 (because x^i;j;+_a = 1)

= E [x^i;j;+_a ja] E [x^i;j;+_a ja⁰j; a _j]

+E [x^i;j;+_a ja⁰j; a j] E [x^i;j;+_a ja⁰j; a j; _j] 4 x^i;j;+_a p ( ja⁰j; a j) p ( ja⁰j; a j; _j) 4 1 +p

2 v_i (s_i; a) 4 2 1 +p

2 4

> 0

We can use the exactly same proof for player j 6= i with qj < 0 (so that x_j := x^i;j;a ) to show that

E ^a;x[x_jja] E ^a;x[x_jja⁰j; a _j; _j] 2 1 +p

2 4

> 0 for any a⁰_j 6= a^j and _j:

Finally for player i; the same proof implies that

E ^a;x[zx^i;j;z_a ja] E ^a;x[zx^i;j;z_a ja⁰i; a _i; _i] 2 1 +p

2 4

> 0

for any j; z = +; , and a⁰_i: Hence, whenever qj 6= 0; we obtain E ^a;x[q_jx^(a;q)_j (y) ja] E ^a;x[q_jx^(a;q)_j (y) ja⁰i; a _i; _i] > 0:

Observe that qj 6= 0 for some j 6= i and qⁱ < 0 by assumption. Therefore it follows that

E ^a;x[x^(a;q)_i (y) ja] E ^a;x[x^(a;q)_i (y) ja⁰i; a _i; _i]

= E ^a;x[ X

j6=i

q_j

q_ix^(a;q)_j (y) ja] E ^a;x[ X

j6=i

q_j

q_ix^(a;q)_j (y) ja⁰i; a _i; _i]

> 0:

Step 5: In this step, we prove that condition (11) holds for x^(a;q) : Y ! Rⁿ if p is regular and the following condition is satis…ed for and :

0 < < 1

(1 )p

jY j + 1 +p 2 2p

jY jn ( ):

If ( ) is satis…ed, it follows directly from Theorem 1 that truthful reporting is a Bayesian Nash equilibrium in the one-shot information revelation game G⁰; p; _a;x; x; a for any x and a: Hence we obtain

E ^a;x[x_jja] E ^a;x[x_jja; ⁰j] for all ⁰_j for any a 2 A:

Step 6: Next consider the case of (iii). Take any q 2 Q such that jqij = 1 for any i. Then it immediately follows from Step 1 and Step 3-5 that we can construct x^(a;q) : Y ! Rⁿ that satisfy (13) and (14) in this case. This is because condition (12), which requires payo¤ pro…les to be on a certain hyperplane, is not imposed this time.

Step 7: Finally, choose and small enough so that ( ) and ( ) are satis…ed in each case. Observe that we can choose and independent of a and q: For each a 2 A and q 2 Q; de…ne ⁰ := _a;x and := x^(a;q) E ⁰[x^(a;q)ja]: Then and ⁰ satisfy (9) in addition to (10)-(12) in the case of (ii) and (13)-(14) in the case of (iii). Therefore the lemma is proved.

We need one more lemma to prove Lemma 4.

Lemma 9 Let M Rⁿ be a closed and convex set with an interior point in Rⁿ. Suppose that each boundary point v 2 M is associated with the unique supporting hyperplane and the unique normal vector _v(6= 0) 2 Rⁿ such that _v v _v x for all x 2 M. Then for any point y 2 Rⁿ such that v v > v y; there exists

2 (0; 1) such that (1 ) v + y is in the interior of M for any 2 (0; ) :

Proof. Suppose that this is not the case, i.e. there does not exist such > 0:

Let W = fx 2 Rⁿj9 2 [0; 1] ; x = (1 ) v + yg.

We …rst show W \ intM = ?: First v is not an interior point of M by de…nition.

If (1 ⁰) v + ⁰y is an interior point for any ⁰ 2 (0; 1]: Then (1 ) v + y is an interior point of M for every 2 (0; ⁰) as it is a strictly positive combination of v 2 M and (1 ⁰) v + ⁰y 2 intM: This is a contradiction. Hence W \ intM = ?:

Since W \ intM = ?; we can apply the separating hyperplane theorem for each x = (1 ) v + y 2 W; obtaining (6= 0) 2 Rⁿ such that (a) x x for all x 2 M: Normalize them so that k k = 1: Since x v; it also follows that (b) y v for every > 0 by the de…nition of x = (1 ) v + y.

Take a sequence of _n such that _n > 0 converges to 0 and _n converges to some (6= 0) 2 Rⁿ: Then v x for all x 2 M (from (a)) and y v (from (b)) by continuity.

Finally 6= v follows from _v v > _v y: Hence and _v are di¤erent normal vectors that separate v from M . This is a contradiction.

We now prove Lemma 4, thus completing the proof of Theorem 3.

.

Proof of Lemma 4

Proof. Choose > 0 satisfying the conditions of Lemma 8 and let W intV (G) be a smooth set:We will show that, for any v 2 W; there exists > 0;

2 (0; 1) and an open set U containing v such that U \ W B ( ; W; ) :

Step 1: Suppose that v is a boundary point of W . Let q 2 Q be the vector of utility weights such that v = arg maxv⁰2W q v⁰ and a = arg max_a2Aq g (a) : We …rst show that v is strictly enforceable for some w⁰ : Y ! Rⁿ such that q v >

q w⁰ (y) for any y:

Let : Y ! Rⁿand ⁰ be the payo¤ function and public coordinating device as de…ned in the conditions of Lemma 8 given q and a . Note that, by (ii), we can …nd c > 0 and ⁰ > 0 such that

g_j(a ) + cE ⁰[ _ija ] ⁰ > g_j a_j; a _j + cE ⁰[ _jja⁰j; a _j; _j] (15) for all a⁰_j; _j with a⁰_j 6= aj and for all j 2 N.

Let u ( ) 2 Rⁿ be the payo¤ vector to satisfy v = (1 ) g (a ) + u ( ) for each 2 (0; 1) : De…ne w⁰ : Y ! Rⁿ as follows.

w⁰(y) := u ( ) +1

c (y) :

Then a ; ⁰; w⁰ clearly (1 ) ⁰-enforces the payo¤ pro…le v for every 2 (0; 1) (by (15) and (iii)).

Since W is in the interior of the feasible set,

q g (a ) > q v = q [(1 ) g (a ) + u ( )] :

Hence q g (a ) > q u ( ) for any 2 (0; 1) : Since q (y) = 0 by construction, this implies the desired inequality:

q v > q u ( ) = q w⁰ (y) for all y 2 Y:

Step 2: We show that w⁰ : Y ! intW for large enough : Fix any : Since v = (1 ) g (a ) + u ( ) ; w⁰(y) can be represented by

w⁰(y) = v (1 ) g (a ) +1

c (y) :

Then for any ⁰ 2 ( ; 1) ; we can represent w⁰0(y) as a positive convex combination of v and w⁰(y) as follows.

w⁰0(y) = 1 ⁰

0(1 )w⁰(y) +

0

0(1 )v for any y 2 Y:

Since Y is a …nite set and q v > q w⁰ (y) by step 1, it directly follows from Lemma 9 that w⁰ takes a value in the interior of W for large enough discount factor.

Step 3: Next suppose that v is an interior point of W: In this case, it is clear that v is strictly enforceable with some w⁰ : Y ! Rⁿ and furthermore w⁰ (y) is in the interior of W for any y 2 Y if is close enough to 1.

Step 4: We have shown that, for any v 2 M; there exists ⁰ 2 (0; 1) ; a 2 A;

0 > 0, ⁰and w⁰₀ : Y ! intW such that a ; ⁰; w⁰₀ 1 ^{0 0}-enforces v: We can now choose " > 0 so that v⁰ 2 U = fz 2 R^Njjjz vjj < "g implies that w⁰₀(y) +^v0₀^v 2 intW for each y. Then it follows that each v⁰ 2 W \ U is (1 ⁰) ⁰ -enforced by a ; ⁰; w⁰₀ +^v0₀^v with respect to W and ⁰: Hence each v⁰2 W \ U is (1 ⁰) ⁰-decomposable with respect to W and ⁰: De…ne by := (1 ⁰) ⁰: Then

0 > 0, ⁰and w⁰₀ : Y ! intW such that a ; ⁰; w⁰₀ 1 ^{0 0}-enforces v: We can now choose " > 0 so that v⁰ 2 U = fz 2 R^Njjjz vjj < "g implies that w⁰₀(y) +^v0₀^v 2 intW for each y. Then it follows that each v⁰ 2 W \ U is (1 ⁰) ⁰ -enforced by a ; ⁰; w⁰₀ +^v0₀^v with respect to W and ⁰: Hence each v⁰2 W \ U is (1 ⁰) ⁰-decomposable with respect to W and ⁰: De…ne by := (1 ⁰) ⁰: Then

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