777 research outputs found

    Complexity of Safety and coSafety Fragments of Linear Temporal Logic

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    Linear Temporal Logic (LTL) is the de-facto standard temporal logic for system specification, whose foundational properties have been studied for over five decades. Safety and cosafety properties define notable fragments of LTL, where a prefix of a trace suffices to establish whether a formula is true or not over that trace. In this paper, we study the complexity of the problems of satisfiability, validity, and realizability over infinite and finite traces for the safety and cosafety fragments of LTL. As for satisfiability and validity over infinite traces, we prove that the majority of the fragments have the same complexity as full LTL, that is, they are PSPACE-complete. The picture is radically different for realizability: we find fragments with the same expressive power whose complexity varies from 2EXPTIME-complete (as full LTL) to EXPTIME-complete. Notably, for all cosafety fragments, the complexity of the three problems does not change passing from infinite to finite traces, while for all safety fragments the complexity of satisfiability (resp., realizability) over finite traces drops to NP-complete (resp., Π2P{\Pi}^P_2-complete)

    A first-order logic characterization of safety and co-safety languages

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    Linear Temporal Logic (LTL) is one of the most popular temporal logics, that comes into play in a variety of branches of computer science. Among the various reasons of its widespread use there are its strong foundational properties: LTL is equivalent to counter-free omega-automata, to star-free omega-regular expressions, and (by Kamp's theorem) to the first-order theory of one successor (S1S[FO]). Safety and co-safety languages, where a finite prefix suffices to establish whether a word does not belong or belongs to the language, respectively, play a crucial role in lowering the complexity of problems like model checking and reactive synthesis for LTL. SafetyLTL (resp., coSafetyLTL) is a fragment of LTL where only universal (resp., existential) temporal modalities are allowed, that recognises safety (resp., co-safety) languages only. The main contribution of this paper is the introduction of a fragment of S1S[FO], called SafetyFO, and of its dual coSafetyFO, which are expressively complete with respect to the LTL-definable safety and co-safety languages. We prove that they exactly characterize SafetyLTL and coSafetyLTL, respectively, a result that joins Kamp's theorem, and provides a clearer view of the characterization of (fragments of) LTL in terms of first-order languages. In addition, it gives a direct, compact, and self-contained proof that any safety language definable in LTL is definable in SafetyLTL as well. As a by-product, we obtain some interesting results on the expressive power of the weak tomorrow operator of SafetyLTL, interpreted over finite and infinite words. Moreover, we prove that, when interpreted over finite words, SafetyLTL (resp. coSafetyLTL) devoid of the tomorrow (resp., weak tomorrow) operator captures the safety (resp., co-safety) fragment of LTL over finite words

    The transposition of council directive 2013/59 into italian law

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    By the European Directive no. 2013/59 Euratom, the European Union has aimed to provide Member States with updated instructions in order to prevent damages possibly arising from radiations in health care, work and social settings. Among the most relevant amendments, the authors have found: a) the introductions of new defining criteria; b) the updating of some dosage related standards, such as the one about the threshold absorbed by the crystalline lens; c) a new set of rules for the measurement of emissions from devices and data management; d) a greater degree of clarity in ascribing liability to anyone involved in utilizing ionizing radiation-emitting devices. The paper outlines the Italian legislative state of affairs by delving into all relevant aspects of the current legislation, what has been put in place in the process of enacting the European Directive and the measures that could be suitable for future improvement

    Ciliate diversity and behavioural observations from the chemoautotrophic cave ecosystem of Frasassi, (Marche region, Italy)

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    Chemoautotrophic cave organisms require specific adaptations to tolerate the stress of living in extreme environmental conditions, such as darkness, nutrient and energy limitations, low temperatures (12-13°C), highly variable sulphide concentrations (from 0 up to 415 μM H2S) and toxic levels of gases (H2S, CO2, CH4). To date, due to the difficulties in sampling in such harsh environment, very few studies were performed in order to describe the ciliate communities from caves; these have remained largely unexplored. Thus, the main aims of our study were to characterize for the first time, the ciliates from the sulfide-rich Frasassi cave complex (Marche region, Italy) and to observe possible behavioural differences with their non-cave-dwelling counterpart ciliate species. Four main sampling sites within Frasassi caves were selected: Pozzo dei Cristalli, Lago Verde, Ramo Solfureo and Grotta Solfurea. The ciliate diversity from the site Pozzo dei Cristalli was studied in greater detail for its spatio-temporal distribution, since it is highly diversified and includes several microhabitats represented by small sulfidic (H2S-rich) ponds, streams and springs as well as, deep and shallow muddy, stagnant lakes. Periodic sampling was realised from 2009 to 2011 in the form of water-sediments, picked up by scraping the surface. Classical culturing, silver staining methods and 18S rRNA gene (for some selected species) for phylogenetic analysis were employed. A total of 31 species belonging to 9 classes, 15 orders and 23 genera were identified. Fluctuation of the ciliate communities were mainly recorded at the Pozzo dei Cristalli sites during various sampling occasions, this could be due to changing environmental conditions (mainly H2S concentrations and water levels). Interestingly, it was observed that some species e.g. Urocentrum turbo, Coleps hirtus hirtus, Oxytricha sp, Euplotes sp, showed adaptation for the cave environment (photo-sensitivity, sulphur tolerance, feeding behaviour, morphological difference). Overall, these results provide a platform for various in-depth studies of ciliates to understand potential role in aquatic microhabitats, nature of chemical compounds secreted, dispersal pattern, and adaptations to cave environment

    Free living ciliated protists from the chemoautotrophic cave ecosystem of Frasassi (Italy)

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    This study provides the first report on a community of free-living ciliated protists from the chemoautotrophic cave ecosystem of Frasassi, Italy. This subterranean groundwater ecosystem represents a hotspot of biodiversity that still needs to be fully explored with particular reference to microbial eukaryotes such as protist ciliates. A total of 33 taxa of ciliates were identified along with one species each of flagellate, heliozoans and naked amoebae, from four main sampling sites, namely, Grotta Solfurea (GSO), Lago Verde (LVE), Ramo Solfureo (RSO), and Pozzo dei Cristalli (PDC). The last consists of small microhabitats/ponds presenting different chemical–physical and biological parameters, such as sulfur and nutrient contents and the presence of bacterial biofilms. Furthermore, an analysis of the cryptic ciliate species biosphere as a ‘seedbank’ of diversity against cave ecosystem disturbance was also performed. This study also highlights some peculiar adaptations of cave-dwelling ciliates not described in their noncave-dwelling conspecifics, such as the extreme photosensitivity of Urocentrum turbo, the cannibalism of Coleps hirtus, the variable number of thorns in Aspidisca species as a defensive response to predation, and the frequent reorganization of ciliary structures in Euplotes aediculatus. The 18S rDNA sequences were generated for five species and were compared with those of the noncave-dwelling conspecifics. Finally, our results shed light on the still largely unknown ciliate diversity in the chemosynthesis-based sulfidic groundwater ecosystem of Frasassi
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