The Complexity of Planning Revisited - A Parameterized Analysis

The early classifications of the computational complexity of planning under various restrictions in STRIPS (Bylander) and SAS+ (Baeckstroem and Nebel) have influenced following research in planning in many ways. We go back and reanalyse their subclasses, but this time using the more modern tool of parameterized complexity analysis. This provides new results that together with the old results give a more detailed picture of the complexity landscape. We demonstrate separation results not possible with standard complexity theory, which contributes to explaining why certain cases of planning have seemed simpler in practice than theory has predicted. In particular, we show that certain restrictions of practical interest are tractable in the parameterized sense of the term, and that a simple heuristic is sufficient to make a well-known partial-order planner exploit this fact.Comment: (author's self-archived copy

Noisy Business Cycles

This paper investigates a real-business-cycle economy that features dispersed information about the underlying aggregate productivity shocks, taste shocks, and—potentially—shocks to monopoly power. We show how the dispersion of information can (i) contribute to significant inertia in the response of macroeconomic outcomes to such shocks; (ii) induce a negative shortrun response of employment to productivity shocks; (iii) imply that productivity shocks explain only a small fraction of high-frequency fluctuations; (iv) contribute to significant noise in the business cycle; (v) formalize a certain type of demand shocks within an RBC economy; and (vi) generate cyclical variation in observed Solow residuals and labor wedges. Importantly, none of these properties requires significant uncertainty about the underlying fundamentals: they rest on the heterogeneity of information and the strength of trade linkages in the economy, not the level of uncertainty. Finally, none of these properties are symptoms of inefficiency: apart from undoing monopoly distortions or providing the agents with more information, no policy intervention can improve upon the equilibrium allocations

Consumption complementarities, monopolies and coordination

Price-coordination and investment coordination are analyzed in a monopolistic multi-sector general equilibrium model with consumption complementarities. Possible solutions to the investment coordination problem are consistent with historical examples of government intervention in investment, the different roles of banking sectors in different countries, and the effect of optimism on the development of new sectors. Price coordination within sectors between monopolists of complementary intermediaries lowers prices and increases welfare because the competition between the final goods of different sectors then becomes the paramount concern of each monopolist. With no price coordination, each monopolist sets infinite prices as the effect of price increases on demand is shared by all other intermediary monopolists due to the complementarities.

Fixed term employment contracts in an equilibrium search model

Fixed term employment contracts have been introduced in number of European countries as a way to provide flexibility to economies with high employment protection levels. We introduce these contracts into the equilibrium search model in Alvarez and Veracierto (1999), a version of the Lucas and Prescott island model, adapted to have undirected search and variable labor force participation. We model a contract of length J as a tax on separations of workers with tenure higher than J. We show a version of the welfare theorems, and characterize the efficient allocations. This requires solving a control problem, whose solution is characterized by two dimensional inaction sets. For J = 1 these contracts are equivalent to the case of firing taxes, and for large J they are equivalent to the laissez- faire case. In a calibrated version of the model, we evaluate to what extent contract lengths similar to those observed in Europe, close the gap between these two extremes.Labor contract ; Employment (Economic theory)

The Influence of k-Dependence on the Complexity of Planning

A planning problem is k-dependent if each action has at most k pre-conditions on variables unaffected by the action. This concept is well-founded since k is a constant for all but a few of the standard planning domains, and is known to have implications for tractability. In this paper, we present several new complexity results for P(k), the class of k-dependent planning problems with binary variables and polytree causal graphs. The problem of plan generation for P(k) is equivalent to determining how many times each variable can change. Using this fact, we present a polytime plan generation algorithm for P(2) and P(3). For constant k> 3, we introduce and use the notion of a cover to find conditions under which plan generation for P(k) is polynomial

Optimality In Reserve Selection Algorithms: When Does It Matter And How Much?

This paper responds to recent criticisms in Biological Conservation of heuristic reserve selection algorithms. These criticisms primarily concern the fact that heuristic algorithms cannot guarantee an optimal solution to the problem of representing a group of targeted natural features in a subset of the sites in a region. We discuss optimality in the context of a range of needs for conservation planning. We point out that classical integer linear programming methods that guarantee an optimal solution, like branch and bound algorithms, are currently intractable for many realistic problems. We also show that heuristics have practical advantages over classical methods and that suboptimality is not necessarily a disadvantage for many real-world applications. Further work on alternative reserve selection algorithms is certainly needed, but the necessary criteria for assessing their utility must be broader than mathematical optimality