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Linh Nguyen

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Papers by Linh Nguyen

Optimised EXPTIME Tableaux for ALC Using Sound Global Caching , Propagation and Cutoffs

We show that global caching can be used with propagation of both satisfiability and unsatisfiabil... more We show that global caching can be used with propagation of both satisfiability and unsatisfiability in a sound manner to give an EXPTIME algorithm for checking satisfiability w.r.t. a TBox in the basic description logic ALC. Our algorithm is based on a simple traditional tableau calculus which builds an and-or graph where no two nodes of the graph contain the same formula set. When a duplicate node is about to be created, we use the pre-existing node as a proxy, even if the proxy is from a different branch of the tableau, thereby building global caching into the algorithm from the start. Doing so is important since it allows us to reason explicitly about the correctness of global caching. We then show that propagating both satisfiability and unsatisfiability via the andor structure of the graph remains sound. We show that various search strategies can be used with global caching, including heuristics ones. To obtain a more efficient decision procedure for ALC, we apply several opti...

On the Complexity of Fragments of Modal Logics

We study and give a summary of the complexity of 15 basic normal monomodal logics under the restr... more We study and give a summary of the complexity of 15 basic normal monomodal logics under the restriction to the Horn fragment and/or bounded modal depth. As new results, we show that: a) the satisfiability problem of sets of Horn modal clauses with modal depth bounded by k 2 in the modal logics K4 and KD4 is PSPACE-complete, in K is NP-complete; b) the satisfiability problem of modal formulas with modal depth bounded by 1 in K4, KD4, and S4 is NP-complete; c) the satisfiability problem of sets of Horn modal clauses with modal depth bounded by 1 in K, K4, KD4, and S4 is PTIME-complete. In this work, we also study the complexity of the multimodal logics Ln under the mentioned restrictions, where L is one of the 15 basic monomodal logics. We show that, for n 2 : a) the satisfiability problem of sets of Horn modal clauses in K5n, KD5n, K45n, and KD45n is PSPACE-complete; b) the satisfiability problem of sets of Horn modal clauses with modal depth bounded by k 2 in Kn, KBn, K5n, K45n, KB5...

Analysing the Influence of Non-Observable and Observable Interviewer Characteristics on Measurement Error: Evidence from Zambia

A Summary of the Habilitation Thesis “ Selected Semantic and Computational Aspects of Modal Logic Programming ”

In [H1], we develop a fixpoint semantics, the least model semantics, and an SLDresolution calculu... more In [H1], we develop a fixpoint semantics, the least model semantics, and an SLDresolution calculus in a direct way for positive modal logic programs in basic serial monomodal logics. In [H2], we extend the results and generalize the methods of [H1] for multimodal logics, giving a framework for developing semantics for positive logic programs in serial multimodal logics. In [H3], we report on our design and implementation of the modal logic programming system [18] based on the framework given in [H2]. The subject of modal deductive databases closely relates to modal logic programming. In [H5], using our theory of modal logic programming [H1, H2], we formulate modal deductive databases and give computational methods for the modal query language MDatalog. In [H5], we also estimate data complexity of MDatalog in some multimodal logics. The work [H4] contains our results on data complexity of MDatalog in basic monomodal logics.

Clausal Tableau Systems and Space Bounds for the Modal Logics

We propose so called clausal tableau systems for the common modal logics K , KD , T , KB , KDB an... more We propose so called clausal tableau systems for the common modal logics K , KD , T , KB , KDB and B . There is a measure such that for each tableau rule of these systems the measure of all its denominators is smaller than the measure of its numerator. Basing on these systems, we give a decision procedure for the logics, which uses O(n)-space for the logics T , KB , KDB and B , and O(n. log n)-space for the logics K and KD . We also show that the problem of checking satisfiability in T , KB , KDB , or B for formulae with finitely bounded modal-depth is decidable in O(n. log n)-space. We are the first who explicitly establish space requirements for the logics KB , KDB and B .

Implementations in the Australian Public Sector

Sad stories about EDRMS implementation failure are often told among records managers. An EDRMS pr... more Sad stories about EDRMS implementation failure are often told among records managers. An EDRMS project, like any other IS implementation, will need key ingredients to be successful. This paper reveals these components of a successful EDRMS implementation from the findings of a Web-based survey on the perspectives of records managers in the three levels of the Australian public sector. It also uncovers these organisations’ attitudes towards digital recordkeeping initiatives and an insight on their EDRMS projects.

Logical Characterizations of Fuzzy Bisimulations in Fuzzy Modal Logics over Residuated Lattices

There are two kinds of bisimulation, namely crisp and fuzzy, between fuzzy structures such as fuz... more There are two kinds of bisimulation, namely crisp and fuzzy, between fuzzy structures such as fuzzy automata, fuzzy labeled transition systems, fuzzy Kripke models and fuzzy interpretations in description logics. Fuzzy bisimulations between fuzzy automata over a complete residuated lattice have been introduced by Ćirić et al. in 2012. Logical characterizations of fuzzy bisimulations between fuzzy Kripke models (respectively, fuzzy interpretations in description logics) over the residuated lattice [0, 1] with the Gödel t-norm have been provided by Fan in 2015 (respectively, Nguyen et al. in 2020). There was the lack of logical characterizations of fuzzy bisimulations between fuzzy graph-based structures over a general residuated lattice, as well as over the residuated lattice [0, 1] with the Lukasiewicz or product t-norm. In this article, we provide and prove logical characterizations of fuzzy bisimulations in fuzzy modal logics over residuated lattices. The considered logics are the...

On the Deterministic Horn Fragment of Test-free PDL

We study the deterministic Horn fragment of test-free proposi- tional dynamic logic (PDL(0)). Thi... more We study the deterministic Horn fragment of test-free proposi- tional dynamic logic (PDL(0)). This fragment adopts the restriction that, in bodies of program clauses and goals, special universal modal operators which are a kind of combination of ! and " are used instead of ! . The fragment contains deterministic positive logic programs and deterministic negative clauses, whose negations form serial positive formulae. A least Kripke model for a deterministic positive logic program in PDL (0) may not exist, because PDL(0) is a non-serial modal logic. In this work, we present an algorithm that, given a deterministic positive logic program P in PDL (0) , constructs a least pseudo-model of P. A pseudo-model is similar to a Kripke model except that it contains two sets of accessibility relations, one for dealing with existential modal operators and the other for dealing with universal modal operators. A least pseudo-model M of P has the property that, for every serial positive formul...

An ExpTime Tableau Method for Dealing with Nominals and Qualified Number Restrictions in Deciding the Description Logic SHOQ

Fundamenta Informaticae

We present the first tableau method with an ExpTime (optimal) complexity for checking satisfiabil... more We present the first tableau method with an ExpTime (optimal) complexity for checking satisfiability of a knowledge base in the description logic SHOQ, which extends ALC with transitive roles, hierarchies of roles, nominals and quantified number restrictions. The complexity is measured using binary representation for numbers. Our procedure is based on global caching and integer linear feasibility checking.

A Framework for Graded Beliefs, Goals and Intentions

Fundamenta Informaticae

In natural language we often use graded concepts, reflecting different intensity degrees of certa... more In natural language we often use graded concepts, reflecting different intensity degrees of certain features. Whenever such concepts appear in a given real-life context, they need to be appropriately expressed in its models. In this paper, we provide a framework which allows for extending the BGI model of agency by grading beliefs, goals and intentions. We concentrate on TEAMLOG [6, 7, 8, 9, 12] and provide a complexity-optimal decision method for its graded version TEAMLOG K by translating it into CPDL reg (propositional dynamic logic with converse and "inclusion axioms" characterized by regular languages). We also develop a tableau calculus which leads to the first EXPTIME (optimal) tableau decision procedure for CPDL reg. As CPDL reg is suitable for expressing complex properties of graded operators, the procedure can also be used as a decision tool for other multiagent formalisms.

Horn Knowledge Bases in Regular Description Logics with PTIME Data Complexity

Fundamenta Informaticae

Developing a good formalism and an efficient decision procedure for the instance checking problem... more Developing a good formalism and an efficient decision procedure for the instance checking problem is desirable for practical application of description logics. The data complexity of the instance checking problem is coNP-complete even for Horn knowledge bases in the basic description logic ALC. In this paper, we present and study weakenings with PTime data complexity of the instance checking problem for Horn knowledge bases in regular description logics. We also study cases when the weakenings are an exact approximation. In contrast to previous related work of other authors, our approach deals with the case when the constructor ∀ is allowed in premises of program clauses that are used as terminological axioms.

Narrowing the Gap Between LIS Research and Practice in Australia

Journal of the Australian Library and Information Association

Concerns about the gap between LIS theory and practice have been voiced by many commentators over... more Concerns about the gap between LIS theory and practice have been voiced by many commentators over the years. A moderate gap was identified between the research priorities of practitioners in the Relevance 2020 project's focus groups, convened across Australia in late 2016, and the LIS research actually being conducted according to the Middleton and Yates (2014) report. A total of 172 Relevance 2020 focus group participants, from a wide range of sectors, identified 96 priority topics, in areas such as the role of libraries and librarians, management, promotion, information service operations, and information literacy. The participants also suggested ways in which LIS research could be better supported and made more relevant to practice. All stakeholders need to play their complementary parts: employers need to support practitioners' involvement in research, as well as their engagement with it; professional networks can help lobby for funds and resources, and facilitate the development of research capacity; academics can identify areas in which their own agendas coincide with those of practitioners, and explain the relevance of their research findings. Strong partnerships between individual practitioners and researchers can be formed by first identifying common ground and aims, through the sort of constructive dialogue generated at the Relevance 2020 events.

Bisimulation and bisimilarity for fuzzy description logics under the Gödel semantics

Fuzzy Sets and Systems

Description logics (DLs) are a suitable formalism for representing knowledge about domains in whi... more Description logics (DLs) are a suitable formalism for representing knowledge about domains in which objects are described not only by attributes but also by binary relations between objects. Fuzzy extensions of DLs can be used for such domains when data and knowledge about them are vague and imprecise. One of the possible ways to specify classes of objects in such domains is to use concepts in fuzzy DLs. As DLs are variants of modal logics, indiscernibility in DLs is characterized by bisimilarity. The bisimilarity relation of an interpretation is the largest auto-bisimulation of that interpretation. In DLs and their fuzzy extensions, such equivalence relations can be used for concept learning. In this paper, we define and study fuzzy bisimulation and bisimilarity for fuzzy DLs under the Gödel semantics, as well as crisp bisimulation and strong bisimilarity for such logics extended with involutive negation. The considered logics are fuzzy extensions of the DL ALC reg (a variant of PDL) with additional features among inverse roles, nominals, (qualified or unqualified) number restrictions, the universal role, local reflexivity of a role and involutive negation. We formulate and prove results on invariance of concepts under fuzzy (resp. crisp) bisimulation, conditional invariance of fuzzy TBoxex/ABoxes under bisimilarity (resp. strong bisimilarity), and the Hennessy-Milner property of fuzzy (resp. crisp) bisimulation for fuzzy DLs without (resp. with) involutive negation under the Gödel semantics. Apart from these fundamental results, we also provide results on using fuzzy bisimulation to separate the expressive powers of fuzzy DLs, as well as results on using strong bisimilarity to minimize fuzzy interpretations. * This is a revised and corrected version of the publication "Bisimulation and bisimilarity for fuzzy description logics under the Gödel semantics", Fuzzy Sets and Systems 388: 146-178 (2020)

An expressive and tractable rule-based description language

Enterprise Information Systems

ABSTRACT is a rule language introduced by Nguyen, Nguyen, and Szałas for the Semantic Web. It has... more ABSTRACT is a rule language introduced by Nguyen, Nguyen, and Szałas for the Semantic Web. It has a PTime data complexity and is more expressive than Horn-. These rule languages, however, do not allow PDL-like role constructors (i.e., union, sequential composition, the reflexive-transitive closure and the test operator). Using the transitive closure of a role is not the same as using a transitive role that is a superrole of . Furthermore, the test operator together with the other PDL-like role constructors would allow users to express sophisticated concepts. In this paper, we extend with PDL-like role constructors to obtain the rule language . We modify the algorithm given by Nguyen, Nguyen, and Szałas (2015) to obtain another one that checks satisfiability of a knowledge base in instead of . The resulting algorithm also has a PTime data complexity and is therefore tractable.

OUP accepted manuscript

Journal Of Logic And Computation

Berman and Paterson proved that test-free propositional dynamic logic (PDL) is weaker than PDL. O... more Berman and Paterson proved that test-free propositional dynamic logic (PDL) is weaker than PDL. One would raise questions: does a similar result also hold for extensions of PDL? For example, is test-free converse-PDL (CPDL) weaker than CPDL? In what circumstances the test operator can be eliminated without reducing the expressive power of a PDL-based logical formalism? These problems have not yet been studied. As the description logics $\mathcal{ALC}_{trans}$ and $\mathcal{ALC}_{reg}$ are, respectively, variants of test-free PDL and PDL, there is a concept of $\mathcal{ALC}_{reg}$ that is not equivalent to any concept of $\mathcal{ALC}_{trans}$. Generalizing this, we prove that there is a concept of $\mathcal{ALC}_{reg}$ that is not equivalent to any concept of the logic that extends $\mathcal{ALC}_{trans}$ with inverse roles, nominals, qualified number restrictions, the universal role and local reflexivity of roles. We also provide some results for the case with RBoxes and TBoxes. ...

Extreme downside risk and market turbulence

Quantitative Finance

We investigate the dynamics of the relationship between returns and extreme downside risk in diff... more We investigate the dynamics of the relationship between returns and extreme downside risk in different states of the market by combining the framework of Bali, Demirtas, and Levy (2009) with a Markov switching mechanism. We show that the risk-return relationship identified by Bali, Demirtas, and Levy (2009) is highly significant in the low volatility state but disappears during periods of market turbulence. This is puzzling since it is during such periods that downside risk should be most prominent. We show that the absence of the risk-return relationship in the high volatility state is due to leverage and volatility feedback effects arising from increased persistence in volatility. To better filter out these effects, we propose a simple modification that yields a positive tail risk-return relationship under all states of market volatility.

Systematic extreme downside risk

Journal of International Financial Markets, Institutions and Money

We propose new systematic tail risk measures constructed using two different approaches. The firs... more We propose new systematic tail risk measures constructed using two different approaches. The first is a non-parametric measure that captures the tendency of a stock to crash at the same time as the market, while the second is based on the sensitivity of stock returns to innovations in market crash risk. Both tail risk measures are associated with a significantly positive risk premium after controlling for other measures of downside risk, including downside beta, coskewness and cokurtosis. Using the new measures, we examine the relevance for investors of the tail risk premium over different horizons.

ExpTime Tableaux with Global Caching for Hybrid PDL

Journal of Automated Reasoning

We present the first direct tableau decision procedure with the ExpTime complexity for HPDL (Hybr... more We present the first direct tableau decision procedure with the ExpTime complexity for HPDL (Hybrid Propositional Dynamic Logic). It checks whether a given ABox (a finite set of assertions) in HPDL is satisfiable. Technically, it combines global caching with checking fulfillment of eventualities and dealing with nominals. Our procedure contains enough details for direct implementation and has been implemented for the TGC2 (Tableaux with Global Caching) system. As HPDL can be used as a description logic for representing and reasoning about terminological knowledge, our procedure is useful for practical applications.

Effect of polyDADMAC on aggregation of clay-size particles in red mud: Implications for immobilization practices

Ecotoxicology and environmental safety, Jan 30, 2018

PolyDADMAC (PD) is a high charge density cationic polymer of diallyldimethylammonium chloride whi... more PolyDADMAC (PD) is a high charge density cationic polymer of diallyldimethylammonium chloride which has been recently developed as a coagulant in water purification. As PD has high affinity to fine negatively-charged colloids, it is worth to ascertain if PD can affect surface properties of clay-size particles in red mud waste and be applied to develop immobilization techniques for red mud storage areas. In the current study, a test tube method in combination with surface charge measurement was used for determination of the colloidal properties of a red mud sample under the variation of PD, soluble Al and Si, and variable pH conditions. Observations for the PD concentration range from 0.25 to 2.0 mM revealed that PD can increase surface charge and enhance aggregation or at least shift the aggregation zone to higher pH. This suggests a possible application of PD for immobilization of red mud in alkaline condition. It was also found that soluble Al and Si acted to modify the effect of ...

A single amino acid substitution in the NS4B protein of Dengue virus confers enhanced virus growth and fitness in human cells in vitro through IFN-dependent host response

The Journal of general virology, Jan 19, 2018

Dengue virus (DENV) replication between mosquito and human hosts is hypothesized to be associated... more Dengue virus (DENV) replication between mosquito and human hosts is hypothesized to be associated with viral determinants that interact in a differential manner between hosts. However, the understanding of inter-host viral determinants that drive DENV replication and growth between hosts is limited. Through the use of clinical isolates, we identified an amino acid variation of Ala, Met and Val at position 116 of DENV-1 NS4B. While the proportion of virus with the NS4B-116V variant remained constantly high in serial passages in a mosquito cell line, populations of the NS4B-116M and NS4B-116A variants became dominant after serial passages in mammalian cell lines. Using recombinant DENV-1 viruses, the Val to Ala or Met alteration at position NS4B-116 (rDENV-1-NS4B-116A and rDENV-1-NS4B-116M) resulted in enhanced virus growth in human cells in comparison to the clone with Val at NS4B-116 (rDENV-1-NS4B-116V). However, the reverse phenomenon was observed in a mosquito cell line. Additiona...