linear logical relations and observational equivalences for session based concurrency we investigate strong normalization confluence and behavioral equality in the realm of session based concurrency these interrelated issues underpin advanced correctness analysis in models of structured communications the starting point for our study is an interpretation of linear logic propositions as session types for communicating processes proposed in prior work strong normalization and confluence are established by developing a theory of logical relations defined upon a linear type structure our logical relations remain remarkably similar to those for functional languages we also introduce a natural notion of observational equivalence for session typed processes strong normalization and confluence come in handy in the associated coinductive reasoning as applications we prove that all proof conversions induced by the logic interpretation actually express observational equivalences and explain how type isomorphisms resulting from linear logic equivalences are realized by coercions between interface types of session based concurrent systems

open bisimulation revisited in the context of the p calculus open bisimulation is prominent and popular due to its congruence properties and its easy implementability motivated by the attempt to generalise it to the spi calculus we offer a new more refined definition and show how far it coincides with the original one

symmetries local names and dynamic de allocation of names the semantics of name passing calculi is often defined employing coalgebraic models over presheaf categories this elegant theory lacks finiteness properties hence it is not apt to implementation coalgebras over named sets called history dependent automata are better suited for the purpose due to locality of names a theory of behavioural functors for named sets is still lacking the semantics of each language has been given in an ad hoc way and algorithms were implemented only for the p calculus existence of the final coalgebra for the p calculus was never proved we introduce a language of accessible functors to specify history dependent automata in a modular way leading to a clean formulation and a generalisation of previous results and to the proof of existence of a final coalgebra in a wide range of cases

on the symbolic reduction of processes with cryptographic functions we study the reachability problem for cryptographic protocols represented as processes relying on perfect cryptographic functions we introduce a symbolic reduction system that can handle hashing functions symmetric keys and public keys desirable properties such as secrecy or authenticity are specified by inserting logical assertions in the processes we show that the symbolic reduction system provides a flexible decision procedure for finite processes and a reference for sound implementations the symbolic reduction system can be regarded as a variant of syntactic unification which is compatible with certain set membership constraints for a significant fragment of our formalism we argue that a dag implementation of the symbolic reduction system leads to an algorithm running in nptime thus matching the lower bound of the problem in the case of iterated or finite control processes we show that the problem is undecidable in general and in ptime for a subclass of iterated processes that do not rely on pairing our technique is based on rational transductions of regular languages and it applies to a class of processes containing the ping pong protocols presented in dolev et al inform comput 55 1982 57

variations on mobile processes the paper investigates a concurrent computation model chi calculus in which communications resemble cut eliminations for classical proofs two bisimilarities local bisimilarity and barbed bisimilarity on chi processes are studied and are shown to be congruence relations the former equivalence turns out to be strictly stronger than the latter it is shown that chi calculus is capable of modeling sequential computation in that it captures the operational semantics of call by name lambda calculus a translation from pi calculus to chi calculus is given demonstrating that practically speaking pi is a sublanguage of chi a higher order version of chi calculus is proposed and examined it combines the communication mechanism of chi calculus and the recursion mechanism of full lambda calculus and therefore extends both

proof nets garbage and computations we study the problem of local and asynchronous computation in the context of multiplicative exponential linear logic mell proof nets the main novelty is in a complete set of rewriting rules for cut elimination in presence of weakening which requires garbage collection the proposed reduction system is strongly normalizing and confluent

bisimulation congruence of ℵ calculus calculus was proposed as a process calculus that has a uniform treatment of names preliminary properties of calculus have been examined in the literature in this paper a more systematic study of bisimilarities for processes is carried out the notion of l bisimilarity is introduced to give a possible classification of bisimilarities on processes it is shown that the set of l bisimilarities form a four element lattice and that well known bisimilarities for processes fit into the lattice hierarchy the four distinct l bisimilarities give rise to four congruence relations complete axiomatization system is given for each of the four congruences

anti self dual four manifolds with a parallel real spinor antiselfdual metrics in the signature that admit a covariantly constant real spinor are studied it is shown that finding such metrics reduces to solving a fourthorder integrable partial differ

what is a good encoding of guarded choice abstract the calculus with synchronous output and mixed guarded choices is strictly more expressive than the calculus with asynchronous output and no choice this result was recently proved by c palamidessi and as a corollary she showed that there is no fully compositional encoding from the former into the latter that preserves divergence freedom and symmetries this paper argues that there are nevertheless good encodings between these calculi in detail we present a series of encodings for languages with 1 input guarded choice 2 both input and output guarded choice and 3 mixed guarded choice and investigate them with respect to compositionality and divergence freedom the first and second encoding satisfy all of the above criteria but various good candidates for the third encoding inspired by an existing distributed implementation invalidate one or the other criterion while essentially confirming palamidessi s result our study suggests that the combination of strong compositionality and divergence freedom is too strong for more practical purposes

domain theory for concurrency a simple domain theory for concurrency is presented based on a categorical model of linear logic and associated comonads it highlights the role of linearity in concurrent computation two choices of comonad yield two expressive metalanguages for higher order processes both arising from canonical constructions in the model their denotational semantics are fully abstract with respect to contextual equivalence one language derives from an exponential of linear logic it supports a straightforward operational semantics with simple proofs of soundness and adequacy the other choice of comonad yields a model of affine linear logic and a process language with a tensor operation to be understood as a parallel composition of independent processes the domain theory can be generalised to presheaf models providing a more refined treatment of nondeterministic branching the article concludes with a discussion of a broader programme of research towards a fully fledged domain theory for concurrency

algebraic theories for name passing calculi in a theory of processes the names are atomic data items which can be exchanged and tested for identify a well known example of a calculus for name passing is the calculus where names are additionally used as communication ports we provide complete axiomatisations of late and early bisimulation equivalences in such calculi since neither of the equivalences is a congruence we also axiomatise the corresponding largest congruences we consider a few variations of the signature of the language among these a calculus of deterministic processes which is reminiscent of sequential functional programs with a conditional construct most of our axioms are shown to be independent the axiom systems differ only by a few simple axioms and reveal the similarities and the symmetries of the calculi and the equivalences

on the existence and decidability of unique decompositions of processes in the applied π calculus unique decomposition has been a subject of interest in process algebra for a long time for example in bpp 1 or ccs 2 3 as it provides a normal form and useful cancellation properties we provide two parallel decomposition results for subsets of the applied calculus we show that every closed normed i e with a finite shortest complete trace process p can be decomposed uniquely into prime factors p i with respect to strong labeled bisimilarity i e such that p l p 1 p n moreover we prove that closed finite processes can be decomposed uniquely with respect to weak labeled bisimilarity we also investigate whether efficient algorithms that compute the unique decompositions exist the simpler problem of deciding whether a process is in its unique decomposition form is undecidable in general in both cases due to potentially undecidable equational theories moreover we show that the unique decomposition remains undecidable even given an equational theory with a decidable word problem

the positive rates problem for attractive nearest neighbor spin systems on ℤ it is shown that a spin system on has only one invariant probability measure if it has attractive or repulsive nearest neighbor flip rates which are strictly positive and periodic under translation along

bisimulation can t be traced in the concurrent language ccs two programs are considered the same if they are bisimilar several years and many researchers have demonstrated that the theory of bisimulation is mathematically appealing and useful in practice however bisimulation makes too many distinctions between programs we consider the problem of adding operations to ccs to make bisimulation fully abstract we define the class of gsos operations generalizing the style and technical advantages of ccs operations we characterize gsos congruence in as a bisimulation like relation called ready simulation bisimulation is strictly finer than ready simulation and hence not a congruence for any gsos language

linear concurrent constraint programming in this paper we give a logical semantics for the class cc of concurrent constraint programming languages and for its extension lcc based on linear constraint systems besides the characterization in intuitionistic logic of the stores of cc computations we show that both the stores and the successes of lcc computations can be characterized in intuitionistic linear logic we illustrate the usefulness of these results by showing with examples how the phase semantics of linear logic can be used to give simple semantical proofs of safety properties of lcc programs

towards a unified approach to encodability and separation results for process calculi we present a unified approach to evaluate the relative expressive power of process calculi in particular we identify a small set of criteria that have already been somehow presented in the literature that an encoding should satisfy to be considered a valid means for language comparison we argue that the combination of such criteria is a valid proposal by noting that i several well known encodings appeared in the literature satisfy them ii this notion is not trivial because some known encodings do not satisfy all the criteria we have proposed iii several well known separation results can be formulated in terms of our criteria and iv some widely believed but never formally proved separation results can be proved by using the criteria we propose moreover the criteria defined induce general proof techniques for separation results that can be easily instantiated to cover known case studies

filter models for conjunctive disjunctive l calculi abstract the distinction between the conjunctive nature of non determinism as opposed to the disjunctive character of parallelism constitutes the motivation and the starting point of the present work calculus is extended with both a non deterministic choice and a parallel operator a notion of reduction is introduced extending reduction of the classical calculus we study type assignment systems for this calculus together with a denotational semantics which is initially defined constructing a set semimodel via simple types we enrich the type system with intersection and union types dually reflecting the disjunctive and conjunctive behaviour of the operators and we build a filter model the theory of this model is compared both with a morris style operational semantics and with a semantics based on a notion of capabilities

behavioral theory for mobile ambients we study a behavioral theory of mobile ambients a process calculus for modelling mobile agents in wide area networks focussing on reduction barbed congruence our contribution is threefold 1 we prove a context lemma which shows that only parallel and nesting contexts need be examined to recover this congruence 2 we characterize this congruence using a labeled bisimilarity this requires novel techniques to deal with asynchronous movements of agents and with the invisibility of migrations of secret locations 3 we develop refined proof methods involving up to proof techniques which allow us to verify a set of algebraic laws and the correctness of more complex examples

unique decomposition of processes abstract in this paper we examine questions about the prime decomposability of processes where we define a process to be prime whenever it cannot be decomposed into nontrivial components we show that any finite process can be uniquely decomposed into prime processes with respect to bisimulation equivalence and demonstrate counterexamples to such a result for both failures testing equivalence and trace equivalence although we show that prime decompositions cannot exist for arbitrary infinite processes we motivate but leave as open a conjecture on the unique decomposability of a wide subclass of infinite behaviours

two decentralized algorithms for strong interaction fairness for systems with unbounded speed variability abstract we present two randomized algorithms one for message passing and the other for shared memory that with probability 1 schedule multiparty interactions in a strongly fair manner both algorithms improve upon a previous result by joung and smolka proposed in a shared memory model along with a straightforward conversion to the message passing paradigm in the following aspects first processes speeds as well as communication delays need not be bounded by any predetermined constant secondly our algorithms are completely decentralized and the shared memory solution makes use of only single writer variables finally both algorithms are symmetric in the sense that all processes execute the same code and no unique identifier is used to distinguish processes

gsos and finite labelled transition systems recently there has been considerable interest in studying formats of plotkin style inference rules which ensure that the induced labelled transition system semantics have certain properties in this note i shall give a contribution to this line of research by giving a restricted version of bloom istrail and meyer s gsos format bloom et al 1988 bloom 1989 which induces finite labelled transition systems

static analysis for the π calculus with applications to security abstract control flow analysis is a static technique for predicting safe and computable approximations to the set of values that the objects of a program may assume during its execution we present an analysis for the calculus that shows how names will be bound to actual channels at run time the result of our analysis establishes a super set of the set of channels to which a given name may be bound and of the set of channels that may be sent along a given channel besides a set of rules that permits one to validate a given solution we also offer a constructive procedure that builds solutions in low polynomial time applications of our analysis include establishing two simple security properties of processes one example is that p has no leaks p offers communication to the external environment through public channels only and confines its secret channels within itself the other example is connected to the no read up no write down property of bell and lapadula once processes are given levels of security clearance we check that a process at a high level never sends channels to processes at a lower level

decomposition orders another generalisation of the fundamental theorem of arithmetic we discuss unique decomposition in partial commutative monoids inspired by a result from process theory we propose the notion of decomposition order for partial commutative monoids and prove that a partial commutative monoid has unique decomposition iff it can be endowed with a decomposition order we apply our result to establish that the commutative monoid of weakly normed processes modulo bisimulation definable in acpe with linear communication with parallel composition as binary operation has unique decomposition we also apply our result to establish that the partial commutative monoid associated with a well founded commutative residual algebra has unique decomposition

more on bisimulations for higher order π calculus in this paper we prove the coincidence between strong weak context bisimulation and strong weak normal bisimulation for higher order p calculus which generalizes sangiorgi s work to achieve this aim we introduce indexed higher order p calculus which is similar to higher order p calculus except that every prefix of any process is assigned indices furthermore we present corresponding indexed bisimulations for this calculus and prove the equivalence between these indexed bisimulations based on this result we prove the main result of this paper i e the equivalence between strong weak context bisimulation and strong weak normal bisimulation

a fully abstract denotational semantics for the calculus of higher order communicating systems abstract in this paper we study the calculus of higher order communicating systems chocs thomsen proc of popl 89 acm 1989 pp 143 154 inform comput 116 1 1995 38 57 in a denotational setting we present a construction of a denotational semantics for chocs which resides in a domain constructed using the standard constructions of separated sum cartesian product the plotkin power domain constructor and recursively defined domains we show under mild restrictions that the denotational semantics and the operational semantics of chocs are fully abstract we have previously proved using bisimulation arguments that processes as first class objects are powerful enough to simulate recursion however the proof is very long and tedious to demonstrate the power of the denotational approach we use it to obtain a very simple proof of the simulation of recursion result

testing equivalences for processes abstract given a set of processes and a set of tests on these processes we show how to define in a natural way three different equivalences on processes these equivalences are applied to a particular language ccs we give associated complete proof systems and fully abstract models these models have a simple representation in terms of trees

stochastic biological modelling in the presence of multiple compartments the application of concurrent calculi to the formalisation of biological systems constitutes a promising approach to the analysis of biological phenomena in silico the peculiar nature of such systems inspired the introduction of specific features in biologically oriented calculi such as compartments to model more faithfully their highly organised structure in this paper we present email protected a conservative extension of the stochastic p calculus which allows an intuitive and concise formalisation of multi compartment systems with dynamic structure despite retaining the simplicity of the original p calculus the possibility to encode into email protected several bio inspired compartmentalised languages demonstrates its expressive power and flexibility the calculus is accompanied by an extended version of gillespie s stochastic simulation algorithm able to handle multiple compartments with varying volumes an enhanced formalisation of the algorithm is also presented in order to provide efficient simulation in the presence of a high number of compartments and reactions

theory of interaction abstract theory of interaction aims to provide a foundational framework for computation and interaction it proposes four fundamental principles that characterize the common features of all models of computation and interaction these principles suffice to support a model independent treatment of the two most important relationships in computer science the equality between processes and the relative expressiveness between models based on the two relationships the theory of equality the theory of expressiveness and the theory of completeness are developed

basic observables for a calculus for global computing we develop the semantic theory of a foundational language for modelling applications over global computers whose interconnection structure can be explicitly manipulated together with process distribution process mobility and remote asynchronous communication through distributed data repositories the language has primitives for explicitly modelling inter node connections and for dynamically activating and deactivating them for the proposed language we define natural notions of extensional observations and study their closure under operational reductions and or language contexts to obtain barbed congruence and may testing equivalence we then focus on barbed congruence and provide an alternative characterisation in terms of a labelled bisimulation to test practical usability of the semantic theory we model a system of communicating mobile devices and use the introduced proof techniques to verify one of its key properties

deciding equivalence based properties using constraint solving formal methods have proved their usefulness for analyzing the security of protocols most existing results focus on trace properties like secrecy or authentication there are however several security properties which cannot be defined or cannot be naturally defined as trace properties and require a notion of behavioural equivalence typical examples are anonymity privacy related properties or statements closer to security properties used in cryptography in this paper we consider three notions of equivalence defined in the applied pi calculus observational equivalence may testing equivalence and trace equivalence first we study the relationship between these three notions we show that for determinate processes observational equivalence actually coincides with trace equivalence a notion simpler to reason with we exhibit a large class of determinate processes called simple processes that capture most existing protocols and cryptographic primitives while trace equivalence and may testing equivalence seem very similar we show that may testing equivalence is actually strictly stronger than trace equivalence we prove that the two notions coincide for image finite processes such as processes without replication second we reduce the decidability of trace equivalence for finite processes to deciding symbolic equivalence between sets of constraint systems for simple processes without replication and with trivial else branches it turns out that it is actually sucient to decide symbolic equivalence between pairs of positive constraint systems thanks to this reduction and relying on a result first proved by m baudet this yields the first decidability result of observational equivalence for a general class of equational theories for processes without else branch nor replication moreover based on another decidability result for deciding equivalence between sets of constraint systems we get decidability of trace equivalence for processes with else branch for standard primitives

model checking mobile processes we introduce a temporal logic for the polyadic calculus based on fixed point extensions of hennessy milner logic features are added to account for parametrisation generation and passing of names including the use following milner of dependent sum and product to account for unlocalised input and output and explicit parametrisation on names using abstraction and application the latter provides a single name binding mechanism supporting all parametrisation needed a proof system and decision procedure is developed based on stirling and walker s approach to model checking the modal calculus using constants one difficulty for both conceptual and efficiency based reasons is to avoid the explicit use of the rule for parametrised processes a key idea following hennessy and lin s approach to deciding bisimulation for certain types of value passing processes is the relativisation of correctness assertions to conditions on names based on this idea a proof system and a decision procedure are obtained for arbitrary calculus processes with finite control calculus correlates of ccs finite state processes avoiding the use of parallel composition in recursively defined processes

dunajski generalization of the second heavenly equation dressing method and the hierarchy dunajski generalization of the second heavenly equation is studied a dressing scheme applicable to the dunajski equation is developed an example of constructing solutions in terms of implicit functions is considered a dunajski equation hierarchy is described and its lax sato form is presented the dunajski equation hierarchy is characterized by the conservation of a three dimensional volume form in which a spectral variable is taken into account

on expressive completeness of duration and mean value calculi the standard way of lifting a binary relation r from closed terms of an algebra to open terms is to define its closed instance extension r ci which holds for a given pair of open terms if and only if r holds for all their closed instantiations in this paper we study alternatives for the case of strong bisimulation we define semantic models for open terms so called conditional transition systems and define bisimulation directly on those models it turns out that this can be done in at least two different ways giving rise to formal hypothesis bisimulation sim fh due to de simone and hypothesis preserving bisimilarity sim hp for open terms we have strict inclusions sim fh subset sim hp subset sim ci for closed terms the three relations coincide we show that each of these relations is a congruence in the usual sense and we give an alternative characterisation of sim hp in terms of non conditional transitions finally we study the issue of recursive congruence we give general theorems for the congruence of each of the above variants with respect to the recursion combinator where however the results we achieve for sim fh and sim hp hold in a more general setting than the one for sim ci

complete inference systems for weak bisimulation equivalences in the π calculus proof systems for weak bisimulation equivalences in the calculus are presented and their soundness and completeness are shown two versions of calculus are investigated one without and the other with the mismatch operator for each version of the calculus proof systems for both late and early weak bisimulation equivalences are studied thus there are four proof systems in all these inference systems are related in a natural way the inference system for early equivalence is obtained from the one for late equivalence by replacing the inference rule for input prefix while the inference system for the version of calculus with mismatch is obtained by adding a single inference rule for mismatch to the one for the version without it the proofs of the completeness results rely on the notion of symbolic bisimulation

a randomized encoding of the π calculus with mixed choice we consider the problem of encoding the calculus with mixed choice i e input output guarded choice into the asynchronous calculus we are interested in a translation which satisfies certain desirable properties namely uniformity and preservation of a reasonable semantics although it has been shown that this is not possible with an exact encoding we suggest a randomized approach using a probabilistic extension of the asynchronous calculus and we show that our solution is correct with probability 1 under any proper adversary wrt a notion of testing semantics this result establishes the basis for a distributed and symmetric implementation of mixed choice which unlike previous proposals in literature does not rely on assumptions on the relative speed of processes and it is robust to attacks of proper adversaries

cpo models for compact gsos languages in this paper we present a general way of giving denotational semantics to a class of languages equipped with an operational semantics that fits the gsos format of bloom istrail and meyer the canonical model used for this purpose will be abramsky s domain of synchronization trees and the denotational semantics automatically generated by our methods will be guaranteed to be fully abstract with respect to the finitely observable part of the bisimulation preorder in the process of establishing the full abstraction result we also obtain several general results on the bisimulation preorder including a complete axiomatization for it and give a novel operational interpretation of gsos languages

symmetric electoral systems for ambient calculi this paper compares the expressiveness of different fragments of ambient calculi via leader election problems we consider mobile ambients ma safe ambients sa and the push and pull ambient calculus pac cardelli and gordon encoded the asynchronous p calculus into ma zimmer has shown that the synchronous p calculus without choice can be encoded in pure no communication sa we show that pure ma without restriction has symmetric electoral systems that is it is possible to solve the problem of electing a leader in a symmetric network by the work of palamidessi this implies that this fragment of ma is not encodable under certain conditions in the p calculus with separate choice moreover we use the same technique to show that fragments of sa and pac are not encodable under certain conditions in the p calculus with separate choice we also show that particular fragments of ambient calculi do not admit a solution to leader election problems in the same way as the p calculus with separate choice this yields a fine grained hierarchy within ambient calculi

a symbolic semantics for the π calculus we use symbolic transition systems as a basis for providing the calculus with an alternative semantics the latter is more amenable to automatic manipulation and sheds light on the logical differences among different forms of bisimulation over algebras of name passing processes symbolic transitions have the formformula where is a boolean combination of equalities on names that has to hold for the transition to take place and is standard a calculus action on top of the symbolic transition system a symbolic bisimulation is defined that captures the standard ones finally a sound and complete proof system is introduced for symbolic bisimulation

ergodicity for spin systems with stirrings or 1 each site after an exponentially distributed random time is updated by a rule that depends on the configuration in the site s neighborhood such processes have been used to study the gibbs states associated with some potential reversible spin flip systems having as invariant measure a gibbs state related to some potential are called stochastic ising models a particle system is ergodic if a there exists a unique invariant measure and b starting from any initial measure the process converges to that invariant measure ergodicity for an attractive ising model is equivalent to absence of phase transition for the related gibbs state indeed the study of gibbs states allowed the proof of ergodicity of attractive short range onedimensional ising models and the absence of ergodicity for nearest neighbor two dimensional ising models two of the most important results on the stochastic ising model a criterion for the ergodicity of spin systems has been given by dobrushin 4 dobrushin established that m e is a sufficient condition for the ergodicity of a spin flip system where m represents the maximal influence of other sites on the spin flip rate at any site and e is in some sense the minimum spin flip rate at any site this approach was also studied by gray and griffeath 11 sullivan 19 and holley and stroock 14 15 a review can be found in liggett 17 recent related results are obtained by aizenman and holley 1 in this paper we exploit a generalized graphical representation of the systems to obtain some results for spin systems and combinations of spin systems with stirring processes in theorem 2 5 we show that m 28 is a sufficient condition for the ergodicity of the one dimensional nearest neighbor ising model the result is interesting because no attractiveness conditions are imposed in the attractive case gray 7 proved that any one dimensional

reformulation of gray s duality for attractive spin systems and its applications duality relations which associate two markov processes in different state spaces have been useful tools in the study of the long term behaviour of the stochastic processes of spin systems in 1986 gray introduced a new duality theory which is applicable to general spin systems with attractive transition rates the theory was developed by making full use of graphical representations in the present paper gray s duality is reformulated by studying the action of the formal generators on a newly chosen duality function and the dual processes defined in the state space upsilon a a is a finite subset of y are discussed where y is a collection of finite subsets of zd as applications of the submodularity of the survival probability sigma a of the dual processes rigorous lower bounds of the critical values are derived for the theta contact process the multi particle creation model and the sexual reproduction process

on quasi open bisimulation quasi open bisimilarity is a variant of the open bisimilarity based on a closer examination of the observationality of local names the paper investigates two alternative characterizations of the quasi open bisimilarity and provides a complete system for the weak quasi open congruence

deciding knowledge in security protocols under equational theories the analysis of security protocols requires precise formulations of the knowledge of protocol participants and attackers in formal approaches this knowledge is often treated in terms of message deducibility and indistinguishability relations in this paper we study the decidability of these two relations the messages in question may employ functions encryption decryption etc axiomatized in an equational theory one of our main positive results says that deducibility and indistinguishability are both decidable in polynomial time for a large class of equational theories this class of equational theories is defined syntactically and includes for example theories for encryption decryption and digital signatures we also establish general decidability theorems for an even larger class of theories these theorems require only loose abstract conditions and apply to many other useful theories for example with blind digital signatures homomorphic encryption xor and other associative commutative functions

behavioral equivalence in the polymorphic pi calculus we investigate parametric polymorphism in message based concurrent programming focusing on behavioral equivalences in a typed process calculus analogous to the polymorphic lambda calculus of girard and reynolds polymorphism constrains the power of observers by preventing them from directly manipulating data values whose types are abstract leading to notions of equivalence much coarser than the standard untyped ones we study the nature of these constraints through simple examples of concurrent abstract data types and develop basic theoretical machinery for establishing bisimilarity of polymorphic processes we also observe some surprising interactions between polymorphism and aliasing drawing examples from both the polymorphic pi calculus and ml

determinancy observation equivalence trace equivalence abstract if an experiment s is conducted on a parallel process p then in general different processes may result from the experiment due to the nondeterministic behaviour of p in the notation of milner 1980 p p for different p process p is called determinate if the resulting processes are all equivalent i e if p p and p p then p and p are equivalent this means that although p behaves nondeterministically this cannot be detected by an observer of p let denote observation equivalence used in ccs milner 1980 let f denote the much weaker failure equivalence used for csp hoare et al 1981 brookes 1983 and let t denote the still weaker trace equivalence we show that the three corresponding notions of determinancy are the same and that for determinate processes f and t are the same determinacy is preserved under and f but not under t

observational congruences for dynamically reconfigurable tile systems the sos formats that ensure that bisimilarity is a congruence tail in the presence ot structural axioms on states dynamic bisimulation introduced to characterize the coarsest congruence for ccs which is also a weak bisimulation reconciles the bisimilarity is a congruence property with structural axioms and also with the specification of open ended systems where states can be reconfigured at runtime we show that the compositional framework offered by tile logic handles structural axioms and specifications of reconfigurable systems successfully this allows for a finitary presentation of dynamic context closure as internalized in the tile language the case study of the calculus illustrates the main features of our approach moreover duality is exploited to model a second kind of reconfiguration dynamic specialization

decoding choice encodings we study two encodings of the asynchronous calculus with input guarded choice into its choice free fragment one encoding is divergence free but refines the atomic commitment of choice into gradual commitment the other preserves atomicity but introduces divergence the divergent encoding is fully abstract with respect to weak bisimulation but the more natural divergence free encoding is not instead we show that it is fully abstract with respect to coupled simulation a slightly coarser but still coinductively defined equivalence that does not enforce bisimilarity of internal branching decisions the correctness proofs for the two choice encodings introduce a novel proof tech nique exploiting the properties of explicit decodings from translations to source terms

hierarchical combination of intruder theories recently automated deduction tools have proved to be very effective for detecting attacks on cryptographic protocols these analysis can be improved for finding more subtle weaknesses by a more accurate modelling of operators employed by protocols several works have shown how to handle a single algebraic operator associated with a fixed intruder theory or how to combine several operators satisfying disjoint theories however several interesting equational theories such as exponentiation with an abelian group law for exponents remain out of the scope of these techniques this has motivated us to introduce a new notion of hierarchical combination for non disjoint intruder theories and to show decidability results for the deduction problem in these theories we have also shown that under natural hypotheses hierarchical intruder constraints can be decided this result applies to an exponentiation theory that appears to be more general than the one considered before

probabilistic bisimulations for quantum processes modeling and reasoning about concurrent quantum systems is very important for both distributed quantum computing and quantum protocol verification as a consequence a general framework formally describing communication and concurrency in complex quantum systems is necessary for this purpose we propose a model named qccs it is a natural quantum extension of classical value passing ccs which can deal with input and output of quantum states and unitary transformations and measurements on quantum systems the operational semantics of qccs is given in terms of probabilistic labeled transition system this semantics has many different features compared with the proposals in the available literature in order to describe the input and output of quantum systems which are possibly correlated with other components based on this operational semantics the notions of strong probabilistic bisimulation and weak probabilistic bisimulation between quantum processes are introduced furthermore some properties of these two probabilistic bisimulations such as congruence under various combinators are examined

a complexity analysis of bisimilarity for value passing processes abstract we study the complexity of deciding bisimilarity between non deterministic processes with explicit primitives for manipulating data values in particular we consider a language with value passing input output of data and parametric definitions of processes we distinguish the case in which data cannot be tested and the case in which a simple equality test over data is permitted in the first case our main result shows that the problem is pspace hard for the full calculus in the second case we first show that the problem is conp complete in the fragment with value passing and no parametric definitions we then define a compositional polynomial time translation of the full calculus to the fragment with parametric definitions but no value passing the translation preserves bisimilarity this fact establishes the decidability of the full calculus and shows that the fragment without value passing is computationally equivalent to the full calculus for the latter bisimilarity is then proved to be exp complete finally we add to our language a parallel composition operator and show that for a certain restricted syntactic format the bisimilarity problem is still decidable and exp complete

on the semantics of durational actions this paper develops a mathematical framework to describe and reason about semantic theories of processes where actions have a non zero duration the framework relies on the notions of reduction and observability and allows one to naturally explain the possible choices to incorporate timing information in terms of process interaction mechanisms the framework provides a parameterized context where well known and new theories can be formally compared and classified by a suitable instantiation of the parameters a proof technique to derive from the reduction semantics an equivalent sos based characterization is also provided

abstract interpretation based verification of temporal properties for bioambients this paper concerns the application of formal methods to biological systems modeled specifically in bioambients a variant of the mobile ambients calculus following the semantic based approach of abstract interpretation we define a new static analysis that computes an abstract transition system our analysis has two main advantages with respect to the analyses appearing in the literature i it is able to address temporal properties which are more general than invariant properties ii it supports by means of a particular labeling discipline the validation of systems where several copies of an ambient may appear we also design new weaker and more efficient analyses by means of simple widening operators

the lambda context calculus extended version we present the lambda context calculus this simple lambda calculus features variables arranged in a hierarchy of strengths such that substitution of a strong variable does not avoid capture with respect to abstraction by a weaker variable this allows the calculus to express both capture avoiding and capturing substitution instantiation the reduction rules extend the vanilla lambda calculus in a simple and modular way and preserve the look and feel of a standard lambda calculus with explicit substitutions good properties of the lambda calculus are preserved the lamcc is confluent and a natural injection into the lamcc of the untyped lambda calculus exists and preserves strong normalisation we discuss the calculus and its design with full proofs in the presence of the hierarchy of variables functional binding splits into a functional abstraction l lambda and a name binder new we investigate how the components of this calculus interact with each other and with the reduction rules with examples in two more extended case studies we demonstrate how global state can be expressed and how contexts and contextual equivalence can be naturally internalised using function application

a fully abstract denotational model for higher order processes a higher order process calculus is defined in which one can describe processes which transmit as messages other processes it may be viewed as a generalisation of the lazy calculus we present a denotational model for the language obtained by generalising the domain equation for abramsky s model of the lazy calculus it is shown to be fully abstract with respect to three different behavioural preorders the first is based on observing the ability of processes to perform an action in all contexts the second on testing and the final one on satisfying certain kinds of modal formulae

a fully abstract denotational semantics for the π calculus this paper describes the construction of two set theoretic denotational models for the p calculus the models are obtained as initial solutions to domain equations in a functor category by associating with each syntactic construct of the p calculus a natural transformation over these models we obtain two interpretations for the language we also show that these models are fully abstract with respect to natural behavioural preorders over terms in the language by this we mean that two terms are related behaviourally if and only if their interpretations in the model are related the behavioural preorders are the standard versions of may and must testing adapted to the p calculus

on the expressiveness of interaction subbisimilarity is proposed as a general tool to classify the relative expressive power of process calculi the expressiveness of several variants of ccs is compared in terms of the subbisimilarity relationship similar investigation is also carried out for the variants of the pi calculus the relative expressiveness of the different forms of the choice operation and the different ways of introducing infinite behaviors are systematically studied in both the frameworks of ccs and pi some issues concerning the expressiveness of both ccs and pi are clarified several open problems are solved along the way the subbisimilarity approach and the relative expressiveness results are applied to show the independence of the operators of the pi calculus the definition of the subbisimilarity relationship can be further strengthened with computational requirement leading to a uniform treatment of computation and interaction

concurrent nets a study of prefixing in process calculi we introduce the calculus of concurrent nets as an extension of the fusion calculus in which usual prefixing is replaced by arbitrary monotonic guards then we use this formalism to describe the prefixing policy of standard calculi as a particular form of communication by developing a graphical syntax we sharpen the geometric intuition and finally we provide an encoding of these guards as causality in the prefix free fragment in the spirit of the encoding of the fusion calculus into solos by laneve and victor proving that communication by fusion is expressive enough to implement arbitrary monotonic guards

a complete symbolic bisimulation for full applied pi calculus symbolic characterisations of bisimilarities for the applied pi calculus proposed so far are sound but incomplete even restricted to the finite fragment of the calculus in this paper we present a novel approach to symbolic semantics for the applied pi calculus leading to a notion of symbolic bisimulation which is both sound and complete with respect to the standard labelled bisimilarity moreover our framework accommodates replications hence works for the full calculus

tau laws for pi calculus the paper investigates the nonsymbolic algebraic semantics of the weak bisimulation congruences on finite pi processes the weak bisimulation congruences are studied both in the absence and in the presence of the mismatch operator some interesting phenomena about the open congruences are revealed several new tau laws are discovered and their relationship is discussed the contributions of the paper are mainly as follows 1 it is proved that milner s three tau laws fail to lift a complete system for the strong open congruence to a complete system for the weak open congruence in the absence of both the mismatch operator and the restriction operator a fourth tau law is proposed to deal with the match operator under the prefix operation it is shown that for this calculus a complete system for the strong open congruence extended with all the four tau laws is complete for the weak open congruence 2 it is verified that the four tau laws are also enough for the weak open congruence of the pi calculus without the mismatch operator a complete system using distinctions is given 3 it is pointed out that the standard definition of the weak open congruence gives rise to a bad equivalence relation in the presence of the mismatch operator two alternatives are proposed these are the late open congruence and the early open congruence their difference is similar to that between the weak late congruence and the weak early congruence complete axiomatic systems for the two weak open congruences are given

a self dual yang mills hierarchy and its reductions to integrable systems in 1 1 and 2 1 dimensions the self dual yang mills equations play a central role in the study of integrable systems in this paper we develop a formalism for deriving a four dimensional integrable hierarchy of commuting nonlinear flows containing the self dual yang mills flow as the first member we show that upon appropriate reduction and suitable choice of gauge group it produces virtually all well known hierarchies of soliton equations in 1 1 and 2 1 dimensions and can be considered as a universal integrable hierarchy prototypical examples of reductions to classical soliton equations are presented and related issues such as recursion operators symmetries and conservation laws are discussed

recursive equations in higher order process calculi regarding behaviour equivalence in higher order process calculi sangiorgi inform and comput 131 1996 141 and thomsen inform and comput 116 1995 38 introduced context and higher order bisimulations respectively in this paper uniqueness of solutions of equations with respect to strong context and higher order bisimilarities and compatibility of strong context and higher order bisimilarities with recursive definitions are shown copyright 2001 elsevier science b v

an exact correspondence between a typed pi calculus and polarised proof nets this paper presents an exact correspondence in typing and dynamics between polarised linear logic and a typed p calculus based on io typing the respective incremental constraints one on geometric structures of proof nets and one based on types precisely correspond to each other leading to the exact correspondence of the respective formalisms as they appear in olivier laurent 2003 27 for proof nets and kohei honda et al 2004 24 for the p calculus

a synchronous π calculus the sl synchronous programming model is a relaxation of the esterel synchronous model where the reaction to the absence of a signal within an instant can only happen at the next instant in previous work we have revisited the sl synchronous programming model in particular we have discussed an alternative design of the model introduced a cps translation to a tail recursive form and proposed a notion of bisimulation equivalence in the present work we extend the tail recursive model with first order data types obtaining a non deterministic synchronous model whose complexity is comparable to the one of the calculus we show that our approach to bisimulation equivalence can cope with this extension and in particular that labelled bisimulation can be characterised as a contextual bisimulation

types and full abstraction for polyadic π calculus a type system for terms of the monadic p calculus is introduced and used to obtain a full abstraction result for the translation of the polyadic p calculus into the monadic calculus well sorted terms of the polyadic calculus are barbed congruent iff their translations are typed barbed congruent

duality for general attractive spin systems with applications in one dimension on developpe une theorie des processus duaux qui s applique aux systemes de spin generaux a taux de transition attractifs on illustre l applicabilite de ces processus en generalisant les resultats de durrett et griffeath 1983

processes as formal power series a coinductive approach to denotational semantics we characterize must testing equivalence on csp in terms of the unique homomorphism from the moore automaton of csp processes to the final moore automaton of partial formal power series over a certain semiring the final automaton is then turned into a csp algebra operators and fixpoints are defined respectively via behavioural differential equations and simulation relations this structure is then shown to be preserved by the final homomorphism as a result we obtain a fully abstract compositional model of csp phrased in purely set theoretical terms

structured coalgebras and minimal hd automata for the π calculus the coalgebraic framework developed for the classical process algebras and in particular its advantages concerning minimal realizations does not fully apply to the calculus due to the constraints on the freshly generated names that appear in the bisimulation in this paper we propose to model the transition system of the calculus as a coalgebra on a category of name permutation algebras and to define its abstract semantics as the final coalgebra of such a category we show that permutations are sufficient to represent in an explicit way fresh name generation thus allowing for the definition of minimal realizations we also link the coalgebraic semantics with a slightly improved version of history dependent hd automata a model developed for verification purposes where states have local names and transitions are decorated with names and name relations hd automata associated with agents with a bounded number of threads in their derivatives are finite and can be actually minimized we show that the bisimulation relation in the coalgebraic context corresponds to the minimal hd antomaton

a congruence rule format for name passing process calculi abstract we introduce a gsos like rule format for name passing process calculi specifications in this format correspond to theories in nominal logic the intended models of such specifications arise by initiality from a general categorical model theory for operational semantics given in this rule format a natural behavioural equivalence a form of open bisimilarity is a congruence

value passing ccs with noisy channels value passing ccs a full version of milner s ccs is a process algebra in which actions consist of sending and receiving values through noiseless communication channels the full calculus is a succinct yet expressive language for the specification and verification of reactive systems taking into account the reality of channel noise in reactive systems in this paper we introduce an extension of value passing ccs called value passing ccs with noisy channels vccs n in which noise is described by a probability distribution over the values after presenting the reduction operational semantics and labelled operational semantics of vccs n we develop the theory of behavioural equivalence by introducing barbed equivalence barbed congruence bisimilarity and full bisimilarity in particular we show that barbed equivalence and barbed congruence coincide with bisimilarity and full bisimilarity respectively based upon the labelled operational semantics of vccs n we establish a probabilistic modal logic for expressing system properties and show its connection with the notion of bisimilarity finally we use vccs n to model a communication protocol for ensuring the reliable transmission of data across an error prone channel

proof techniques for cryptographic processes contextual equivalences for cryptographic process calculi like the spi calculus can be used to reason about correctness of protocols but their definition suffers from quantification over all possible contexts here we focus on two such equivalences namely may testing and barbed equivalence and investigate tractable proof methods for them to this aim we design an enriched labelled transition system where transitions are constrained by the knowledge the environment has of names and keys the new transition system is then used to define a trace equivalence and a weak bisimulation equivalence that avoid quantification over contexts our main results are soundness and completeness of trace and weak bisimulation equivalence with respect to may testing and barbed equivalence respectively they lead to more direct proof methods for equivalence checking the use of these methods is illustrated with a few examples concerning implementation of secure channels and verification of protocol correctness