Coordination via Interaction Constraints I: Local Logic

Wegner describes coordination as constrained interaction. We take this approach literally and define a coordination model based on interaction constraints and partial, iterative and interactive constraint satisfaction. Our model captures behaviour described in terms of synchronisation and data flow constraints, plus various modes of interaction with the outside world provided by external constraint symbols, on-the-fly constraint generation, and coordination variables. Underlying our approach is an engine performing (partial) constraint satisfaction of the sets of constraints. Our model extends previous work on three counts: firstly, a more advanced notion of external interaction is offered; secondly, our approach enables local satisfaction of constraints with appropriate partial solutions, avoiding global synchronisation over the entire constraints set; and, as a consequence, constraint satisfaction can finally occur concurrently, and multiple parts of a set of constraints can be solved and interact with the outside world in an asynchronous manner, unless synchronisation is required by the constraints. This paper describes the underlying logic, which enables a notion of local solution, and relates this logic to the more global approach of our previous work based on classical logic

Parameterized models for on-line and off-line use

The Monadic Constraint Programming framework leverages Haskell’s rich static type system and powerful abstraction mechanisms to implement an embedded domain specific language (EDSL) for constraint programming. In this paper we show how the same constraint model expressed in the EDSL can be processed in various modes by external constraint solvers. We distinguish between on-line and off-line use of solvers. In off-line mode, the model is not solved; instead it is compiled to lower-level code that will search for solutions when compiled and run. For on-line use, the search can be handled by either the framework or in the external solver. Off-line mode requires recompilation after each change to the model. To avoid repeated recompilation, we separate model from data by means of parameters that need not be known at compile time. Parametrization poses several challenges, which we resolve by embedding the EDSL more deeply.status: publishe

External constraint and financial crises with balance sheet effects

This paper investigates the dynamic implications of Krugman’s (1999) model of financial crises with balance-sheet effects, which has a considerable impact on the literature as well as the teaching of international financial crisis. By explicitly taking account of wealth accumulation and external equilibrium condition, it is shown that a financial crisis in emerging market economies, instead of being interpreted as a jump from a good to a bad equilibrium with zero investment and zero foreign debt, could be explained as a jump from an unstable dynamic trajectory to a stable one. The dynamic framework illustrates well the analysis of different factors at the origin of financial vulnerability and crisis. By discriminating the financial crises according to the severity of their negative impacts on the domestic economy, the present study also adds some insights in the analysis of policy implications.Financial crisis, currency crisis, balance sheet effect, external solvency constraint.

International Finance

This essay written for The New Palgrave dictionary of Ecnomics provides a selective and interpretive account of the development of thought on international financial questions. Attention is focused on the process of international adjustment and on the proper definition of external balance. Since the first descriptions of the price-specie-flow mechanism in Humes time, the definition of external balance has evolved in response to changes in the world economy's structure. The foreign reserve constraint so central under the gold standard or in the early Bretton Woods years is less important under conditions of high international capital mobility. Increasingly, the current account and the national intertemporal budget constraint are emphasized in discussions of international adjustment. In analogy with the idea of a high-employment government budget surplus, a working definition of external balance might be a current account that maintains the highest possible steady consumption level consistent with the economy's expected intertemporal budget constraint. Intertemporal approaches to external balance become more difficult to apply when countries face credit rationing as a result of nonrepayment risk.

Rotational invariance as an additional constraint on local realism

Rotational invariance of physical laws is a generally accepted principle. We show that it leads to an additional external constraint on local realistic models of physical phenomena involving measurements of multiparticle spin 1/2 correlations. This new constraint rules out such models even in some situations in which standard Bell inequalities allow for explicit construction of such models. The whole analysis is performed without any additional assumptions on the form of local realistic models.Comment: 4 page

External constraint and financial crises with balance sheet effects.

This paper examines a model of financial and exchange crises with balance-sheet effects by explicitly taking account of wealth accumulation and external equilibrium condition. We have found that, in a general equilibrium analysis, there are two stationary equilibria. Since foreign debt is always zero at these equilibria, financial crises in emerging market economies cannot be interpreted as jumps between equilibria but between trajectories leading to one equilibrium or another one. The mechanisms of financial crises due to monsoon or spill-over effects are also analysed in this framework.Financial crisis, exchange crisis, balance sheet effect, external solvency constraint.

The External Debt-Servicing Constraint and Public Expenditure Composition: Evidence from African Economies

The paper explores the impact of a binding external debt-servicing constraint on the sectoral composition of government expenditures in the economies of Africa, where this constraint has traditionally been most prevalent. Applying seemingly unrelated regression (SUR) to 1975-94 five-year panel data for 35 countries, the paper finds that the implied debt service burden adversely affects the share of public spending in the social sector, with similar impacts on education and health. Despite evidence that such a burden might also negatively influence public investment, the deleterious implications of debt servicing appear to be primarily a social-sector phenomenon. The partial elasticity on the sector's expenditure share is estimated at 1.5, which is by far the highest among all the explanatory variables considered, including external aid, whose estimated effect on the social sector is positive but with an elasticity of only 0.2. Aid also positively affects public investment with a similar elasticity of 0.2. Constraint on the executive exercises significant positive and negative impacts, respectively, on ...external debt servicing, public expenditure composition, seemingly unrelated regression, African economies

Programming complex shapes in thin nematic elastomer and glass sheets

Nematic elastomers and glasses are solids that display spontaneous distortion under external stimuli. Recent advances in the synthesis of sheets with controlled heterogeneities have enabled their actuation into non-trivial shapes with unprecedented energy density. Thus, these have emerged as powerful candidates for soft actuators. To further this potential, we introduce the key metric constraint which governs shape changing actuation in these sheets. We then highlight the richness of shapes amenable to this constraint through two broad classes of examples which we term nonisometric origami and lifted surfaces. Finally, we comment on the derivation of the metric constraint, which arises from energy minimization in the interplay of stretching, bending and heterogeneity in these sheets

Collinear and Soft Limits of Multi-Loop Integrands in N=4 Yang-Mills

It has been argued in arXiv:1112.6432 that the planar four-point integrand in N=4 super Yang-Mills theory is uniquely determined by dual conformal invariance together with the absence of a double pole in the integrand of the logarithm in the limit as a loop integration variable becomes collinear with an external momentum. In this paper we reformulate this condition in a simple way in terms of the amplitude itself, rather than its logarithm, and verify that it holds for two- and three-loop MHV integrands for n>4. We investigate the extent to which this collinear constraint and a constraint on the soft behavior of integrands can be used to determine integrands. We find an interesting complementarity whereby the soft constraint becomes stronger while the collinear constraint becomes weaker at larger n. For certain reasonable choices of basis at two and three loops the two constraints in unison appear strong enough to determine MHV integrands uniquely for all n.Comment: 27 pages, 14 figures; v2: very minor change