Dynamic reasoning in a knowledge-based system

Any space based system, whether it is a robot arm assembling parts in space or an onboard system monitoring the space station, has to react to changes which cannot be foreseen. As a result, apart from having domain-specific knowledge as in current expert systems, a space based AI system should also have general principles of change. This paper presents a modal logic which can not only represent change but also reason with it. Three primitive operations, expansion, contraction and revision are introduced and axioms which specify how the knowledge base should change when the external world changes are also specified. Accordingly the notion of dynamic reasoning is introduced, which unlike the existing forms of reasoning, provide general principles of change. Dynamic reasoning is based on two main principles, namely minimize change and maximize coherence. A possible-world semantics which incorporates the above two principles is also discussed. The paper concludes by discussing how the dynamic reasoning system can be used to specify actions and hence form an integral part of an autonomous reasoning and planning system

Tunneling through two resonant levels: fixed points and conductances

We study point contact tunneling between two leads of a Tomonaga-Luttinger liquid through two degenerate resonant levels in parallel. This is one of the simplest cases of a quantum junction problem where the Fermi statistics of the electrons plays a non-trivial role through the Klein factors appearing in bosonization. Using a mapping to a `generalized Coulomb model' studied in the context of the dissipative Hofstadter model, we find that any asymmetry in the tunneling amplitudes from the two leads grows at low temperatures, so that ultimately there is no conductance across the system. For the symmetric case, we identify a non-trivial fixed point of this model; the conductance at that point is generally different from the conductance through a single resonant level.Comment: 6 pages, 3 figure

Creep Tension Tests on Hastelloy Alloy X Bar, Predicting its Behavior and Applying the Data in a Design Problem

Creep is a continuing deformation at elevated temperatures under constant stress. Creep data are important in the design of many machine members subjected to high temperatures. Examples of these would be steam and gas turbine components, thermal cracking equipment in oil refineries, jet air-craft, missiles, rockets and mobile reactors. The second law of thermodynamics shows us that the efficiency of an ideal heat engine can be increased by increasing the temperature of the working fluid. The optimum strength-to-weight ratio is an essential requirement in aircraft structural design. But all these facts bring us the problem of higher creep rates for the same stresses. In some applications, the permissible creep deformations are critical, e.g. Allowance for creep is important in turbine blades for proper operation to maintain small clearances between the moving and stationary parts and in others no significance. The creep phenomenon has been known for the last 80 years, but knowledge of the phenomenon has advanced rapidly since World War II, due to more demand for high temperature materials, resulting in today’s successful new alloys. Creep deformation can be determined in the laboratory with a testing machine but conducting such tests for the entire life of the machine element is impracticable. As we know, the operating characteristics of an original model can be obtained from a small scale model by means of dimensional analysis. The idea was to have lead creep curves similar to those of the original material in order to obtain direct extrapolation from low temperature lead tests to operating conditions. Cases occur where parts are subjected to combine bending and direct load. For example, turbine blades are subjected to a b3ending action due to the transverse loading applied by the steam, and to a direct load due to centrifugal force. Under creep conditions, parts subjected to this combined straining action when the predominant action is bending are much more favorably stressed than would be the case if the material were elastic and when the predominant action is direct load the stress conditions will approach those for elastic condition. So analyzing the forces acting on a machine element is an important step in a design problem

Anomalous microwave response of high-temperature superconducting thin-film microstrip resonator in weak dc magnetic fields

We have studied an anomalous microwave (mw) response of superconducting YBa_{2}Cu_{3}O_{7-delta} (YBCO) microstrip resonators in the presence of a weak dc magnetic field, H_{dc}. The surface resistance (R_{s}) and reactance (X_{s}) show a correlated non-monotonic behaviour as a function of H_{dc}. R_{s} and X_{s} were found to initially decrease with elevated H_{dc} and then increase after H_{dc} reaches a crossover field, H_{c}, which is independent of the amplitude and frequency of the input mw signal within the measurements. The frequency dependence of R_{s} is almost linear at fixed H_{dc} with different magnitudes (H_{c}). The impedance plane analysis demonstrates that r_{H}, which is defined as the ratio of the change in R_{s}(H_{dc}) and that in X_{s}(H_{dc}), is about 0.6 at H_{dc}<H_{c} and 0.1 at H_{dc}>H_{c}. The H_{dc} dependence of the surface impedance is qualitatively independent of the orientation of H_{dc}.Comment: REVTex 3.1, 5 pages, 6 EPS figures, submitted to Physica

Comment on ''the controlled charge ordering and evidence of the metallic state in Pr0.65_{0.65}Ca0.35_{0.35}MnO3_{3} films''

In a recent paper (2000 \QTR{it}{J. Phys.: Condens. Matter} \QTR{bf}{12} L133) Lee \QTR{it}{et al.} have studied the transport properties of Pr$_{0.65}$Ca$_{0.35}$MnO$_{3}$ thin films. They claimed that they are able to controlled the charge-ordered (CO) state by the lattice strains. We propose herein another alternative since another indexation of the orientation of the film can be found leading to almost no distortion of the cell, as compared to the bulk compound.Comment: 2 page

Indian Tropical Storms and Zones of Heavy Rainfall

The accepted explanation for the heavy rainfall in southwest monsoon depressions in India on the basis of fronter structure is not supported by radio-sonde data. An alternative explanation on the basis of convergence derived from the isobaric patterns is offered. It has also been applied to satisfactorily explain the distribution of heavy rain in the post-monsoon depressions