Proper belief revision and rationalizability in dynamic games

In this paper we develop an epistemic model for dynamic games in which players may revise their beliefs about the opponents’ utility functions as the game proceeds. Within this framework, we propose a rationalizability concept that is based upon the following three principles: (1) at every instance of the game, a player should believe that his opponents are carrying out optimal strategies, (2) a player, at information set h, should not change his belief about an opponent’s relative ranking of two strategies s and s′ if both s and s′ could have led to h, and (3) the players’ initial beliefs about the opponents’ utility functions should agree on a given profile u of utility functions. Common belief in these events leads to the concept of persistent rationalizability for the profile u of utility functions. It is shown that for a given game tree with observable deviators and a given profile u of utility functions, every properly point-rationalizable strategy is a persistently rationalizable strategy for u. This result implies that persistently rationalizable strategies always exist for all game trees with observable deviators and all profiles of utility functions. We provide an algorithm that can be used to compute the set of persistently rationalizable strategies for a given profile u of utility functions. For generic games with perfect information, persistent rationalizability uniquely selects the backward induction strategy for every player.     

Stochastic dominance equilibria in two-person noncooperative games

Two-person noncooperative games with finitely many pure strategies are considered, in which the players have linear orderings over sure outcomes but incomplete preferences over probability distributions resulting from mixed strategies. These probability distributions are evaluated according to t-degree stochastic dominance. A t-best reply is a strategy that induces a t-degree stochastically undominated distribution, and a t-equilibrium is a pair of t-best replies. The paper provides a characterization and an existence proof of t-equilibria in terms of representing utility functions, and shows that for large t behavior converges to a form of max–min play. Specifically, increased aversion to bad outcomes makes each player put all weight on a strategy that maximizes the worst outcome for the opponent, within the supports of the strategies in the limiting sequence of t-equilibria.The paper has benefitted from the comments of four referees and an associate editor.     

The influence of the boundary behavior on isometric immersions in the hyperbolic space

This paper studies how the behavior of a proper isometric immersion into the hyperbolic space is influenced by its behavior at infinity. Our first result states that a proper isometric minimal immersion into the hyperbolic space with the asymptotic boundary contained in a sphere reduces codimension. This result is a corollary of a more general one that establishes a sharp lower bound for the sup-norm of the mean curvature vector of a Proper isometric immersion into the Hyperbolic space whose Asymptotic boundary is contained in a sphere. We also prove that a properly immersed hypersurface with mean curvature satisfying sup _{p∈Σ ||H(p)|| < 1} has no isolated points in its asymptotic boundary. Our main tool is a Tangency principle for isometric immersions of arbitrary codimension. This work is partially supported by CAPES, Brazil.     

Schubert cycles, differential forms and D-modules on varieties of unseparated flags

Proper homogeneous G-spaces (where G is semisimple algebraic group) over positive characteristic fields k can be divided into two classes, the first one being the flag varieties G/P and the second one consisting of varieties of unseparated flags (proper homogeneous spaces not isomorphic to flag varieties as algebraic varieties). In this paper we compute the Chow ring of varieites of unseparated flags, show that the Hodge cohomology of usual flag varieties extends to the general setting of proper homogeneous spaces and give an example showing (by geometric means) that the D -affinity of Beilinson and Bernstein fails for varieties of unseparated flags.     

Learning of winning strategies for terminal games with linear-size memory

We prove that if one or more players in a locally finite positional game have winning strategies, then they can find it by themselves, not losing more than a bounded number of plays and not using more than a linear-size memory, independently of the strategies applied by the other players. We design two algorithms for learning how to win. One of them can also be modified to determine a strategy that achieves a draw, provided that no winning strategy exists for the player in question but with properly chosen moves a draw can be ensured from the starting position. If a drawing- or winning strategy exists, then it is learnt after no more than a linear number of plays lost (linear in the number of edges of the game graph). Z. Tuza’s research has been supported in part by the grant OTKA T-049613.     

General Theorem on a Finite Support of Mixed Strategy in the Theory of Zero-Sum Games

A theorem related to the theory of zero-sum games is proved. Rather general assumptions on the payoff function are found that are sufficient for an optimal strategy of one of the players to be chosen in the class of mixed strategies concentrated in at most m + 1 points if the opponent chooses a pure strategy in a finite-dimensional convex compact set and m is its dimension. This theorem generalizes results of several authors, starting from Bohnenblust, Karlin, and Shapley (1950).     

Semi‐proper forcing,remarkable cardinals,and Bounded Martin's Maximum

We show that L(?) absoluteness for semi‐proper forcings is equiconsistent with the existence of a remarkable cardinal, and hence by [6] with L(?) absoluteness for proper forcings. By [7], L(?) absoluteness for stationary set preserving forcings gives an inner model with a strong cardinal. By [3], the Bounded Semi‐Proper Forcing Axiom (BSPFA) is equiconsistent with the Bounded Proper Forcing Axiom (BPFA), which in turn is equiconsistent with a reflecting cardinal. We show that Bounded Martin's Maximum (BMM) is much stronger than BSPFA in that if BMM holds, then for every X ∈ V , X^# exists. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Two-noisy-versus-one-silent duel with equal accuracy functions

This paper deals with the two-noisy-versus-one-silent duel which is still open, as pointed out by Styszyński (Ref. 1). Player I has a noisy gun with two bullets, and player II has a silent gun with one bullet. Each player fires his bullets aiming at his opponent at any time in [0, 1]. The accuracy function (the probability that one player hits his opponent if he fires at timet) isp(t)=t for each player. If player I hits player II, without being hit himself before, the payoff of the duel is +1; if player I is hit by player II, without hitting player II before, the payoff is taken to be ?1. In this paper, we determine the optimal strategies and the value of the game. The strategy for player II depends explicitly on the firing moment of player I's first shot.     

An adaptive algorithm for discrete-time differential games

This paper presents an algorithm for a player to improve his performance by adapting optimally over his non-optimally playing opponent in discrete-time differential games. The algorithm first estimates the opponent's actual strategies and then constructs an adaptive strategy for the player. The adaptive strategy is periodically updated according to the opponent's behavior using the neighboring optimal closed-loop solution technique. An example is given which demonstrates the superiority of this algorithm over the conventional one which assumes that the opponent plays optimally.     

Suboptimality of M-step back strategies in Bayesian games

We consider the Bayes optimal strategy for repeated two player games where moves are made simultaneously. In these games we look at models where one player assumes that the other player is employing a strategy depending only on the previousm-move pairs (as discussed in Wilson, 1986). We show that, under very unrestrictive conditions, such an assumption is not consistent with the assumption of rationality of one's opponent. Indeed, we show that by employing such a model a player is implicitly assuming that his opponent is not playing rationally,with probability one. We argue that, in the context of experimental games, thesem-step back models must be inferior to models which are consistent with the assumption that an opponent can be rational.     

Expenses and underwriting strategy in competition

An earlier paper [Taylor (1986)] developed optimum underwriting strategies under the assumption of constant unit expense rates. In the discussion of the paper, it was suggested that such a strategy might be substantially affected if marginal expense rates were property taken into account. The theory for doing so is developed in Section 2, and further in Section 4.It is noted that the optimal strategy is not affected by the introduction of a component of fixed expenses, irrespective of the size of that component (Section 2.2).Sections 3 and 4 set limits on the effect of expenses on optimal underwriting strategy. The sharpness of these limits depends on:

• 1.the extent of variation in marginal expense rates as demand varies;
• 2.the price-elasticity of demand.

The case in which the marginal expense rate is constant and price-elasticity is directly proportional with price is a particularly simple one. In this case, the optimal premium rates taking expenses into account are precisely equal to the optimal premium rates ignoring expenses, increased by the marginal expense rates.As factors (i) and (ii) depart from this particular case, the behaviour of optimal premium rates with expenses taken properly into account becomes less predictable relative to the optimal rates ignoring expenses.Some numerical examples are examined in Section 6. The empirical results obtained suggest that the general ‘shape’ of the optimal strategy, in terms of the optimal premium rate as a function of time measured between the present and the planning time horizon, is to a large extent unaffected by whether they incorporate proper allowance for (possibly varyong) marginal expenses or approximate these by assumed constant unit expenses.The general level of optimal premiums may, however, be shifted to a material extent by the proper recognition of expenses. It is found that in examples in which the assumption of constant unit expense rates leads to optimal premium rates of substantial negative profitability, the adjustment to reflect marginal expenses properly can cause very significant changes to these low premium rates.     

Auswahl einer optimalen Ersatzstrategie mit Hilfe eines Entscheidungsdiagramms

Zusammenfassung Ein System besteht ausN Geräten, von denenn N gleichzeitig eingesetzt sind. Das System ist nur funktionsfähig, solange allen Geräte arbeiten. Beim Ausfall eines oder mehrerer Geräte muß durch eine geeignete optimale Wartungsstrategie für alsbaldige Wiederaufnahme der Systemfunktion gesorgt werden.Hierfür stehen zwei Strategien zur Auswahl, von denen mit Hilfe eines Entscheidungsdiagramms die billigere ausgewählt wird.Die Strategien werden durch physikalische, strategische und Kostenparameter beschrieben. Ändern sich allein die Kostenparameter, so erspart das Entscheidungsdiagramm die Durchrechnung der abzulehnenden Strategie. Ändern sich außerdem die physikalischen Parameter, so muß ein neues Diagramm errechnet werden.Zur Optimierung der Strategien, d. h. zur Ermittlung der strategischen Parameter, werden analytische Verfahren zitiert. Ein numerisches Beispiel wird vorgeführt.

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Summary A system consists ofN equipments of whichn N are used simultaneously. It is only operational, if all equipments are working properly. In case of failure of one or more equipments a proper optimal strategy of maintenance has to be established in order to make the system operational again as soon as possible.There are two strategies to obtain this. The cheaper one is chosen by use of a decision diagram.The strategies are described by physical, strategic, and cost parameters. If the cost parameters are changing the decision diagram serves to save the computation of the strategy to be rejected. If furthermore the physical parameters are changing a new diagram has to be computed.For optimization of the strategies, i.e. finding out the strategic parameters, analytical methods are cited. A numerical example is given.

     

Strategies in competing subset selection

We address an optimization problem in which two agents, each with a set of weighted items, compete in order to minimize the total weight of their solution sets. The latter are built according to a sequential procedure consisting in a fixed number of rounds. In every round each agent submits one item that may be included in its solution set. We study two natural rules to decide which item between the two will be included. We address the problem from a strategic point of view, that is finding the best moves for one agent against the opponent, in two distinct scenarios. We consider preventive or minimax strategies, optimizing the objective of the agent in the worst case, and best-response strategies, where the items submitted by the opponent are known in advance in each round.     

三步矩阵对策上的无中生有计及其实例

基于一个历史实例及假定:①三步矩阵对策中赢得矩阵都不变,②每步都是局中人1先行动,③对于每步对策,局中人2观测不到对手究竟使用了何策略;但局中人1可以观测到对手所用的策略,建立了三步矩阵对策上的无中生有计(《三十六计》中的第七计)的对策模型.研究了当局中人2中计,半识破和完全识破对手的无中生有计时的赢得和所用的策略的情况.并用上述实例对模型作了说明.     

A characterization of intrinsic reciprocity

This paper studies a game-theoretic model in which players have preferences over their strategies. These preferences vary with the strategic context. The paper further assumes that each player has an ordering over an opponent’s strategies that describes the niceness of these strategies. It introduces a condition that insures that the weight on an opponent’s utility increases if and only if the opponent chooses a nicer strategy.     

Refinements of rationalizability for normal-form games

There exist three equivalent definitions of perfect Nash equilibria which differ in the way “best responses against small perturbations” are defined. It is shown that applying the spirit of these definitions to rationalizability leads to three different refinements of rationalizable strategies which are termed perfect (Bernheim, 1984), weakly perfect and trembling-hand perfect rationalizability, respectively. We prove that weakly perfect rationalizability is weaker than both perfect and proper (Schuhmacher, 1995) rationalizability and in two-player games it is weaker than trembling-hand perfect rationalizability. By means of examples, it is shown that no other relationships can be found. Received: January 1997/final version: August 1998     

Bezout's theorem and Cohen-Macaulay modules

We define very proper intersections of modules and projective subschemes. It turns out that equidimensional locally Cohen-Macaulay modules intersect very properly if and only if they intersect properly. We prove a Bezout theorem for modules which meet very properly. Furthermore, we show for equidimensional subschemes X and Y: If they intersect properly in an arithmetically Cohen-Macaulay subscheme of positive dimension then X and Y are arithmetically Cohen-Macaulay. The module version of this result implies splitting criteria for reflexive sheaves. Received August 26, 1999 / Published online March 12, 2001     

Some Optimal Strategies for Bandit Problems with Beta Prior Distributions

A bandit problem with infinitely many Bernoulli arms is considered. The parameters of Bernoulli arms are independent and identically distributed random variables from a common distribution with beta(a, b). We investigate the k-failure strategy which is a modification of Robbins's stay-with-a-winner/switch-on-a-loser strategy and three other strategies proposed recently by Berry et al. (1997, Ann. Statist., 25, 2103–2116). We show that the k-failure strategy performs poorly when b is greater than 1, and the best strategy among the k-failure strategies is the 1-failure strategy when b is less than or equal to 1. Utilizing the formulas derived by Berry et al. (1997), we obtain the asymptotic expected failure rates of these three strategies for beta prior distributions. Numerical estimations and simulations for a variety of beta prior distributions are presented to illustrate the performances of these strategies.     

On a noisy-silent-versus-silent duel with equal accuracy functions

This paper deals with the noisy-silent-versus-silent duel with equal accuracy functions. Player I has a gun with two bullets and player II has a gun with one bullet. The first bullet of player I is noisy, the second bullet of player I is silent, and the bullet of player II is silent. Each player can fire their bullets at any time in [0, 1] aiming at his opponent. The accuracy function ist for both players. If player I hits player II, not being hit himself before, the payoff of the duel is +1; if player I is hit by player II, not hitting player II before, the payoff is –1. The optimal strategies and the value of the game are obtained. Although optimal strategies in past works concerning games of timing does not depend on the firing moments of the players, the optimal strategy obtained for player II depends explicitly on the firing moment of player I's noisy bullet.     

Extending immersions of curves to properly immersed surfaces

Proper generic immersions of compact one-dimensional manifolds in surfaces are studied. Suppose an immersion γ of a collection of circles is given with an even number of double points in a closed surface G. Then γ extends to various proper immersions of surfaces in three-manifolds that are bounded by G. Some of these extensions do not have triple points. The minimum of the genera of the triple point free surfaces is an invariant of the curve. An algorithm to compute this invariant is given.Necessary and suffecient conditions determine if a given collection δ of immersed arcs in a surface F maps to the double points set of a proper immersion. In case the conditions are satisfied, an immersion of F into a three-manifold that depends on δ is constructed explicitly. In the process, the possible triple points of immersed surfaces in three-manifolds are categorized.The techniques are applied to find examples of curves in surfaces that do not bound immersed disks in any three-manifold.