Natalia López

An Overview of Probabilistic Process Algebras and Their Equivalences

In order to describe probabilistic processes by means of a formal model, some considerations have to be taken into account. In this paper we present some of the ideas appeared in the literature that could help to define appropriate formal frameworks for the specification of probabilistic processes. First, we will explain the different interpretations of the probabilistic information included in this kind of models. After that, the different choice operators used in the most common probabilistic languages are enumerated. Once we have an appropriate language, we have to give its semantics. Thus, we will review some of the theories based on bisimulation and testing semantics. We will conclude by studying the extensions of the chosen languages with other operators such as parallel composition and hiding.

MDR1 polymorphisms and response to azathioprine therapy in patients with Crohn's disease.

Background To investigate the contribution of multidrug resistance 1 (MDR1) gene pharmacogenetics (G2677T/A and C3435T) to the efficacy of azathioprine in inducing remission in patients with Crohns disease (CD). Methods A cohort of 327 unrelated Spanish patients with CD recruited from a single center was studied. All patients were rigorously followed up for at least 2 years (mean time, 11.5 years). A case‐control analysis of MDR1 G2677T/A and C3435T SNPs and 2 loci haplotypes in 112 steroid‐dependent CD patients treated with azathioprine was performed. Patients were classified on the basis of response to azathioprine. Results A total 76 patients treated with azathioprine for longer than 3 months were included. Remission was achieved in 42 CD patients (55.3%). A higher frequency of the 2677TT genotype was found in nonresponders than in responders (17.65% versus 7.14%; OR = 2.8; 95% CI; 0.6–12.1; P = 0.11). Nonresponders to azathioprine were found to have a higher frequency of the 3435TT genotype than did CD patients who had achieved clinical remission (17.64% versus 4.76%; OR = 4.3; 95% CI, 0.8–22.8; P = 0.06). The 2677T/3435T haplotype was also more abundant in nonresponders (29.4% versus 20.2%), whereas the 2677G/3435C haplotype was more frequent in responders (58.3% versus 47.1%). Lack of response to azathioprine therapy in CD patients was 1.8‐fold greater in carriers of the 2677T/3435T haplotype than in carriers of the 2677G/3435C haplotype (OR = 1.8; 95% CI, 0.82–3.9; P = 0.14). Conclusions The results of our study indicate higher frequencies of the 2677TT and 3435TT genotypes and the 2677T/3435T haplotype in CD patients who did not respond to azathioprine. Additional replications in independent populations would confirm the real impact of these polymorphisms in response to azathioprine therapy. (Inflamm Bowel Dis 2007)

Specification, testing and implementation relations for symbolic-probabilistic systems

We consider the specification and testing of systems where probabilistic information is not given by means of fixed values but as intervals of probabilities. We will use an extension of the finite state machines model where choices among transitions labelled by the same input action are probabilistically resolved. We will introduce our notion of test and we will define how tests are applied to implementations under test. We will also present implementation relations to assess the conformance, up to a level of confidence, of an implementation to a specification. In order to define these relations we will take finite samples of executions of the implementation and compare them with the probabilistic constraints imposed by the specification. Finally, we will give an algorithm for deriving sound and complete test suites.

A Testing Theory for Generally Distributed Stochastic Processes

In this paper we present a testing theory for stochastic processes. This theory is developed to deal with processes which probability distributions are not restricted to be exponential. In order to define this testing semantics, we compute the probability with which a process passes a test before an amount of time has elapsed. Two processes will be equivalent if they return the same probabilities for any test T and any time t. The key idea consists in joining all the random variables associated with the computations that the composition of process and test may perform. The combination of the values that this random variable takes and the probabilities of executing the actions belonging to the computation will give us the desired probabilities. Finally, we relate our stochastic testing semantics with other notions of testing.

A Formal Framework for E-Barter Based on Microeconomic Theory and Process Algebras

In this paper we present a formal framework for the definition of e-barter architectures. By e-barter we mean the possibility of (electronically) exchanging goods without reducing transactions to money. Actually, in our setting, money can be considered just as another good, so that e-barter generalizes seller/buyer architectures. An advantage of e-barter systems, in contrast with most current systems, is that multilateral exchanges can be performed. Customers are first grouped into local markets, according mainly to their localities. Next, a higher order construction allows to compose markets, so that a global market takes a tree-like shape.In order to methodically build our systems, we consider a process algebraic notation. This allows us to specify all the stages of a system (from customers to markets, markets of markets, etc). We introduce an operational semantics for our language so that exchanges of goods are formally defined. Besides, we use some concepts borrowed from microeconomic theory. Specifically, we consider utility functions (i.e. functions returning the valuation that customers/markets give to goods), exchange of goods, and equilibria.We will show that the integration of microeconomic theory and process algebras provides two important gains. Firstly, it allows to avoid ambiguity in the understanding of the behavior of systems. Secondly, it gives a scheme to appropriately structure, in a bottom-up way, e-barter systems.

Including Malicious Agents into a Collaborative Learning Environment

In this paper we introduce a collaborative environment for the development of medium-size programming projects. Our system provides the usual facilities for communication among members of the group as well as a friendly programming environment for the functional programming language Haskell. A relevant feature of our learning environment is that some of the students may be, in fact, virtual students. It is worth to point out that these agents will not always behave as helpers. On the contrary, it can happen that they produce, on purpose, wrong programs. By doing so, we pretend that students get the abilities to detect mistakes not only in their own code, but also in the code generated by other team-mates.

Global Timed Bisimulation: An Introduction

Bisimulations are a broadly used formalism to define the semantics of process algebras. In particular, by means of weak bisimulation most of the internal activity of processes may be abstracted. Unfortunately, this is not fully accomplished: for instance, the internal choice operator becomes non-associative since bisimulation can see the branching structure of processes. In this paper we propose global timed bisimulation as a weakening of weak timed bisimulation. Global timed bisimulation is defined exactly as weak timed bisimulation once ordinary transitions are replaced by the adequate notions of generalized transitions. In order to asses the definition of our global timed bisimulation we present a collection of small examples that illustrate each of the clauses of that definition. Finally, a more elaborated example is presented to summarize the main properties of that notion.

Weak stochastic bisimulation for non-markovian processes

In this paper we introduce a novel notion of bisimulation to properly capture the behavior of stochastic systems with general distributions. The key idea consists in the identification of different sequences of random variables if the additions of the random variables of each sequence are identically distributed. That is, we will not only identify sequences of internal actions with one of them (as it is usually done in weak bisimulations) but we will also reduce (in some conditions) sequences of stochastic transitions to only one transition. Therefore, we will identify processes that are considered non-equivalent in previous notions of bisimulation for this kind of languages.

An integrated framework for the performance analysis of asynchronous communicating stochastic processes

Abstract.In this paper we present a design framework containing a process algebra and the concurrent functional programming language Eden. In order to study properties of a specification written in our process algebraic notation, we provide a translation mechanism to generate Eden programs. Once we have a translation, we may use the Eden tools to study the performance of the (simulation of the) system. In order to add expressiveness to our design language we use a very powerful process algebra. First, we allow the specification of delays induced by general random variables. We also consider value passing. Finally, the communication between concurrent processes is asynchronous. The usefulness of our framework is presented by two examples featuring all the characteristics of our process algebraic model, we give the corresponding translations, and we provide some performance measures obtained by using Eden tools.

A multi-agent system for e-barter including transaction and shipping costs

An e-barter multi-agent system consists of a set of agents exchanging goods. In contrast to e-commerce systems, transactions do not necessarily involve the exchange of money. Agents are equipped with a utility function to simulate the preferences of the customers that they are representing. They are grouped into local markets, according to the localities of the corresponding customers. Once these markets are saturated (i.e. no more exchanges can be performed) new agents, representing those local markets, are generated and combined into new markets. By reiteratively applying this process we finally get a global market.Even though a formalism to define e-barter architectures has been already introduced, that framework had a strong drawback: Neither transaction nor shipping costs were considered. In this paper we extend that framework to deal with systems where fees have to be paid to the owner of the system. These fees depend on the goods involved in the corresponding exchanges. In addition, shipping costs have also to be paid. These modifications complicate the setting because the utility that customers receive after exchanging goods is not directly given by the original utility function. That is, the returned utility after an exchange is performed has to be computed as a combination of the former utility and the derived costs. In particular, some exchanges may be disallowed because those costs exceed the increase of utility returned by the new basket of goods.