Stratos Kontopoulos | Centre for Research and Technology, Hellas (Greece) (original) (raw)

Papers by Stratos Kontopoulos

The emergence of Web 2.0 has drastically altered the way users perceive the Internet, by improvin... more The emergence of Web 2.0 has drastically altered the way users perceive the Internet, by improving information sharing, collaboration and interoperability. Micro-blogging is one of the most popular Web 2.0 applications and related services, like Twitter, have evolved into a practical means for sharing opinions on almost all aspects of everyday life. Consequently, micro-blogging web sites have since become rich data sources for opinion mining and sentiment analysis. Towards this direction, text-based sentiment classifiers often prove inefficient, since tweets typically do not consist of representative and syntactically consistent words, due to the imposed character limit. This paper proposes the deployment of original ontology-based techniques towards a more efficient sentiment analysis of Twitter posts. The novelty of the proposed approach is that posts are not simply characterized by a sentiment score, as is the case with machine learning-based classifiers, but instead receive a sentiment grade for each distinct notion in the post. Overall, our proposed architecture results in a more detailed analysis of post opinions regarding a specific topic.

Defeasible reasoning is a rule-based approach for efficient reasoning with incomplete and inconsi... more Defeasible reasoning is a rule-based approach for efficient reasoning with incomplete and inconsistent information. Such reasoning is useful for many applications in the Semantic Web, such as policies and business rules, agent brokering and negotiation, ontology and knowledge merging, etc. However, the syntax of defeasible logic may appear too complex for many users. In this paper we present a graphical authoring tool for defeasible logic rules that acts as a shell for the DR-DEVICE defeasible reasoning system over RDF metadata. The tool helps users to develop a rule base using the OO-RuleML syntax of DR-DEVICE rules, by constraining the allowed vocabulary through analysis of the input RDF namespaces, so that the user does not have to type-in class and property names. Rule visualization follows the tree model of RuleML. The DR-DEVICE reasoning system is implemented on top of the CLIPS production rule system and builds upon an earlier deductive rule system over RDF metadata that also supports derived attribute and aggregate attribute rules.

This work presents a visualization algorithm for defeasible logic rule bases as well as a softwar... more This work presents a visualization algorithm for defeasible logic rule bases as well as a software tool that applies this algorithm, according to which, a directed graph is produced that represents the rule base. The graph features distinct node types for rules and atomic formulas and distinct connection types for the various rule types of defeasible logic.

The Semantic Web (SW) is an extension to the current Web, enhancing the available information wit... more The Semantic Web (SW) is an extension to the current Web, enhancing the available information with semantics. RDF, one of the most prominent standards for representing meaning in the SW, offers a data model for referring to objects and their interrelations. Managing RDF documents, however, is a task that demands experience and expert understanding. Tools have been developed that alleviate this drawback and offer an interactive graphical visualization environment. This paper studies the visualization of RDF documents, a domain that exhibits many applications. The most prominent approaches are presented and a novel graph-based visualization software application is also demonstrated.

Ontologies are the primary knowledge representation tool in the Semantic Web and are mainly used ... more Ontologies are the primary knowledge representation tool in the Semantic Web and are mainly used in defining common vocabularies, used in the exchange of information among Semantic Web applications. In the process of encoding ontologies, appropriate ontology languages are applied; such a language is RDF Schema, one of the dominant standards. A variety of commercial and educational tools that address the tasks of developing and manipulating RDF Schema ontologies has been developed. None of them, however, is specifically destined for the inexperienced Semantic Web user. In this paper we present RDFSbuilder, a Java-built visual authoring tool for developing RDF Schema ontologies. The system helps users to develop their model quickly and efficiently, without being concerned about syntax or semantic errors. Furthermore, it adopts a purely object-oriented representation of the RDF Schema model, emphasizing on functional flexibility and simplicity of use. As a result, the model produced is easy to understand and equally easy to handle.

Defeasible reasoning is a rule-based approach for efficient reasoning with incomplete and inconsi... more Defeasible reasoning is a rule-based approach for efficient reasoning with incomplete and inconsistent information. Such reasoning is useful for many applications in the Semantic Web. However, the RuleML syntax of defeasible logic may appear too complex for many users. Furthermore, the interplay between various technologies and languages, such as defeasible reasoning, RuleML, and RDF impose a demand for using multiple, diverse tools for building rule-based applications for the Semantic Web. In this paper we present VDR-Device, a visual integrated development environment for developing and using defeasible logic rule bases on top of RDF ontologies. VDR-Device integrates in a user-friendly graphical shell, a visual RuleML-compliant rule editor that constrains the allowed vocabulary through analysis of the input RDF ontologies and a defeasible reasoning system that processes RDF data and RDF Schema ontologies.

Non-monotonic reasoning constitutes an approach to reasoning with incomplete or changing informat... more Non-monotonic reasoning constitutes an approach to reasoning with incomplete or changing information and is significantly more powerful than standard reasoning, which simply deals with universal statements. Defeasible reasoning, a member of the non-monotonic reasoning family, offers the extra capability of dealing with conflicting information and can represent facts, rules and priorities among rules. The main advantages of defeasible reasoning, however, are not only limited to its enhanced representational capabilities, but also feature low computational complexity compared to mainstream non-monotonic reasoning. This paper presents a system for non-monotonic reasoning on the Semantic Web called VDR-Device, which is capable of reasoning about RDF metadata over multiple Web sources using defeasible logic rules. It is implemented on top of the CLIPS production rule system and features a RuleML compatible syntax. The operational semantics of defeasible logic are implemented through compilation into a generic deductive rule language. Since the RuleML syntax may appear complex for many users, we have also implemented a graphical authoring tool for defeasible logic rules that acts as a shell for the defeasible reasoning system. The tool constrains the allowed vocabulary through analysis of the input RDF documents, so that the user does not have to type-in class and property names.

This chapter is concerned with the visualization of defeasible logic rules in the Semantic Web do... more This chapter is concerned with the visualization of defeasible logic rules in the Semantic Web domain. Logic plays an important role in the development of the Semantic Web and defeasible reasoning seems to be a very suitable tool. However, it is too complex for an end-user, who often needs graphical trace and explanation mechanisms for the derived conclusions. Directed graphs can assist in this affair, by offering the notion of direction that appears to be extremely applicable for the representation of rule attacks and superiorities in defeasible reasoning. Their applicability, however, is balanced by the fact that it is difficult to associate data of a variety of types with the nodes and the connections between the nodes in the graph. In this chapter we try to utilize digraphs in the graphical representation of defeasible rules, by exploiting the expressiveness and comprehensibility they offer, but also trying to leverage their major disadvantages. Finally, the chapter briefly presents a tool that implements this representation methodology.

Defeasible reasoning is a rule-based approach for efficient reasoning with incomplete and conflic... more Defeasible reasoning is a rule-based approach for efficient reasoning with incomplete and conflicting information. Nevertheless, it is based on solid mathematical formulations and is not fully comprehensible by end users, who often need graphical trace and explanation mechanisms for the derived conclusions. Directed graphs (or digraphs) can assist in this affair, but their applicability is balanced by the fact that it is difficult to associate data of a variety of types with the nodes and the connections in the graph. In this paper we try to utilize digraphs in the graphical representation of defeasible rules, by exploiting their expressiveness, but also trying to counter their major disadvantage, by defining multiple node and connection types.

Personal knowledge management has been studied from various angles, one of which is the Semantic ... more Personal knowledge management has been studied from various angles, one of which is the Semantic Web. Ontologies, the primary knowledge representation tool for the Semantic Web, can play a significant role in semantically managing personal knowledge. The scope of this paper focuses on addressing the issue of effective personal knowledge management, by proposing an ontology for modelling the domain of biographical events. The proposed ontology also undergoes a thorough evaluation, based on specific criteria presented in the literature.

Logic and proofs constitute key factors in increasing the user trust towards the Semantic Web. De... more Logic and proofs constitute key factors in increasing the user trust towards the Semantic Web. Defeasible reasoning is a useful tool towards the development of the Logic layer of the Semantic Web architecture. However, having a solid mathematical notation, it may be confusing to end users, who often need graphical trace and explanation mechanisms for the derived conclusions. In a previous work of ours, we outlined a methodology for representing defeasible logic rules, utilizing directed graphs that feature distinct node and connection types. However, visualizing a defeasible logic rule base also involves the placement of the multiple graph elements in an intuitive way, a non-trivial task that aims at improving user comprehensibility. This paper presents a stratification algorithm for visualizing defeasible logic rule bases that query and reason about RDF data as well as a tool that applies this algorithm.

International Journal on Artificial Intelligence Tools, 2008

The standardization of the Semantic Web has reached as far as ontologies and ontology languages. ... more The standardization of the Semantic Web has reached as far as ontologies and ontology languages. However, in order for the full potential of the Semantic Web to be achieved, the ability of reasoning over the available information is also essential. Rules can assist in this affair and various logics have been proposed for the Semantic Web domain. One of them is defeasible reasoning that deals with incomplete and conflicting information. However, despite its solid mathematical notation, it may be confusing to end users. To confront this downside, we proposed a representation schema for defeasible logic rule bases, which is based on directed graphs that feature distinct node and connection types. This paper presents DR-VisMo, a defeasible logic rule base editor and visualization system that implements this representation approach. The system also features a stratification algorithm for visualizing rule bases that deals with decisions, regarding the arrangement of the various elements in the graph. DR-VisMo is implemented as part of VDR-DEVICE, an environment for modeling and deploying defeasible logic rule bases on top of RDF ontologies.

The amount of unstructured information on the Web has reached a critical level, suggesting the ne... more The amount of unstructured information on the Web has reached a critical level, suggesting the need for the Semantic Web (SW). The SW expresses an initiative to improve the World Wide Web, by adding semantic content to the provided information, thus making the data accessible not only to humans but to machines as well. The fulfilment of this goal requires the contribution of a variety of research areas and tools: reasoning techniques that give the opportunity to agents/machines to act autonomously on the environment of the Web in order to fulfil the users' needs and tools for assisting the end users in expressing these needs and comprehending the derived results. This paper presents two systems, namely O-DEVICE and VDR-DEVICE, developed by the Intelligent Systems and Knowledge Processing (ISKP) group 1 at the Department of Informatics, Aristotle University of Thessaloniki, Greece. These two systems are designed to function in the SW environment. O-DEVICE is a system for querying and inferencing about ontologies expressed in the OWL Lite sublanguage and VDR-DEVICE is a visual integrated development environment for developing, using and visualizing defeasible logic rule bases on top of RDF ontologies.

The Semantic Web represents an initiative to improve the current Web, by augmenting content with ... more The Semantic Web represents an initiative to improve the current Web, by augmenting content with semantics and encouraging cooperation among human and software agents. The development of the logic and proof layers of the Semantic Web is currently concentrating the related research effort and is vital, since these layers allow systems to infer new knowledge from existing information, assisting them in explaining their actions and, ultimately, increasing user trust towards the Semantic Web. However, there is a lack of applications that could contribute towards developing logic-based applications. Consequently, users resort to inadequate tools that offer syntactic support, without being able to support the user semantically as well. This work presents S 2 DRREd, a software tool that introduces a supplementary level of semantic assistance during rule base development. The tool allows creating meta-models of the main notions of the loaded rule sets and assists the user in authoring rule bases, independently of the explicitly chosen rule language syntax. The domain of application is defeasible logic, a type of logic that allows reasoning with incomplete and conflicting information and, as such, it can play an increasingly important role in a drastically dynamic environment like the Web.

Public institutes often face the challenge of managing vast volumes of administrative documents, ... more Public institutes often face the challenge of managing vast volumes of administrative documents, a need that is often met via Content Management Systems (CMSs). CMSs offer various advantages, like separation of data structure from presentation and variety in user roles, but also present certain disadvantages, like inefficient keyword-based search facilities. The new generation of content management solutions imports the notion of semantics and is based on Semantic Web technologies, such as metadata and ontologies. The benefits include semantic interoperability, competitive advantages and dramatic cost reduction. In this paper a leading Enterprise CMS is extended with semantic capabilities for automatically importing and exporting ontologies. This functionality enables reuse of repository content, semantically-enabled search and interoperability with third-party applications. The extended system is deployed in semantically managing the large volumes of documents for a state university.

The Semantic Web aims at augmenting the WWW with meaning, assisting people and machines in compre... more The Semantic Web aims at augmenting the WWW with meaning, assisting people and machines in comprehending Web content and better satisfying their requests. Intelligent agents are considered to be greatly favored by Semantic Web technologies, because of the interoperability the latter will achieve. One of the main problems in agent interoperation is the great variety in reasoning formalisms, as agents do not necessarily share a common rule or logic formalism. This paper reports on the implementation of EMERALD, a knowledge-based framework for interoperating intelligent agents in the Semantic Web. More specifically, a multi-agent system was developed on top of JADE, featuring trusted, third party reasoning services, a reusable agent prototype for knowledge-customizable agent behavior, as well as a reputation mechanism for ensuring trust in the framework. Finally, a use case scenario is presented that illustrates the viability of the proposed framework.

Informatica (slovenia), 2010

The Semantic Web aims at enriching information with well-defined semantics, making it possible bo... more The Semantic Web aims at enriching information with well-defined semantics, making it possible both for people and machines to understand Web content. Intelligent agents are the most prominent approach towards realizing this vision. Nevertheless, agents do not necessarily share a common rule or logic formalism, neither would it be realistic to attempt imposing specific logic formalisms in a rapidly changing world like the Web. Thus, based on the plethora of proposals and standards for logic-and rule-based reasoning for the Semantic Web, a key factor for the success of Semantic Web agents lies in the interoperability of reasoning tasks. This paper reports on the implementation of trusted, third party reasoning services wrapped as agents in a multi-agent system framework. This way, agents can exchange their arguments, without the need to conform to a common rule or logic paradigm -via an external reasoning service, the receiving agent can grasp the semantics of the received rule set. Finally, a use case scenario is presented that illustrates the viability of the proposed approach.

Based on the plethora of proposals and standards for logic- and rulebased reasoning for the Seman... more Based on the plethora of proposals and standards for logic- and rulebased reasoning for the Semantic Web (SW), a key factor for SW agents is reasoning task interoperability. This paper reports on a framework for interoperable reasoning among agents in the SW that deploys third-party trusted reasoning services. This way, agents can exchange arguments, without conforming to a common rule or logic paradigm; via an external reasoning service, the receiving agent can grasp the semantics of the received rule set. The paper presents how a multi-agent system was extended with a third-party trusted defeasible reasoning service, which offers agents the ability of reasoning with incomplete and inconsistent information. In addition, a brokering trade scenario is presented that illustrates the usability of the approach.

Data & Knowledge Engineering, 2008

Logic is currently the target of the majority of the upcoming efforts towards the realization of ... more Logic is currently the target of the majority of the upcoming efforts towards the realization of the Semantic Web vision, namely making the content of the Web accessible not only to humans, as it is today, but to machines as well. Defeasible reasoning, a rule-based approach to reasoning with incomplete and conflicting information, is a powerful tool in many Semantic Web applications. Despite its strong mathematical background, logic, in general, and defeasible logic, in particular, may overload the user with tons of additional complex semantic relationships among data and metadata of the Semantic Web. To this end, a comprehensible, visual representation of these semantic relationships (rules) would help users understand them and make more use of them. This paper presents VDR-DEVICE, a defeasible reasoning system, designed specifically for the Semantic Web environment. VDR-DEVICE is an integrated development environment for deploying and visualizing defeasible logic rule bases on top of RDF Schema ontologies. The system consists of a number of subcomponents, which, though developed autonomously, are combined efficiently, forming a flexible framework. The system employs a defeasible reasoning system that supports direct importing and processing of RDF data and RDF Schema ontologies as well as a number of user-friendly rule base and ontology visualization modules.

The Semantic Web aims at making Web content understandable both for people and machines. Although... more The Semantic Web aims at making Web content understandable both for people and machines. Although intelligent agents can assist towards this vision, they do not have to conform to a common rule or logic paradigm. This paper reports on the first steps towards a framework for interoperating knowledge-based intelligent agents. A multiagent system was extended with defeasible reasoning and a reusable agent model is proposed for customizable agents, equipped with a knowledge base and a Jess rule engine. Two use case scenarios display the integration of these technologies.

The emergence of Web 2.0 has drastically altered the way users perceive the Internet, by improvin... more The emergence of Web 2.0 has drastically altered the way users perceive the Internet, by improving information sharing, collaboration and interoperability. Micro-blogging is one of the most popular Web 2.0 applications and related services, like Twitter, have evolved into a practical means for sharing opinions on almost all aspects of everyday life. Consequently, micro-blogging web sites have since become rich data sources for opinion mining and sentiment analysis. Towards this direction, text-based sentiment classifiers often prove inefficient, since tweets typically do not consist of representative and syntactically consistent words, due to the imposed character limit. This paper proposes the deployment of original ontology-based techniques towards a more efficient sentiment analysis of Twitter posts. The novelty of the proposed approach is that posts are not simply characterized by a sentiment score, as is the case with machine learning-based classifiers, but instead receive a sentiment grade for each distinct notion in the post. Overall, our proposed architecture results in a more detailed analysis of post opinions regarding a specific topic.

Defeasible reasoning is a rule-based approach for efficient reasoning with incomplete and inconsi... more Defeasible reasoning is a rule-based approach for efficient reasoning with incomplete and inconsistent information. Such reasoning is useful for many applications in the Semantic Web, such as policies and business rules, agent brokering and negotiation, ontology and knowledge merging, etc. However, the syntax of defeasible logic may appear too complex for many users. In this paper we present a graphical authoring tool for defeasible logic rules that acts as a shell for the DR-DEVICE defeasible reasoning system over RDF metadata. The tool helps users to develop a rule base using the OO-RuleML syntax of DR-DEVICE rules, by constraining the allowed vocabulary through analysis of the input RDF namespaces, so that the user does not have to type-in class and property names. Rule visualization follows the tree model of RuleML. The DR-DEVICE reasoning system is implemented on top of the CLIPS production rule system and builds upon an earlier deductive rule system over RDF metadata that also supports derived attribute and aggregate attribute rules.

This work presents a visualization algorithm for defeasible logic rule bases as well as a softwar... more This work presents a visualization algorithm for defeasible logic rule bases as well as a software tool that applies this algorithm, according to which, a directed graph is produced that represents the rule base. The graph features distinct node types for rules and atomic formulas and distinct connection types for the various rule types of defeasible logic.

The Semantic Web (SW) is an extension to the current Web, enhancing the available information wit... more The Semantic Web (SW) is an extension to the current Web, enhancing the available information with semantics. RDF, one of the most prominent standards for representing meaning in the SW, offers a data model for referring to objects and their interrelations. Managing RDF documents, however, is a task that demands experience and expert understanding. Tools have been developed that alleviate this drawback and offer an interactive graphical visualization environment. This paper studies the visualization of RDF documents, a domain that exhibits many applications. The most prominent approaches are presented and a novel graph-based visualization software application is also demonstrated.

Ontologies are the primary knowledge representation tool in the Semantic Web and are mainly used ... more Ontologies are the primary knowledge representation tool in the Semantic Web and are mainly used in defining common vocabularies, used in the exchange of information among Semantic Web applications. In the process of encoding ontologies, appropriate ontology languages are applied; such a language is RDF Schema, one of the dominant standards. A variety of commercial and educational tools that address the tasks of developing and manipulating RDF Schema ontologies has been developed. None of them, however, is specifically destined for the inexperienced Semantic Web user. In this paper we present RDFSbuilder, a Java-built visual authoring tool for developing RDF Schema ontologies. The system helps users to develop their model quickly and efficiently, without being concerned about syntax or semantic errors. Furthermore, it adopts a purely object-oriented representation of the RDF Schema model, emphasizing on functional flexibility and simplicity of use. As a result, the model produced is easy to understand and equally easy to handle.

Defeasible reasoning is a rule-based approach for efficient reasoning with incomplete and inconsi... more Defeasible reasoning is a rule-based approach for efficient reasoning with incomplete and inconsistent information. Such reasoning is useful for many applications in the Semantic Web. However, the RuleML syntax of defeasible logic may appear too complex for many users. Furthermore, the interplay between various technologies and languages, such as defeasible reasoning, RuleML, and RDF impose a demand for using multiple, diverse tools for building rule-based applications for the Semantic Web. In this paper we present VDR-Device, a visual integrated development environment for developing and using defeasible logic rule bases on top of RDF ontologies. VDR-Device integrates in a user-friendly graphical shell, a visual RuleML-compliant rule editor that constrains the allowed vocabulary through analysis of the input RDF ontologies and a defeasible reasoning system that processes RDF data and RDF Schema ontologies.

Non-monotonic reasoning constitutes an approach to reasoning with incomplete or changing informat... more Non-monotonic reasoning constitutes an approach to reasoning with incomplete or changing information and is significantly more powerful than standard reasoning, which simply deals with universal statements. Defeasible reasoning, a member of the non-monotonic reasoning family, offers the extra capability of dealing with conflicting information and can represent facts, rules and priorities among rules. The main advantages of defeasible reasoning, however, are not only limited to its enhanced representational capabilities, but also feature low computational complexity compared to mainstream non-monotonic reasoning. This paper presents a system for non-monotonic reasoning on the Semantic Web called VDR-Device, which is capable of reasoning about RDF metadata over multiple Web sources using defeasible logic rules. It is implemented on top of the CLIPS production rule system and features a RuleML compatible syntax. The operational semantics of defeasible logic are implemented through compilation into a generic deductive rule language. Since the RuleML syntax may appear complex for many users, we have also implemented a graphical authoring tool for defeasible logic rules that acts as a shell for the defeasible reasoning system. The tool constrains the allowed vocabulary through analysis of the input RDF documents, so that the user does not have to type-in class and property names.

This chapter is concerned with the visualization of defeasible logic rules in the Semantic Web do... more This chapter is concerned with the visualization of defeasible logic rules in the Semantic Web domain. Logic plays an important role in the development of the Semantic Web and defeasible reasoning seems to be a very suitable tool. However, it is too complex for an end-user, who often needs graphical trace and explanation mechanisms for the derived conclusions. Directed graphs can assist in this affair, by offering the notion of direction that appears to be extremely applicable for the representation of rule attacks and superiorities in defeasible reasoning. Their applicability, however, is balanced by the fact that it is difficult to associate data of a variety of types with the nodes and the connections between the nodes in the graph. In this chapter we try to utilize digraphs in the graphical representation of defeasible rules, by exploiting the expressiveness and comprehensibility they offer, but also trying to leverage their major disadvantages. Finally, the chapter briefly presents a tool that implements this representation methodology.

Defeasible reasoning is a rule-based approach for efficient reasoning with incomplete and conflic... more Defeasible reasoning is a rule-based approach for efficient reasoning with incomplete and conflicting information. Nevertheless, it is based on solid mathematical formulations and is not fully comprehensible by end users, who often need graphical trace and explanation mechanisms for the derived conclusions. Directed graphs (or digraphs) can assist in this affair, but their applicability is balanced by the fact that it is difficult to associate data of a variety of types with the nodes and the connections in the graph. In this paper we try to utilize digraphs in the graphical representation of defeasible rules, by exploiting their expressiveness, but also trying to counter their major disadvantage, by defining multiple node and connection types.

Personal knowledge management has been studied from various angles, one of which is the Semantic ... more Personal knowledge management has been studied from various angles, one of which is the Semantic Web. Ontologies, the primary knowledge representation tool for the Semantic Web, can play a significant role in semantically managing personal knowledge. The scope of this paper focuses on addressing the issue of effective personal knowledge management, by proposing an ontology for modelling the domain of biographical events. The proposed ontology also undergoes a thorough evaluation, based on specific criteria presented in the literature.

Logic and proofs constitute key factors in increasing the user trust towards the Semantic Web. De... more Logic and proofs constitute key factors in increasing the user trust towards the Semantic Web. Defeasible reasoning is a useful tool towards the development of the Logic layer of the Semantic Web architecture. However, having a solid mathematical notation, it may be confusing to end users, who often need graphical trace and explanation mechanisms for the derived conclusions. In a previous work of ours, we outlined a methodology for representing defeasible logic rules, utilizing directed graphs that feature distinct node and connection types. However, visualizing a defeasible logic rule base also involves the placement of the multiple graph elements in an intuitive way, a non-trivial task that aims at improving user comprehensibility. This paper presents a stratification algorithm for visualizing defeasible logic rule bases that query and reason about RDF data as well as a tool that applies this algorithm.

International Journal on Artificial Intelligence Tools, 2008

The standardization of the Semantic Web has reached as far as ontologies and ontology languages. ... more The standardization of the Semantic Web has reached as far as ontologies and ontology languages. However, in order for the full potential of the Semantic Web to be achieved, the ability of reasoning over the available information is also essential. Rules can assist in this affair and various logics have been proposed for the Semantic Web domain. One of them is defeasible reasoning that deals with incomplete and conflicting information. However, despite its solid mathematical notation, it may be confusing to end users. To confront this downside, we proposed a representation schema for defeasible logic rule bases, which is based on directed graphs that feature distinct node and connection types. This paper presents DR-VisMo, a defeasible logic rule base editor and visualization system that implements this representation approach. The system also features a stratification algorithm for visualizing rule bases that deals with decisions, regarding the arrangement of the various elements in the graph. DR-VisMo is implemented as part of VDR-DEVICE, an environment for modeling and deploying defeasible logic rule bases on top of RDF ontologies.

The amount of unstructured information on the Web has reached a critical level, suggesting the ne... more The amount of unstructured information on the Web has reached a critical level, suggesting the need for the Semantic Web (SW). The SW expresses an initiative to improve the World Wide Web, by adding semantic content to the provided information, thus making the data accessible not only to humans but to machines as well. The fulfilment of this goal requires the contribution of a variety of research areas and tools: reasoning techniques that give the opportunity to agents/machines to act autonomously on the environment of the Web in order to fulfil the users' needs and tools for assisting the end users in expressing these needs and comprehending the derived results. This paper presents two systems, namely O-DEVICE and VDR-DEVICE, developed by the Intelligent Systems and Knowledge Processing (ISKP) group 1 at the Department of Informatics, Aristotle University of Thessaloniki, Greece. These two systems are designed to function in the SW environment. O-DEVICE is a system for querying and inferencing about ontologies expressed in the OWL Lite sublanguage and VDR-DEVICE is a visual integrated development environment for developing, using and visualizing defeasible logic rule bases on top of RDF ontologies.

The Semantic Web represents an initiative to improve the current Web, by augmenting content with ... more The Semantic Web represents an initiative to improve the current Web, by augmenting content with semantics and encouraging cooperation among human and software agents. The development of the logic and proof layers of the Semantic Web is currently concentrating the related research effort and is vital, since these layers allow systems to infer new knowledge from existing information, assisting them in explaining their actions and, ultimately, increasing user trust towards the Semantic Web. However, there is a lack of applications that could contribute towards developing logic-based applications. Consequently, users resort to inadequate tools that offer syntactic support, without being able to support the user semantically as well. This work presents S 2 DRREd, a software tool that introduces a supplementary level of semantic assistance during rule base development. The tool allows creating meta-models of the main notions of the loaded rule sets and assists the user in authoring rule bases, independently of the explicitly chosen rule language syntax. The domain of application is defeasible logic, a type of logic that allows reasoning with incomplete and conflicting information and, as such, it can play an increasingly important role in a drastically dynamic environment like the Web.

Public institutes often face the challenge of managing vast volumes of administrative documents, ... more Public institutes often face the challenge of managing vast volumes of administrative documents, a need that is often met via Content Management Systems (CMSs). CMSs offer various advantages, like separation of data structure from presentation and variety in user roles, but also present certain disadvantages, like inefficient keyword-based search facilities. The new generation of content management solutions imports the notion of semantics and is based on Semantic Web technologies, such as metadata and ontologies. The benefits include semantic interoperability, competitive advantages and dramatic cost reduction. In this paper a leading Enterprise CMS is extended with semantic capabilities for automatically importing and exporting ontologies. This functionality enables reuse of repository content, semantically-enabled search and interoperability with third-party applications. The extended system is deployed in semantically managing the large volumes of documents for a state university.

The Semantic Web aims at augmenting the WWW with meaning, assisting people and machines in compre... more The Semantic Web aims at augmenting the WWW with meaning, assisting people and machines in comprehending Web content and better satisfying their requests. Intelligent agents are considered to be greatly favored by Semantic Web technologies, because of the interoperability the latter will achieve. One of the main problems in agent interoperation is the great variety in reasoning formalisms, as agents do not necessarily share a common rule or logic formalism. This paper reports on the implementation of EMERALD, a knowledge-based framework for interoperating intelligent agents in the Semantic Web. More specifically, a multi-agent system was developed on top of JADE, featuring trusted, third party reasoning services, a reusable agent prototype for knowledge-customizable agent behavior, as well as a reputation mechanism for ensuring trust in the framework. Finally, a use case scenario is presented that illustrates the viability of the proposed framework.

Informatica (slovenia), 2010

The Semantic Web aims at enriching information with well-defined semantics, making it possible bo... more The Semantic Web aims at enriching information with well-defined semantics, making it possible both for people and machines to understand Web content. Intelligent agents are the most prominent approach towards realizing this vision. Nevertheless, agents do not necessarily share a common rule or logic formalism, neither would it be realistic to attempt imposing specific logic formalisms in a rapidly changing world like the Web. Thus, based on the plethora of proposals and standards for logic-and rule-based reasoning for the Semantic Web, a key factor for the success of Semantic Web agents lies in the interoperability of reasoning tasks. This paper reports on the implementation of trusted, third party reasoning services wrapped as agents in a multi-agent system framework. This way, agents can exchange their arguments, without the need to conform to a common rule or logic paradigm -via an external reasoning service, the receiving agent can grasp the semantics of the received rule set. Finally, a use case scenario is presented that illustrates the viability of the proposed approach.

Based on the plethora of proposals and standards for logic- and rulebased reasoning for the Seman... more Based on the plethora of proposals and standards for logic- and rulebased reasoning for the Semantic Web (SW), a key factor for SW agents is reasoning task interoperability. This paper reports on a framework for interoperable reasoning among agents in the SW that deploys third-party trusted reasoning services. This way, agents can exchange arguments, without conforming to a common rule or logic paradigm; via an external reasoning service, the receiving agent can grasp the semantics of the received rule set. The paper presents how a multi-agent system was extended with a third-party trusted defeasible reasoning service, which offers agents the ability of reasoning with incomplete and inconsistent information. In addition, a brokering trade scenario is presented that illustrates the usability of the approach.

Data & Knowledge Engineering, 2008

Logic is currently the target of the majority of the upcoming efforts towards the realization of ... more Logic is currently the target of the majority of the upcoming efforts towards the realization of the Semantic Web vision, namely making the content of the Web accessible not only to humans, as it is today, but to machines as well. Defeasible reasoning, a rule-based approach to reasoning with incomplete and conflicting information, is a powerful tool in many Semantic Web applications. Despite its strong mathematical background, logic, in general, and defeasible logic, in particular, may overload the user with tons of additional complex semantic relationships among data and metadata of the Semantic Web. To this end, a comprehensible, visual representation of these semantic relationships (rules) would help users understand them and make more use of them. This paper presents VDR-DEVICE, a defeasible reasoning system, designed specifically for the Semantic Web environment. VDR-DEVICE is an integrated development environment for deploying and visualizing defeasible logic rule bases on top of RDF Schema ontologies. The system consists of a number of subcomponents, which, though developed autonomously, are combined efficiently, forming a flexible framework. The system employs a defeasible reasoning system that supports direct importing and processing of RDF data and RDF Schema ontologies as well as a number of user-friendly rule base and ontology visualization modules.

The Semantic Web aims at making Web content understandable both for people and machines. Although... more The Semantic Web aims at making Web content understandable both for people and machines. Although intelligent agents can assist towards this vision, they do not have to conform to a common rule or logic paradigm. This paper reports on the first steps towards a framework for interoperating knowledge-based intelligent agents. A multiagent system was extended with defeasible reasoning and a reusable agent model is proposed for customizable agents, equipped with a knowledge base and a Jess rule engine. Two use case scenarios display the integration of these technologies.