Computing stable models by program transformation

In analogy to the Davis--Putnam procedure we develop a new procedure for computing stable models of propositional normal disjunctive logic programs, using case analysis and simplification. Our procedure enumerates all stable mofels without repetition and without the need for a minimality check. Since it is not necessary to store the set of stable models explicitly, the procedure runs in polynomial space. We allow clauses with empty heads, in order to represent truth or falsity of a proposition as a one--literal clause. In particular, a clause of form $\sim A \rightarrow$ expresses that $A$ is contrained to be true, without providing a justification for $A$. Adding this clause to a program restricts its stable models to those containing A, without introducing new stable models. Together with $A \rightarrow$ this provides the basis for case analysis. We present our procedure as a set of rules which transform a program into a set of solved forms, which resembles the standard method for presenting unification algorithms. Rules are sound in the sense that they preserve the set of stable models. $A$ subset of the rules is shown to be complete in the sense that for each stable model a solved form can be obtained. The method allows for concise presentation, flexible choice of a control strategy and simple correctness proofs

The Changing Effects of Energy-Price Shocks on Economic Activity and Inflation

In this article the author examines the effects that major changes in energy prices in recent years have had on inflation and on the pace of economic expansion. These are then compared with the effects of the oil-price shocks that occurred in the 1970s and early 1980s. Changes in the intensity of energy use are examined, as well as developments in Canada's merchandise trade surplus in energy commodities and products. The author also considers the effects that a monetary policy anchored to low and stable inflation could have on price-setting behaviour and thus on the pass-through of higher energy costs to core inflation in Canada and in other industrial countries.

Transition from order to chaos in the wake of an airfoil

An experimental effort is presented here that examines the nonlinear interaction of multiple frequencies in the forced wake of an airfoil. Wakes with one or two distinct frequencies behave in an ordered manner – being either locked or quasi-periodic. When a third incommensurate frequency is added to the system, the flow demonstrates chaotic behaviour. Previously, the existence of the three-frequency route to chaos has been reported only for closed system flows. It is important to note that this chaotic state is obtained at a low Reynolds number. However, the chaotic flow shows localized characteristics similar to those of high Reynolds number turbulent flows. The degree of chaotic behaviour is verified by applying ideas from nonlinear dynamics (such as Lyapunov exponents and Poincaré sections) to the experimental data, thus relating the basic physics of the system to the concepts of mode interaction and chaos. Significant changes to the vortex configuration in the wake and to the r.m.s. velocity profile occur during the transition from order to chaos

Experiments on the forced wake of an airfoil

The effect of initial flow conditions on the wake of an airfoil is examined in an experiment which uses the ‘strip heater’ technique to externally force the airfoil wake. The strip heaters are used to introduce waves into the top and bottom boundary layers of a thin symmetric airfoil which are subsequently amplified and introduced to the wake. The evolution and interaction of the waves in the wake is the primary interest of this study. A linear stability analysis is applied to the mean velocity profiles in order to understand the frequency selection process in the wake. It is seen that the mean velocity profile adjusts itself in order to become more receptive to the forced frequency of oscillation, resulting in the suppression of previously existing frequencies. The amplitude of oscillations in the wake can be controlled by varying the phase relation between two input signals. In this respect, cancellation and enhancement of the oscillations is possible. The linear stability analysis is applied to the cancellation/enhancement flow to verify the level of cancellation achieved. The receptivity of the system to external forcing is established. A substantial reduction in drag is achieved for forcing frequencies near the centre of the receptivity range

Recent Developments in Global Commodity Prices: Implications for Canada

The authors examine the recent evolution of commodity prices. They discuss the factors behind the price declines that occurred between the summer of 1997 and the end of 1998, including the key supply factors and the drop in Asian demand caused by that region's concurrent financial and economic crisis. They then review the effects of the reduction in world commodity prices on economic activity in Canada. They point out that the depreciation of the Canadian dollar against the U.S.dollar, together with the continued strength of the U.S.economy, has partly offset the negative effects on Canadian aggregate demand.

Reference catalogue for ICT services in healthcare : model for ICT service management, controlling and benchmarking : version 1.0

Translation of the German original

A Distributed Parameters Model for Solar Cells

Distributive circuit element model for solar cells with series and shunt path

The combline filter and phase-lock loop. A new technique to improve FM television reception

Development and performance of combline filter phase locked loop combination for television receptio

Lehrmittel für technisches und textiles Gestalten

Der Beitrag zeigt, wie spezielle Rahmenbedingungen des Schulfachs ‹technisches und textiles Gestalten› die Innovation des Bereichs beeinflusst haben. Am Beispiel der zwei neueren Lehrmittel Werkweiser und Phänomenales Gestalten wird das Konzept der Lehrmittel, das mit einem Paradigmenwechsel verbunden ist, dargestellt. Erfahrungen mit den Lehrmitteln werden diskutiert.The article shows how particular parameters of the school subject Technical and Textile Design have had an influence on innovation in this area. The concept of teaching aids, which has been associated with a paradigm shift, is illustrated using the examples of the recent materials Work Manner and Phenomenal Design. Experiences with these teaching materials are discussed