Occurrence Counting Analysis for the π-calculus

We propose an abstract interpretation-based analysis for automatically proving non-trivial properties of mobile systems of processes. We focus on properties relying on the number of occurrences of processes during computation sequences, such as mutual exclusion and non-exhaustion of resources.We design a non-standard semantics for the π-calculus in order to explicitly trace the origin of channels and to solve efficiently problems set by α-conversion and non-deterministic choices. We abstract this semantics into an approximate one. The use of a relational domain for counting the occurrences of processes allows us to prove quickly and efficiently properties such as mutual exclusion and non-exhaustion of resources. At last, dynamic partitioning allows us to detect some configurations by which no infinite computation sequences can pass.     

A spatial equational logic for the applied π-calculus

Spatial logics have been proposed to reason locally and modularly on algebraic models of distributed systems. In this paper we define the spatial equational logic A π L whose models are processes of the applied π-calculus. This extension of the π-calculus allows term manipulation and records communications as aliases in a frame, thus augmenting the predefined underlying equational theory. Our logic allows one to reason locally either on frames or on processes, thanks to static and dynamic spatial operators. We study the logical equivalences induced by various relevant fragments of A π L, and show in particular that the whole logic induces a coarser equivalence than structural congruence. We give characteristic formulae for some of these equivalences and for static equivalence. Going further into the exploration of A π L’s expressivity, we also show that it can eliminate standard term quantification.     

Characterisations of testing preorders for a finite probabilistic π-calculus

We consider two characterisations of the may and must testing preorders for a probabilistic extension of the finite π-calculus: one based on notions of probabilistic weak simulations, and the other on a probabilistic extension of a fragment of Milner–Parrow–Walker modal logic for the π-calculus. We base our notions of simulations on similar concepts used in previous work for probabilistic CSP. However, unlike the case with CSP (or other non-value-passing calculi), there are several possible definitions of simulation for the probabilistic π-calculus, which arise from different ways of scoping the name quantification. We show that in order to capture the testing preorders, one needs to use the “earliest” simulation relation (in analogy to the notion of early (bi)simulation in the non-probabilistic case). The key ideas in both characterisations are the notion of a “characteristic formula” of a probabilistic process, and the notion of a “characteristic test” for a formula. As in an earlier work on testing equivalence for the π-calculus by Boreale and De Nicola, we extend the language of the π-calculus with a mismatch operator, without which the formulation of a characteristic test will not be possible.     

A logical encoding of the π-calculus: model checking mobile processes using tabled resolution

We present MMC, a model checker for mobile systems specified in the style of the -calculus. MMCs development builds on that of XMC, a model checker for an expressive extension of Milners value-passing calculus implemented using the XSB tabled logic-programming engine. MMC addresses the salient issues that arise in the -calculus, including scope extrusion and intrusion and dynamic generation of new names to avoid name capture. We show that logic programming provides an efficient implementation platform for model checking -calculus specifications and can be used to obtain an exact encoding of the -calculuss transitional semantics. Moreover, MMC is easily extended to handle process expressions in the spi-calculus of Abadi and Gordon. Our experimental data show that MMC outperforms other known tools for model checking the -calculus.     

On the contact detection for contact-impact analysis in multibody systems

One of the most important and complex parts of the simulation of multibody systems with contact-impact involves the detection of the precise instant of impact. In general, the periods of contact are very small and, therefore, the selection of the time step for the integration of the time derivatives of the state variables plays a crucial role in the dynamics of multibody systems. The conservative approach is to use very small time steps throughout the analysis. However, this solution is not efficient from the computational view point. When variable time-step integration algorithms are used and the preimpact dynamics does not involve high-frequencies, the integration algorithms may use larger time steps and the contact between two surfaces may start with initial penetrations that are artificially high. This fact leads either to a stall of the integration algorithm or to contact forces that are physically impossible which, in turn, lead to post-impact dynamics that is unrelated to the physical problem. The main purpose of this work is to present a general and comprehensive approach to automatically adjust the time step, in variable time-step integration algorithms, in the vicinity of contact of multibody systems. The proposed methodology ensures that for any impact in a multibody system the time step of the integration is such that any initial penetration is below any prescribed threshold. In the case of the start of contact, and after a time step is complete, the numerical error control of the selected integration algorithm is forced to handle the physical criteria to accept/reject time steps in equal terms with the numerical error control that it normally uses. The main features of this approach are the simplicity of its computational implementation, its good computational efficiency, and its ability to deal with the transitions between non-contact and contact situations in multibody dynamics. A demonstration case provides the results that support the discussion and show the validity of the proposed methodology.     

Full order H ∞ filtering for linear systems in the frequency domain

This paper proposes an easier frequency domain solution to the standard H _∞ filtering problem using a polynomial approach. The design of the H _∞ filter in the frequency domain is first obtained from the time domain solution which is related to a Riccati equation, and then by the use of the connecting relationship between the time and frequency domain approach given by Hippe [8], its representation in the frequency domain is derived. The filter is easy to calculate as it requires the computation of a single gain and it is easily implementable also. A numerical example is given to illustrate the presented approach.     

On the choice of boundary conditions for mode shapes in flexible multibody systems

The modal representation of the deformation field is a widespread and efficient approach in the analysis of flexible multibody systems. However, it requires a pre-processing in advance to the actual multibody survey that includes the imposition of boundary conditions for the evaluation of the mode functions as an essential user input. Quite often the appropriateness of these boundary conditions is a point of discussion. Therefore the present paper reviews the theoretical background and the implications of this task. Then, a consistent and comprehensive proposal is made how these boundary conditions may be chosen. The suggestion is justified by theoretical considerations and compared to alternative approaches from the literature in a simulation study with three representative examples. It may be concluded that several approaches lead to reasonable results for a sufficient number of mode functions. However, the proposed approach turned out to be the most efficient one and provides a consistent framework.     

Information systems ‘in the wild’: supporting activity in the world

One of the most important determinants of managerial effectiveness in achieving organisational objectives is the success in influencing subordinates, lateral peers and supervisors through influence tactics. However, little attention has been paid to the use of a communication medium in the context of influence tactics. Our objective is to study the frequency of these dyadic influence tactics on diverse communication media. The study involves a questionnaire-based survey conducted on Spanish post-graduate students who had been working in medium-sized or large companies during the last 2 years or more. The results suggest that the richness of the medium explains most similarities and differences in the frequency of influence tactics for different communication media. Furthermore, in the study of hard influence tactics in communication media with similar richness, it is necessary to introduce social aspects and the level of surveillance in order to explain it fully. This study helps managers to understand the relationships between influence tactics and the use of communication media in order to improve their communication effectiveness.     

On the state estimation for non-linear dynamic systems †

The object of this paper is to present an approximate technique for state estimation of non-linear dynamical systems under noisy observations. The conditional cumulant is introduced, by which the conditional probability density can be characterized. A set of dynamical equations satisfied by conditional cumulants is derived, and an approximate method is proposed for computing the cumulants. The relation of the cumulant method to the stochastic linearization technique is also discussed. Finally the state estimation problem for linear stochaatic system with state-dependent disturbance is solved to illustrate the use of the Gaussian approximation.     

On the abs algorithms for perturbed linear systems ∗

One of the most important problems in numerical analysis and numerical optimization is to solve a linear system of equations. Sometimes it should be repeated when one of the equations is replaced by a new one. In this paper as a result of theoretical analysis, an algorithm model and a particular algorithm which are based on the ABS class are proposed. After the original linear system has been solved by the ABS class, the algorithms proposed here can efficiently solve the new system which is obtained from the original system by replacing one of its equations by using information obtained in the previous computation. These algorithms can be used continually when some equations of the original system are replaced by new equations successively with less computation effort.     

On the estimation of stochastic sensitivity for non-linear dynamical systems in state estimation problems†

This paper consists of two main parts. Recognizing the existence of identification errors due to variations of system parameters, the first part is devoted to the verification of the existence of a unique continuous solution of a non-linear vector stochastic differential equation with a random parameter and to the establishment of the stochastic sensitivity equation. Both the a and β-stochastic sensitivity equations are established through the precise definition of stochastic sensitivity.

The remainder of this paper deals with evaluation of quantitative aspects of the sensitivity in the state estimation by using the stochastic sensitivity equation.     

Comments on ‘Parallel procedure for the design of observers in high-order multivariable systems’

Shafai (1988) proposed a state observer design algorithm which is very slightly different from an earlier algorithm of Tsui (1985). Furthermore, the change made by Shafai severely worsens the parallel computation property, the only key property of his algorithm claimed by Shafai. It is important to mention that this comment is obviously valid for the dual version of Shafai's algorithm (state feedback design for pole assignment), which has appeared elsewhere several times recently.     

Job roles of systems analysts in the “profit” vs the “not for profit” sectors

Study of the personal characteristics and job development roles of a nationwide sample of 104 systems analysts in 20 companies in both the “profit” and “not for profit” sectors indicates that there are significant differences in responsibilities. The range of duties is more narrowly defined for the systems analyst in the “not for profit” sector. The analyst in the “not for profit” sector is more likely to write documentation and perform end user training. Maintenance duties are about the same. The analyst in the “profit sector” tends to be younger, has less formal education and is paid more than his “not for profit” counterpart.The paper specifically covers (1) system development roles, (2) training and documentation duties, (3) systems maintenance duties, (4) staff functions, and (5) professional development activities.     

Levenberg–Marquardt Algorithm for Acousto-Electric Tomography based on the Complete Electrode Model

Journal of Mathematical Imaging and Vision - The inverse problem in acousto-electric tomography concerns the reconstruction of the electric conductivity in a body from knowledge of the power...