Solubility and diffusion of oxygen in tantalum

Solubility of oxygen in tantalum determined by resistivity techniqu

Topographical Surveying

A study of the use of surveying and topography to develop maps

Exploiting Behavioral Hierarchy for Efficient Model Checking

Inspired by the success of model checking in hardware and protocol verification, model checking techniques for software have been the focus of a lot of research in the last few years [5,3,2,6]. Model checking can be applied only to relatively small models due to its inherently high computational requirements, and there are two complementary trends to address scalability. The model extraction approach, exemplified by projects such as Bandera [6] and SLAM [3], involves constructing inputs to model checkers by abstracting programs written in languages such as C and Java. The model-based design approach, exemplified by modeling notations such as Statecharts [7], promotes design using high-level models that are compiled into code. Our research agenda is to develop model checking techniques for model-based design of software. Modern software design languages promote hierarchy as one of the key constructs for structuring complex specifications. The input language to our model checker is based on hierarchic reactive modules [1]. This choice was motivated by the fact that, unlike STATECHARTS and other languages, in hierarchic reactive modules, the notion of hierarchy is semantic with an observational trace-based semantics and a notion of refinement with assume-guarantee rules. The first contribution of this paper is the Hermes toolkit that implements hierarchic reactive modules. Our implementation has a visual front-end and XML-based back-end, consistent with modern software design tools, and is in Java. There are two basic techniques for reachability analysis. Enumerative model checkers such as SPIN [8] perform an on-the-fly exploration of the state-space using a depth-first search, while symbolic model checkers such as SMV [9] perform a breadth-first search by manipulating sets of states, rather than individual states, encoded typically by ordered binary (or multi-valued) decision diagrams. Since the two approaches are incomparable, and have been shown to be successful, Hermes supports both enumerative and symbolic reachability analysis. In this paper, we report progress on exploiting the structuring information in the behavioral hierarchy of the input model to speed up the exploration of reachable state-space of the model for both the approaches. More information about the tool is available at http://www.cis.upenn.edu/sdrl/hermes

Artificial Intelligence

Contains research objectives and reports on five research projects.Computation Center, M.I.T

Software Model Checking with Explicit Scheduler and Symbolic Threads

In many practical application domains, the software is organized into a set of threads, whose activation is exclusive and controlled by a cooperative scheduling policy: threads execute, without any interruption, until they either terminate or yield the control explicitly to the scheduler. The formal verification of such software poses significant challenges. On the one side, each thread may have infinite state space, and might call for abstraction. On the other side, the scheduling policy is often important for correctness, and an approach based on abstracting the scheduler may result in loss of precision and false positives. Unfortunately, the translation of the problem into a purely sequential software model checking problem turns out to be highly inefficient for the available technologies. We propose a software model checking technique that exploits the intrinsic structure of these programs. Each thread is translated into a separate sequential program and explored symbolically with lazy abstraction, while the overall verification is orchestrated by the direct execution of the scheduler. The approach is optimized by filtering the exploration of the scheduler with the integration of partial-order reduction. The technique, called ESST (Explicit Scheduler, Symbolic Threads) has been implemented and experimentally evaluated on a significant set of benchmarks. The results demonstrate that ESST technique is way more effective than software model checking applied to the sequentialized programs, and that partial-order reduction can lead to further performance improvements.Comment: 40 pages, 10 figures, accepted for publication in journal of logical methods in computer scienc

Colloidal suspensions in modulated light fields

Periodically-modulated potentials in the form of light fields have previously been applied to induce reversible phase transitions in dilute colloidal systems with long-range interactions. Here we investigate whether similar transitions can be induced in very dense systems, where inter-particle contacts are important. Using microscopy we show that particles in such systems are indeed strongly affected by modulated potentials. We discuss technical aspects relevant to generating the light-induced potentials and to imaging simultaneously the particles. We also consider what happens when the particle size is comparable with the modulation wavelength. The effects of selected modulation wavelengths as well as pure radiation pressure are illustrated.Comment: Accepted by J. Phys.: Condens. Matter, CODEF II Special Issue. 23 pages, 12 figure

A comparative modeling of supernova 1993J

The light curve of Supernova (SN) 1993J is calculated using two approaches to radiation transport as exemplified by the two computer codes, STELLA and EDDINGTON. Particular attention is paid to shock breakout and the photometry in the U, B, and V bands during the first 120 days. The hydrodynamical model, the explosion of a 13 Msun star which had lost most of its hydrogenic envelope to a companion, is the same in each calculation. The comparison elucidates differences between the approaches and also serves to validate the results of both. STELLA includes implicit hydrodynamics and is able to model supernova evolution at early times, before the expansion is homologous. STELLA also employs multi-group photonics and is able to follow the radiation as it decouples from the matter. EDDINGTON uses a different algorithm for integrating the transport equation, assumes homologous expansion, and uses a finer frequency resolution. Good agreement is achieved between the two codes only when compatible physical assumptions are made about the opacity. A new result for SN 1993J is a prediction of the continuum spectrum near the shock breakout (calculated by STELLA) which is superior to the results of other standard single energy group hydrocodes such as VISPHOT or TITAN. Based on the results of our independent codes, we discuss the uncertainties involved in the current time dependent models of supernova light curves.Comment: 43 pages with 22 eps figures, aaspp4.sty + epsf.sty, Accepted by ApJ, to appear in March 20, 1998 issue, Vol. 49