85 research outputs found
Surjective H-Colouring over reflexive digraphs
The Surjective H-Colouring problem is to test if a given graph allows a vertex-surjective homomorphism to a fixed graph H. The complexity of this problem has been well studied for undirected (partially) reflexive graphs. We introduce endo-triviality, the property of a structure that all of its endomorphisms that do not have range of size 1 are automorphisms, as a means to obtain complexity-theoretic classifications of Surjective H-Colouring in the case of reflexive digraphs. Chen (2014) proved, in the setting of constraint satisfaction problems, that Surjective H-Colouring is NP-complete if H has the property that all of its polymorphisms are essentially unary. We give the first concrete application of his result by showing that every endo-trivial reflexive digraph H has this property. We then use the concept of endo-triviality to prove, as our main result, a dichotomy for Surjective H-Colouring when H is a reflexive tournament: if H is transitive, then Surjective H-Colouring is in NL; otherwise, it is NP-complete. By combining this result with some known and new results, we obtain a complexity classification for Surjective H-Colouring when H is a partially reflexive digraph of size at most 3
A Calculus for Orchestration of Web Services
Service-oriented computing, an emerging paradigm for distributed computing based on the use of services, is calling for the development of tools and techniques to build safe and trustworthy systems, and to analyse their behaviour. Therefore, many researchers have proposed to use process calculi, a cornerstone of current foundational research on specification and analysis of concurrent, reactive, and distributed systems. In this paper, we follow this approach and introduce CWS, a process calculus expressly designed for specifying and combining service-oriented applications, while modelling their dynamic behaviour. We show that CWS can model all the phases of the life cycle of service-oriented applications, such as publication, discovery, negotiation, orchestration, deployment, reconfiguration and execution. We illustrate the specification style that CWS supports by means of a large case study from the automotive domain and a number of more specific examples drawn from it
Stance Classification for Rumour Analysis in Twitter: Exploiting Affective Information and Conversation Structure
Analysing how people react to rumours associated with news in social media is
an important task to prevent the spreading of misinformation, which is nowadays
widely recognized as a dangerous tendency. In social media conversations, users
show different stances and attitudes towards rumourous stories. Some users take
a definite stance, supporting or denying the rumour at issue, while others just
comment it, or ask for additional evidence related to the veracity of the
rumour. On this line, a new shared task has been proposed at SemEval-2017 (Task
8, SubTask A), which is focused on rumour stance classification in English
tweets. The goal is predicting user stance towards emerging rumours in Twitter,
in terms of supporting, denying, querying, or commenting the original rumour,
looking at the conversation threads originated by the rumour. This paper
describes a new approach to this task, where the use of conversation-based and
affective-based features, covering different facets of affect, has been
explored. Our classification model outperforms the best-performing systems for
stance classification at SemEval-2017 Task 8, showing the effectiveness of the
feature set proposed.Comment: To appear in Proceedings of the 2nd International Workshop on Rumours
and Deception in Social Media (RDSM), co-located with CIKM 2018, Turin,
Italy, October 201
Contracts in distributed systems
We present a parametric calculus for contract-based computing in distributed
systems. By abstracting from the actual contract language, our calculus
generalises both the contracts-as-processes and contracts-as-formulae
paradigms. The calculus features primitives for advertising contracts, for
reaching agreements, and for querying the fulfilment of contracts. Coordination
among principals happens via multi-party sessions, which are created once
agreements are reached. We present two instances of our calculus, by modelling
contracts as (i) processes in a variant of CCS, and (ii) as formulae in a
logic. With the help of a few examples, we discuss the primitives of our
calculus, as well as some possible variants.Comment: In Proceedings ICE 2011, arXiv:1108.014
Denotational Semantics of Mobility in Unifying Theories of Programming (UTP)
UTP promotes the unification of programming theories and has been used successfully
for giving denotational semantics to Imperative Programming, CSP process algebra,
and the Circus family of programming languages, amongst others.
In this thesis, we present an extension of UTP-CSP (the UTP semantics for CSP)
with the concept of mobility. Mobility is concerned with the movement of an entity
from one location (the source) to another (the target). We deal with two forms of
mobility:
ā¢ Channel mobility, concerned with the movement of links between processes,
models networks with a dynamic topology; and
ā¢ Strong process mobility, which requires to suspend a running process first, and
then move both its code and its state upon suspension, and finally resume the
process on the target upon reception.
Concerning channel mobility:
ā¢ We model channels as concrete entities in CSP, and show that it does not affect
the underlying CSP semantics.
ā¢ A requirement is that a process may not own a channel prior to receiving it. In
CSP, the set of channels owned by a process (called its interface) is static by
definition. We argue that making the interface variable introduces a paradox.
We resolve this by introducing a new concept: the capability of a process, and
show how it relates to the interface.
We then define channel mobility as the operation that changes the interface of a process,
but not its capability. We also provide a functional link between static CSP and its
mobile version.
Concerning strong mobility, we provide:
ā¢ The first extension of CSP with jump features, using the concept of continuations.
ā¢ A novel semantics for the generic interrupt (a parallel-based interrupt operator),
using the concept of Bulk Synchronous Parallelism.
We then define strong mobility as a specific interrupt operator in which the interrupt
routine migrates the suspended program
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