Hopf bifurcation and quasi-periodic dynamics in discrete multisector optimal growth models

This paper discusses the asymptotic stability of the steady state and the existence of a Hopf bifurcation in discrete time multisector optimal growth models. We obtain on the one hand a local turnpike theorem which guarantees the saddle point property for all discount rates. On the other hand, we provide a new proposition which gives some conditions ensuring local stability of the steady state if the impatience rate is not too high. A characterization of the boundδ*, above which the steady state is saddle-point stable, is also proposed in terms of indirect utility function's concavity properties. On this basis, some sufficient conditions for the existence of a Hopf bifurcation are stated. We thus prove the existence of quasi-periodic optimal paths in asymmetric models.     

On the indeterminacy of capital accumulation paths

In neoclassical optimal growth models the stability of the accumulation paths depends on the discount parameter. We prove that, for discount factors small enough, the policy function which describes an optimal path can be of any type. The result is achieved using the notion of α-concavity. We adopt a constructive approach. Given any twice differentiable map we show how to construct an optimal growth problem which produces that map as the optimal policy function. A consequence is that “chaos” can appear in these models. We also provide bounds on the values of the discount parameter for which “indeterminacy” is possible.     

A new turnpike theorem for discounted programs

Summary In this paper we present new results on the local and global convergence property of solutions to an optimization model where the objective function is a discounted sum of stationary one-period utilities. The asymptotic local turnpike is given without differentiability assumptions but imposing some mild curvature restrictions on the utility function. This approach allows us to get easy estimates on the range of discount factors and the size of the neighborhood for which the asymptotic property occurs. The paper concludes by providing two global turnpike theorems. The first one is an asymptotic theorem derived from a result similar to Scheinkman's visit lemma. The second one turns out to be a restatement of McKenzie's neighborhood turnpike theorem.This research was partially supported by MURST, National Group on Nonlinear Dynamics in Economics and Social Sciences.     

Conditions for Sustainable Optimal Economic Development

This paper shows that, for dynamic optimizing economies with different types of natural resource, environmental, and human‐made capital stocks, a necessary and sufficient condition for permanently sustaining an optimal utility/consumption level is the stationarity of the current‐value Hamiltonian. For economies whose development is not exogenously and directly affected by time (i.e., time‐autonomous economies), this stationarity condition generalizes Dixit et al.’s (1980 ) “zero‐net‐aggregate‐investment” rule of sustain‐ability, which in turn generalizes Solow‐Hartwick’s sustainability rule. For non‐autonomous economies, the stationarity condition is not generally fulfilled, and the current‐value Hamiltonian under (over) estimates the true welfare level by an amount equal to the discounted value of the net “pure time effect.” For the non‐autonomous case of a time‐dependent utility discount rate, a general condition on the discount rate function (of which the hyperbolic discount rate function is a special case) upholds the results obtained for autonomous cases. The paper concludes with a discussion of policies that promote both optimality and sustainability objectives.     

Global stability and the “turnpike” in optimal unbounded growth models

This study proves various global stability results for unbounded optimal growth models. The main theorem states that any optimal path will eventually be in the neighborhood of a balanced growth path if future utility is sufficiently weakly discounted. The assumptions allow for non-smooth technologies, joint production, and production in independent sectors. Hence, the results form the integration of new growth and turnpike theory sought by McKenzie (1998) [31] in his Ely lecture. The applicability of the results is exemplified by means of a number of cases from growth theory and other areas of economics.     

The Ethics of Intertemporal Distribution in a Warming Planet

This paper evaluates, from the ethical viewpoint, current work by economists on intergenerational resource allocation in the presence of global warming. We begin by attempting to elucidate the debate that has recently occurred on the appropriate choice of the discount rate. We offer three justifications for maximizing the discounted sum of generational utilities, and find only one of these to be a satisfactory justification of that practice: the possibility that the human species may become extinct. This implies that a very small discount rate (large discount factor) should be used. We argue that the justification for discounting, inherent in the approaches taken by many economists, is that of ‘the present generation of hegemon,’ which is unacceptable. The role of the Ramsey equation in deducing the discount rate in these theories is explained. As an alternative to discounted utilitarianism, we propose a principle of sustainability; we describe optimal paths that have been calculated for the sustainabilitiarian (Rawlsian) objective function, and paths that will sustain growth in welfare, at a positive rate. We report results concerning optimal paths when the uncertainty of existence of future generations is taken into account. In sharp contrast to the utilitarian model, it turns out that under some conditions, the ‘sustainabilitarian’ can ignore the uncertainty regarding the date at which humans become extinct. There is a striking difference between the solutions of the discounted utilitarian program and the sustainabilitarian program under uncertainty.     

SOCIAL NORMS,LOCAL INTERACTION,AND NEIGHBORHOOD PLANNING*

This article examines optimal social linkage when each individual's repeated interaction with each of his neighbors creates spillovers. Each individual's discount factor is randomly determined. A planner chooses a local interaction network or neighborhood design before the discount factors are realized. Each individual then plays a repeated Prisoner's Dilemma game with his neighbors. A local trigger strategy equilibrium (LTSE) describes an equilibrium in which each individual conditions his cooperation on the cooperation of at least one “acceptable” group of neighbors. Our main results demonstrate a basic trade‐off in the design problem between suboptimal punishment and social conflict. Potentially suboptimal punishment arises in designs with local interactions since in this case monitoring is imperfect. Owing to the heterogeneity of discount factors, however, greater social conflict may arise in more connected networks. When individuals' discount factors are known to the planner, the optimal design exhibits a cooperative “core” and an uncooperative “fringe.”“Uncooperative” (impatient) types are connected to cooperative ones who tolerate their free riding so that social conflict is kept to a minimum. By contrast, when the planner knows only the ex ante distribution over individual discount factors, then in some cases the optimal design partitions individuals into maximally connected cliques (e.g., cul‐de‐sacs), whereas in other cases incomplete graphs with small overlap (e.g., grids) are possible.     

Global asymptotic stability of a competitive equilibrium with recursive preferences

Summary. This paper demonstrates global stability of a competitive equilibrium in a multi-sector model of many firms, each of which exhibits constant returns to scale technology, and of infinitely lived consumers, whose preferences are recursive but not necessarily additively separable. In the topology induced by a sup-norm, the dominant diagonal blocks condition (Araujo and Scheinkman (Econometrica 45, 1977)) allows us to apply the implicit function theorem to obtain continuity of the equilibrium path. If a stationary equilibrium is locally asymptotically stable, then the continuity of the equilibrium path and smoothness of a weight function on heterogeneous consumers imply that all equilibrium paths converge to the steady state. The dominant diagonal blocks condition is also shown to be sufficient for the local asymptotic turnpike property. Received: December 13, 1996; revised version: June 2, 1999     

Risk aversion,impatience, and optimal timing decisions

In this paper, a theory measuring a decision maker's aversion to temporal risks is developed in the context of a simple choice framework that admits the interpretation “time varying utility of wealth.” A relation “more temporally risk averse” is defined and characterized in terms of instantaneous risk aversion (the usual single variable case) and impatience. A further characterization of this relation is obtained in the context of a class of action timing problems known as optimal stopping problems and is applied to preference based investigations of information production, incentives to innovate, and job search.     

A local analysis of stability and regularity of stationary states in discrete symmetric optimal capital accumulation models

In discrete time optimal capital accumulation models, where the period utilities have a certain “symmetry” property, a complete characterization of local stability of stationary states is obtained in terms of the characteristic roots of the product of certain submatrices of the Hessian of the utility function. This is used to analyze the connection between stability and properties of the effects of parameter changes on the stationary state. Certain differences in the results in discrete and continuous models are noted. A special case where utilities are “separable” in initial stocks and net investment is also treated.     

The optimal quantity of money: A reconciliation

Despite the apparent disagreement of Samuelson's (1958) overlapping generations model and Townsend's (1980) turnpike model on the optimal quantity of money, in both: (i) those endowed at the start with goods prefer a constant money stock, and (ii) those endowed at the start with fiat money prefer deflation.     

Optimal growth and cycles in overlapping generations models

Summary This paper investigates the dynamical properties of optimal paths in one-sector overlapping generations models without assuming that the utility function of the representative agent is separable. When the utility function is separable, the optimal growth paths monotonically converges toward the modified golden rule steady state. In the non-separable case, we show that the optimal growth path may be oscillating and optimal two-period cycles may exist. Applying these results to the model with altruism, we show that the condition of operative bequest is fully compatible with endogeneous fluctuations provided that the discount factor is close enough to one. All our results are illustrated using Cobb-Douglas utility and production functions.We thank C. Blackorby, J. Blot, P. Cartigny and one anonymous referee for helpful comments and suggestions which generally improved the exposition of the paper. We would also like to thank the participants of the Population and demography session of the European Economic Association 10th Annual Congress (Prague, The Czech Republic, September 1–4, 1995).     

Turnpike theory, discounted utility, and the von Neumann facet

The convergence of infinite optimal paths to stationary optimal paths is proved in models of capital accumulation whose discount factors are near 1, where strict concavity is not required for utility functions and production functions. The critical assumptions are unique support prices for points of the von Neumann facet, where is near 1, a unique optimal stationary stock when is equal to 1, and the absence of cyclic paths on the von Neumann facet when is equal to 1. The results are illustrated in generalizing a model provided by Weitzman and Samuelson.     

Equilibrium and optimality: Some imprints of David Gale

The paper is a review of some of the themes to which David Gale made lasting contributions. It touches upon a number of the fundamental issues in the Walrasian equilibrium theory (existence, uniqueness and stability), the overlapping generations model (non-optimality and indeterminacy of competitive equilibria), the von Neumann equilibrium (as a turnpike), and in the theory of decentralized intertemporal allocation through competitive prices (efficiency and golden rules, duality and existence of optimal programs).     

On income fluctuations and capital gains

We consider the infinite time horizon problem of asymptotically maximizing the expected accumulated discounted utility in a one-good production economy. The available capital in a given period is given by the production of the previous period plus a random variable. The product of the discount and interest factors is either (1) greater than or (2) equal to one. Under (1) the optimal policy exists under certain conditions and always under (2). The optimal capital sequence almost surely goes to infinity. Under (1) with conditions on the utility one almost surely reaches a capital level above which the sequence is increasing.     

Rational demand and expenditures patterns under habit formation

This paper proposes a model in which habit formation is present in a relatively general but tractable way. The consumer's problem is transformed into a sequence of two-period Fisherian problems by introducing a “reduced utility function” to ensure full dynamic rationality of the consumer. By making preferences dependent on past real expenditure levels rather than past consumption bundles, it is possible to characterize the long-run behavior of the consumer. Stability analysis is performed. The cases of “immediate habit formation” and “delayed habit formation” are discussed.     

Reputation and imperfect information

A common observation in the informal literature of economics (and elsewhere) is that is multistage “games,” players may seek early in the game to acquire a reputation for being “tough” or “benevolent” or something else. But this phenomenon is not observed in some formal game-theoretic analyses of finite games, such as Selten's finitely repeated chain-store game or in the finitely repeated prisoners' dilemma. We reexamine Selten's model, adding to it a “small” amount of imperfect (or incomplete) information about players' payoffs, and we find that this addition is sufficient to give rise to the “reputation effect” that one intuitively expects.     

Nash equilibrium search for the best alternative

In a recent paper, Weitzman (Econometrica47 (1979) 641–654) described a policy of “optimal search for the best alternative.” The present paper is concerned with the development and characterization of a policy of “Nash equilibrium search for the best alternative.” Specifically, it is shown that, under certain monotonicity assumptions, and under the assumption that firms have incomplete information regarding the results of rivals' search behavior, a Nash equilibrium search policy exists and has the same form as Weitzman's optimal search policy.     

Multi-unit auctions with budget limits

We study multi-unit auctions for bidders that have a budget constraint, a situation very common in practice that has received relatively little attention in the auction theory literature. Our main result is an impossibility: there is no deterministic auction that (1) is individually rational and dominant-strategy incentive-compatible, (2) makes no positive transfers, and (3) always produces a Pareto optimal outcome. In contrast, we show that Ausubel?s “clinching auction” satisfies all these properties when the budgets are public knowledge. Moreover, we prove that the “clinching auction” is the unique auction that satisfies all these properties when there are two players. This uniqueness result is the cornerstone of the impossibility result. Few additional related results are given, including some results on the revenue of the clinching auction and on the case where the items are divisible.     

Aggregation and pairwise aggregation of demand when the distribution of income is fixed

This paper derives necessary and sufficient conditions for “pairwise aggregation” of the demand functions of a group of consumers (conditions under which the mean demand for each pair of consumers satisfies the Slutsky restrictions) when the distribution of income is fixed. The sufficient conditions imply existence of a “representative” competitive consumer whose demand is the mean demand of the group. The necessary conditions imply that such a representative consumer exists for every fixed income distribution only if the consumers have homothetic preferences or, alternatively, if for each price vector, all the consumers' income expansion paths lie in the same plane.