<!DOCTYPE article PUBLIC "-//NLM//DTD JATS (Z39.96) Journal Archiving and Interchange DTD v1.0 20120330//EN" "JATS-archivearticle1.dtd">
<article xmlns:xlink="http://www.w3.org/1999/xlink">
  <front>
    <journal-meta />
    <article-meta>
      <contrib-group>
        <aff id="aff0">
          <label>0</label>
          <institution>04060</institution>
          ,
          <country country="UA">Ukraine</country>
        </aff>
        <aff id="aff1">
          <label>1</label>
          <institution>Institute for Digitalization of Education, National Academy of Educational Sciences of Ukraine</institution>
          ,
          <addr-line>9 M. Berlynskoho Str., Kyiv</addr-line>
        </aff>
        <aff id="aff2">
          <label>2</label>
          <institution>Institute of Software Systems of the National Academy of Sciences of Ukraine</institution>
          ,
          <addr-line>40, Ave Glushkov, Kyiv, 03181</addr-line>
          ,
          <country country="UA">Ukraine</country>
        </aff>
        <aff id="aff3">
          <label>3</label>
          <institution>International Research and Training Center for Information Technologies and Systems under NAS and MES of Ukraine</institution>
          ,
          <addr-line>40, Ave Glushkov, Kyiv, 03680 GSP</addr-line>
          ,
          <country country="UA">Ukraine</country>
        </aff>
      </contrib-group>
      <fpage>119</fpage>
      <lpage>133</lpage>
      <abstract>
        <p>This paper addresses the challenge of reusing ontological knowledge in semantic web-oriented systems and analyzes the issues encountered in retrieval of some structured objects in external ontologies. We propose the model of repository oriented on representation of complex information objects that enhances the functionality of ontology repository search services at the content level, taking into account the structure elements of these objects and the semantic relations between them. The paper outlines the fundamental requirements for repository and provides an example of its practical application: the development of a semantic web-oriented system called ActiveBook, designed to retrieve educational materials integrated from external sources. The prototype of this system facilitates the selection of relevant textbooks for lecturers and students in educational institutions, aligning with academic programs. It is implemented on base of semantic extension of Wiki technology, with the structural elements of complex information objects sourced from relevant external ontologies.</p>
      </abstract>
      <kwd-group>
        <kwd>Ontology</kwd>
        <kwd>complex information object</kwd>
        <kwd>semantic Wiki</kwd>
        <kwd>learning object1</kwd>
      </kwd-group>
    </article-meta>
  </front>
  <body>
    <sec id="sec-1">
      <title>1. Introduction</title>
      <p>The development of various information analytical systems (IASs) increasingly relies on obtaining
information about subject domains from external information resources (IRs) with dynamic structures
and content. To automate the acquisition and structuring of information from these IRs, we propose
an ontological approach that facilitates the extraction of domain knowledge from external knowledge
bases (KBs) for web-oriented applications of varying complexity. Formal models of information objects
(IOs) processed by IASs help standardize and provide a clear interpretation of their semantics and
content.</p>
      <p>
        Ontologies serve as a theoretical foundation for defining the domain-specific structure of IOs and
the relations between them that are critical for retrieval and comparison. They establish a unified
terminology for content processing and define semantic connections with other information sources,
such as encyclopedias, regulatory documents, and classifications [
        <xref ref-type="bibr" rid="ref1">1</xref>
        ].
      </p>
    </sec>
    <sec id="sec-2">
      <title>2. Complex Information Objects</title>
      <p>
        IO, in its most general sense, represents a formalized abstraction of data that describes various types
of material and virtual entities characterized by different properties. The choice of characteristics
and methods of representation of IOs depend on purposes of their use and capabilities for processing
them. From the perspective of ontological analysis [
        <xref ref-type="bibr" rid="ref2">2</xref>
        ], IOs are represented by ontology classes and
class instances. Ontology classes are defined by their structure represented as a set of properties and
their characteristics, as well as possible relations with other classes. Instances of ontology classes can
also have values of properties.
      </p>
      <p>
        Many practical tasks require analyzing complex sets of information where IOs are interconnected
through specific relations and constraints. Some of these tasks align with the concept of semantic
search [
        <xref ref-type="bibr" rid="ref3">3</xref>
        ], where the goal is to generate a non-empty set of IOs belonging to the same class that meet
specific conditions, possibly ranked by relevance. In more generalized cases, the result is a set of
complex information objects (CIOs) — collections of IOs of different types that adhere to certain
conditions regarding the relationships between the IOs within these collections and their properties.
      </p>
      <p>CIO models can be categorized into four groups based on their sources:
 manually created by IAS developers based on their understanding of the problem, without
external knowledge;</p>
      <p> manually created by IAS developers on base of some external information sources such as
relevant KBs and IRs (ontologies and semantically annotated IRs).</p>
      <p> generated automatically by processing of external IRs (for instance, such as ontologies
generated on base of the semantically marked Wiki pages);</p>
      <p> obtained from external repositories with expressive metadata that allows automated
matching of problem constraints with contextual elements.</p>
      <p>Each CIO instance consists of two or more IO instances linked by domain ontology-defined
relations, satisfying requirements for the structure and property values of the IOs within the CIO.
Examples of CIOs are:</p>
      <p> An organization, its employees, and the projects it undertakes, where IO types are
“organization,” “project,” and “employee”;</p>
      <p>
         Hierarchically related organizational units performing common tasks using shared technical
resources, with IO types like “organization,” “task,” and “equipment”;
 The infrastructure of a settlement, including its support systems and personnel;
 An educational institution, its staff, equipment, offered specialties, disciplines, and
competencies, with primary IO types such as “competence,” “discipline,” “person,” and “specialty” [
        <xref ref-type="bibr" rid="ref4">4</xref>
        ].
      </p>
      <p>We use formal model of domain ontology Odomain to define CIO formal model:</p>
      <p>Odomain =&lt; T, R, F &gt; ,
where</p>
      <p>
instances Tind ;</p>
      <p>T contains a finite non-empty set of domain concepts, divided into classes Tcl and class
</p>
      <p>R defines a finite set of relations between class instances from Tind ;
 F is a finite set of interpretation functions defined on the terms and relations of ontology from
T and R.</p>
      <p>This CIO model uses non-empty subset of elements of the ontology Odomain (1) that separates a
set of instances Tind from selected subset of ontology classes from Tcl on base of subset semantic
relations from R between these instances.</p>
      <p>We propose to use CIO model with the following structure:
(1)
(2)</p>
      <p>C =&lt;TC  T,NC ,RC  R,{ (t j ,tk ,rm ),t j Tind ,tk Tind ,rm  R } &gt;
where TC = ti ,i = 1, p T ; N C { ni ,i  1, p } .</p>
      <p>Main difference of CIO model (2) from model of domain ontology (1) is fixation of positions of
classes and class instances into CIO structure. These positions indicate all CIO structural elements by
unique names from N C { ni ,i  1, p } . Each CIO element has some fixed set of characteristics that
define mandatory and multi–values properties, restrictions on intersection with values of some other
CIO elements, etc.. If separation of IO positions is not important for domain, several formally different
CIOs are joined in single object.</p>
      <p>For instance, if structure of CIO “Discipline textbook” contains mandatory IOs of class "Person"
denoted by the names "Author of the textbook" that can have more than one instance, their order is
not significant for the search, i.e. textbooks that differ only by order of information about the authors
are not different at the content level. Conversely, in CIO “Researcher Publications,” where order of
authors is significant (e.g., first or second author), such distinctions are crucial because they differ
papers where some person is a first author, a second one, etc.</p>
    </sec>
    <sec id="sec-3">
      <title>3. External Sources of Knowledge About CIO Structure</title>
      <p>A wide range of ontologies, each characterized by varying levels of expressiveness and detail based
on their development goals, is currently available. These ontologies often use open formats and
provide open access, with numerous tools supporting the extraction of information about the
structure and instances of complex information objects (CIOs) to facilitate knowledge reuse.</p>
      <p>
        The primary challenge lies in search expressiveness: ontology metadata typically describes the
ontology as a whole, limiting searches for CIOs within ontology to name-based queries. However,
many tasks require more advanced search capabilities to find not only individual ontology classes or
their instances but also sets of instances linked by specific relationships. This need highlights the
importance of creating CIO repositories that support detailed search functionalities, similar to those
found in ontology or document repositories [
        <xref ref-type="bibr" rid="ref5">5</xref>
        ].
      </p>
      <p>These repositories can be enriched with information from external ontologies and semantically
annotated documents, provided there is a formal mapping between semantic markup elements and
ontology elements. Additionally, domain experts can manually enter or edit information about CIOs.</p>
      <p>From an ontological analysis perspective, the following functions are essential for CIO
repositories:
 search for classes with some defined set of object properties;
 search for class instances with specific property values;
 search for instances of selected classes that have some defined relations with instances of
other selected classes;
 check for the existence of class instances that meet specific conditions;
 search for semantically similar classes and compare their instances using various measures
of semantic similarity.</p>
      <p>
        An essential condition for the effective operation of such repositories is their openness and the
ability to be populated by various external users and information resources (IRs). The technological
foundation for these repositories should adhere to FAIR principles [
        <xref ref-type="bibr" rid="ref6">6</xref>
        ], which emphasize data that is
Findable, Accessible, Interoperable, and Reusable. Unlike Open Data, that is available to everyone
without restrictions, FAIR allows for controlled access under specific conditions, offering greater
flexibility in managing data throughout its lifecycle. Although FAIR is primarily aimed at scientific
research data, it can also be applied to other IRs, such as educational materials (textbooks, manuals,
and reference books) and online encyclopedias and vocabularies.
      </p>
      <p>
        Therefore, the technological basis for CIO repository can use semantically extended wiki
technology because this software provides development of IRs that satisfy FAIR requirements [
        <xref ref-type="bibr" rid="ref7">7</xref>
        ] and
support semantic search by the categories and values of properties of IOs represented in these IRs.
      </p>
    </sec>
    <sec id="sec-4">
      <title>4. Scope of CIO Repositories</title>
      <p>The development of a CIO repository should provide the infrastructure and automate the semantic
analysis of the ontologies it contains. The main objectives of the CIO repository are:
 to reduce the time required to retrieve information about the structure and properties of IOs
that can be utilized by applied IASs;</p>
      <p> to harmonize the terminological basis and enhance the interoperability of knowledge
processed and created within such systems;
 to improve the reuse of previously acquired knowledge and increase the relevance of search
results.</p>
      <p>The functionality of CIO repository differs from ontology repository by providing:
 search capabilities for ontological classes that are similar to selected ones (by structure, name,
instances, superclasses, subclasses, etc.);</p>
      <p> search for all classes across different ontologies that contain one or more selected instances
(search by sample);
 search for instances of similar classes from different ontologies;
 finding classes and instances that can define values for instance properties of a selected class;
 finding ontologies that contain instances of selected classes connected by certain relations
(CIO prototypes).</p>
      <p>The functions of the CIO repository are not limited to this list, but most of them can be considered
as different subtypes of semantic search, where constraints and results are specified in terms of classes
and class instances of domain ontologies and relations between them.</p>
    </sec>
    <sec id="sec-5">
      <title>5. Functions of ActiveBook System</title>
      <p>The web-oriented information system ActiveBook is designed to find and provide access to
educational materials – such as textbooks, reference books, and methodological instructions – to
students and lecturers at Ukrainian universities and other persons that take part in learning process.
The objects of its operations can be considered as CIOs that contain information about IOs of various
types like individuals, disciplines, competencies, and educational organizations. This system
addresses a practical task in IAS development, requiring external knowledge that cannot be fully
satisfied by the functionalities provided by ontology repositories.</p>
      <p>The users of this system are divided into several groups:
 students searching for educational literature that meets their current information needs
(pertinent to specific educational module, discipline, or thesis in a particular specialty, etc.);
 lecturers selecting the most relevant and up-to-date textbooks for their courses or analyzing
the need for creating their own educational materials;</p>
      <p> other university department employees selecting appropriate textbooks for each module,
determining the need for textbooks in the university library, or planning the development of original
materials;</p>
      <p> book publishers (both private and state publishing houses) who can provide information
about textbooks they have published or analyze the need for new editions.</p>
      <p>The content for the "e-Textbook" system can be provided by:
 book publishers and authors, who can register textbooks they publish and set access terms;
 lecturers and other university department employees, who can register textbooks in their
specialty (whether created by them personally, in co-authorship, or found in open access).</p>
      <p>Main CIOs processed by the ActiveBook system are: "textbook", "author", "publisher", "specialty",
"competency", "learning course (module)".</p>
      <p>Currently, various educational portals and electronic libraries of educational institutions,
information about LO ("Textbook", "Monograph" CIOs, etc.) and the field of their use ("Discipline",
"Competence", "Specialty" CIOs, etc.) are represented by different sets of properties, and their similar
properties are represented by similar but different names. The use of external knowledge bases is
caused by the need to unambiguous and interoperable definition of the structure of the main objects
and subjects of this IAS in order to enable the exchange of information with other applications and
its automated export from various IRs (such as libraries of other universities). External ontologies can
help to unify the semantics of metadata about the LOs submitted in the system.</p>
      <p>ActiveBook should support semantic-level retrieval and comparison of CIOs that include attributes
of IOs, such as "Textbook," "Specialty," "Competence," and more. This allows searches not only by
textbook titles or specialty codes but also by additional parameters such as publication year, language,
difficulty level, and integration with other disciplines.</p>
      <p>We propose implementing ActiveBook as an analytical web portal, where all content additions
and updates are automatically reflected across related pages. Semantic markup ensures content
findability and integration, facilitating meaningful navigation. Access to personal information is
restricted to specific user groups based on their access levels. For example, a general textbook rating
is visible to all users, but only portal administrators can see which users submitted specific ratings.</p>
    </sec>
    <sec id="sec-6">
      <title>6. Libraries of Educational Institutions and Learning Content</title>
      <p>Most higher education institutions currently maintain their own electronic libraries, which contain
various educational materials corresponding to the specialties offered by this institution. We analyzed
several university libraries and found that they have different structure of knowledge base, use
different technological frameworks, and offer different navigation approaches. These differences
complicate integration efforts and global searches across multiple systems</p>
      <p>For instance, the institutional repository of Borys Grinchenko Kyiv University
(https://elibrary.kubg.edu.ua/) hosts electronic training courses, personal profiles of students and
lecturers, evaluation journals, and a catalog of proposed disciplines. The available mobile application
allows task processing offline and is designed to accumulate, systematize, and electronically store the
intellectual products of the university's scientific community, making them accessible via Internet
technologies. This open-access resource is hosted on the university's server and is available globally,
at any time. It is registered in the Registry of Open Access Archives (ROAR) and the Directory of
Open Access Repositories (OpenDOAR), and is indexed by the European search service BASE
(Bielefeld Academic Search Engine). The repository provides a complex search function
(elibrary.kubg.edu.ua/cgi/search/advanced) that allows searching by names, titles, subjects, and
keywords (Fig. 1). However, this search is not integrated with an ontological representation, and it
does not allow the selection of search results based on their form or composition. Additionally, the
system only offers full-text documents, not metadata, and lacks the ability to export results into
Semantic Web formats.</p>
      <p>National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute” (NTU-KPI) also
offers several scientific and educational information resources. The electronic catalog
(https://kpi.ua/1338-1) provides an advanced search system for accessing electronic textbooks.
NTUKPI’s institutional repository (https://ela.kpi.ua/) serves as an electronic archive of scientific and
educational materials.</p>
      <p>Search capabilities include filtering by authors, departments, directions, years of publication, and
keywords, but it does not support complex searches based on multiple parameters. Furthermore, the
software implementation lacks a clear visualization of the textbook information structure or the
relationships between them.</p>
      <p>Another example is Dmytro Motornyi Tavria State Agrotechnological
University (http://www.tsatu.edu.ua/biblioteka/), which integrates scientific libraries of its territorial
units. The library (http://elar.tsatu.edu.ua/) contains about 12000 documents, including educational
and methodological materials, monographs, conference materials, and lecture notes, indexed by
external search engines. However, it does not support searching by arbitrary parameters and only
offers full-text access in PDF format to registered users.</p>
      <p>Most Ukrainian higher education libraries are not integrated with each other, are implemented on
different platforms, and differ significantly in interface and structure. These issues hinder semantic
search, making it difficult for students or lecturers to find and reuse materials across institutions.</p>
      <p>
        It should be noted that some existing systems oriented on support for the learning process partially
or fully solve these problems. Currently, a number of such integrated applicable systems are
developed for LO search. They differ significantly with functionality, knowledge representation
models, and the scope and focus of content. For instance, [
        <xref ref-type="bibr" rid="ref8">8</xref>
        ] describes the online service Evdoxus
designed to provide university textbooks to students. It provides: a) exact search of information about
textbooks for each course/module; b) simple access to textbooks for students; c) relevant mechanisms
for needs of publishers; d) dynamic and actual distribution of free e-books and applications; e)
prevention of abuse of state resources; and f) more transparency and less bureaucracy. Service can be
used by book publishers who register their textbooks, by lecturers who search for appropriate
textbooks for their courses, by department staff who register textbooks selected by lecturers for each
module of the curriculum or course, and finally, by students after registering in the university
information system and choice of appropriate learning modules. But they do not have a
Ukrainianlanguage interface, they do not support the existing Ukrainian means of classification of learning
disciplines and specialties, and these features complicate the possibility of their integration with LOs
in Ukrainian.
      </p>
    </sec>
    <sec id="sec-7">
      <title>7. Problem Definition</title>
      <p>The analysis of university libraries highlights significant variability in metadata
structures and descriptive parameters for learning objects. Libraries use different parameters to
describe the same of similar their content and such differences cause a need to build a more universal
structure of main CIOs to integrate all information available for search without loss of important
parameters.</p>
      <p>Therefore, we consider the existing approaches to CIO structuring in various ontologies to choose
such representation form that integrates main significant properties of conventional structures.
Search expressiveness proposed by repositories of ontologies is not sufficient to find structural
elements that correspond to some selected CIOs (or this task is not trivial). Key Issues:
 Inconsistent Metadata: Different libraries use varied parameters to describe similar content,
complicating integration.</p>
      <p> Search Limitations: Ontology repositories often lack the expressiveness needed to locate CIOs
based on complex structural relations. Manual analysis of classes and subclasses is frequently required
to map relevant CIOs.
 Interoperability Gaps: Current systems do not fully support semantic integration or
automated data exchange across repositories.</p>
      <p>Proposed solution is based on developing a universal CIO structure that incorporates essential
properties from existing metadata models and supports semantic search to enhance integration and
retrieval capabilities.</p>
    </sec>
    <sec id="sec-8">
      <title>8. Search for CIO Structure Knowledge in Ontology Repositories and</title>
    </sec>
    <sec id="sec-9">
      <title>Knowledge Graphs</title>
      <p>
        The development of CIO repository relies on key theoretical and technological advancements in
intelligent information processing. Key technologies supporting this development are:
 standards and toolkit of the Semantic Web project that provide interoperable means of
representation and processing of distributed knowledge on base of the ontological approach (OWL,
RDF, SPARQL);
 Linked Data concept [
        <xref ref-type="bibr" rid="ref9">9</xref>
        ];
 FAIR concept for providing access, search and reuse of data and documents;
 existing ontological models of CIOs that formalize LO structure;
 knowledge graphs;
 semantic extensions of wiki technology that provide the possibility of semantic markup of
natural language texts and multimedia with tags of an arbitrary ontological structure and creating
templates for representing instances of such CIOs that can be used as a source of information about
the structure of CIOs.
      </p>
      <p>
        All these elements can be used by CIO repositories for representation of content elements and for
unification of its structure. RDF (Resource Description Framework) is a standard that provides the
ability to formulate statements as suitable for computer processing based on the
"object-attributevalue" data model for metadata [
        <xref ref-type="bibr" rid="ref10">10</xref>
        ]. Widely used in a variety of fields, RDF Schema enables
developers to define a specific vocabulary for RDF data and the kinds of objects that can use these
attributes. In other words, the RDF Schema engine provides an underlying type system for RDF
models. On its basis, such large knowledge bases as Dbpedia and Wikidata become available. In
addition, search engines such as Google and Bing also support RDF. OWL (Web Ontology Language)
ontology is defined as an ordered set of axioms, facts and links to other ontologies that can be defined
by their URIs. OWL extends the capabilities of such formal languages as XML, RDF, RDF Schema and
DAML+OIL [
        <xref ref-type="bibr" rid="ref11">11</xref>
        ].
      </p>
      <p>Linked Data is a set of advanced methods for publishing and combining structured data on the
Web. These methods that integrate various best practices selected by data providers are aimed on
development of global information space named the Web of Data. Linked Data is based on two
fundamental web technologies: Uniform Resource Identifiers (URIs) and Hypertext Transfer Protocol
(HTTP). As opposed to URLs, use of URI provides a more general means of entity identifying.</p>
      <p>URIs and HTTP are complemented by RDF technology critical to the Web of Data. While HTML
provides means of structuring and linking documents on the Web, RDF provides a general
graphbased data model that can be used to structure and link data describing a variety of entities. Data
representation of RDF model is based on triples “subject-predicate-object” where subject and object
use URIs for resource identification or contain constants. Triple predicate defines relation between
subject and object, and it also uses URI for identification of predicate resource.</p>
      <p>
        Knowledge graphs (KGs) provide a powerful way of representation structured knowledge and
integrating information from different sources [
        <xref ref-type="bibr" rid="ref12">12</xref>
        ]. Every KG consists of non–empty sets of nodes
and edges: every KG node represents some concept, and each KG edge represents some connection
between two concepts.
      </p>
      <p>KGs are used in several of tasks that require semantic processing of information. Examples of these
tasks are semantic retrieval, data analysis, generation of recommendations, natural language
processing and pattern recognition. One of the important key aspects of KG approach is the use of
ontologies to provide a formal representation of entities and their relations. Ontologies provide a
logical derivation of KGs, as well as a check of their consistency. In addition, integration of KGs with
different sources on base of ontological analysis provide interaction schemas and common term
systems of domain.</p>
      <p>The use of KG supports the Linked Data concept by defining relations linking IO instances of the
system with corresponding DBpedia records, using the Wikipedia and DBpedia SPARQL search
engine. Such KGs correspond to lightweight ontologies that can be processed by many open tools.
Therefore KGs can be used for development of various repositories for integration of information
from various external sources. In order to automatically integrate data from such different sources
such as libraries of educational institutions and catalogs of publishing houses, KGs and RDF format
can be used as technological means of converting them into open formats of knowledge
representation.</p>
      <p>
        Linked open data (LOD) [
        <xref ref-type="bibr" rid="ref13">13</xref>
        ] is an expansion of Linked data concept based on KGs. Main aim of
LOD is to transform the Web data into more interoperable, accessible and reusable representation
according to Linked Data principles: concepts are identified by URIs, HTTP provides their search,
and RDF is used for data structuring with controlled vocabularies [
        <xref ref-type="bibr" rid="ref14">14</xref>
        ]. LOD supports data combining
from heterogeneous sources and domains, performs semantic queries. LOD datasets examples:
DBpedia (https://www.dbpedia.org/), Wikidata (https://www.wikidata.org/), and GeoNames
(https://www.geonames.org/).
      </p>
    </sec>
    <sec id="sec-10">
      <title>9. Semantic Information Resources for CIO Structure</title>
      <p>The development of the knowledge base structure for any IAS can benefit from the use of external
ontologies to define the structure of CIOs in accordance with established standards. For instance, LO
search can leverage external ontologies that represent various aspects of knowledge about the
educational process, subject domains and learning specialties. Unfortunately, direct search for such
information in relevant ontologies and in ontology repositories is not a simple task that can be
performed automatically.</p>
      <p>
        Currently, a lot of ontologies with various characteristics describes research activities, education
and scientific publications [
        <xref ref-type="bibr" rid="ref15">15</xref>
        ], but most of such academic ontologies are related only to scientific
research and publications (for instance, Microsoft Academic Knowledge Graph [
        <xref ref-type="bibr" rid="ref16">16</xref>
        ]) and Open
Research Knowledge Graph [
        <xref ref-type="bibr" rid="ref17">17</xref>
        ]), and only a few of them include aspects related to educational
process and specifics of LO search for learning courses with selected properties, educational
management, and learning technologies [
        <xref ref-type="bibr" rid="ref18">18</xref>
        ].
      </p>
      <p>
        For instance, the Linked Open Vocabularies (LOV) repository (https://lov.linkeddata.es/dataset/lov)
that ensures the reuse of information resources in the field of scientific research and education [
        <xref ref-type="bibr" rid="ref19">19</xref>
        ]
currently contains 782 ontologies, and only 6 of these ontologies pertain to educational activities,
while the rest focus on research and publishing activities. Furthermore, not all these educational
ontologies include information in English. For instance, the Education
Ontology (https://schema.edu.ee) may not have comprehensive English-language support, and some
ontologies reflect education systems specific to certain countries, such as the EduProgression
Ontology (http://ns.inria.fr/semed/eduprogression/).
      </p>
      <p>
        VIVO ontology (http://vivoweb.org/ontology/core) focuses on academic research, publication
activities and relations between researchers. While it partially covers the structure of universities and
study modules, it does not link these modules to curricula or hierarchically connect academic units
to each other [
        <xref ref-type="bibr" rid="ref20">20</xref>
        ]. Thus, VIVO is not fully suited to describe educational structures in a way that
supports detailed course content or materials.
      </p>
      <p>AIISO ontology (https://vocab.org/aiiso/schema-20080925.html) characterizes the internal
organizational structure of academic institutions. It provides classes and properties for describing
universities, departments, curricula, and courses. However, AIISO does not cover textbooks and their
relationships with courses and modules, limiting its utility in contexts requiring educational content
details.</p>
      <p>Teaching Core Vocabulary TEACH (https://lov.linkeddata.es/dataset/lov/vocabs/teach) is a
lightweight ontology that enables educators to link various elements within their courses. This
ontology facilitates the description of courses/modules and learning materials (e.g., textbooks, books),
but it does not address the relations between educational institutions. TEACH is based on practice
requirements established by providing workshop and course descriptions as linked data (therefore
users can see all relations between main terms – Fig.2). This ontology allows the description of
courses/modules and LOs (e.g. textbooks, books), but it does not extend to university-wide structures.</p>
      <p>
        Some ontologies specialize in modeling of learning infrastructure. For instance, the ReSIST
ontology (https://lov.linkeddata.es/dataset/lov/vocabs/crsw) represents educational courses and
learning resources with focus on the internal structure of learning modules and the software that
supports their use. The Bologna Educational Ontology (https://gist.github.com/lsarni) models the
academic environment proposed by the Bologna reform principles [
        <xref ref-type="bibr" rid="ref21">21</xref>
        ]. It characterizes
administrative procedures in European universities and concepts for describing learning programs.
Such ontologies can be used as a source of information about the structure and properties of learning
programs and specialties.
      </p>
      <p>It is advisable to use ontologies that formalize learning outcomes for describing specialties and
related educational modules. ESCO ontology (European Skills, Competences, Qualifications and
Occupations, https://ec.europa.eu/esco/portal/home)【22】, which classifies professions, skills, and
qualifications related to the European labor and education market.</p>
      <p>
        It is also advisable to use ontologies that formalize learning outcomes for description of specialties
and related to them learning modules. An example is the ESCO ontology (European Skills,
Competences, Qualifications and Occupations) (https:/ /ec.europa.eu/esco/portal/home) [
        <xref ref-type="bibr" rid="ref22">22</xref>
        ] that
describes, defines and classifies profession, skills and qualifications related to the European Union
(EU) labor and education market. ESCO can be used as a dictionary of terms applied for describing
and matching of competencies, skills qualifications and vacancies. Currently, ESCO contains about
3000 descriptions of professions and about 15 000 descriptions of skills related to these professions.
This information is translated in 27 languages (all official languages of EU and some additional ones):
each concept is associated with at least one term in every language but some different several terms
can be used for the same concept. Within the ESCO data model, terms are linked with other ones by
pertinent relations. ESCO uses Linked Open Data formats (SKOS-RDF, CSV) that developers of
various applications can use to provide services such as job search, career guidance and
selfassessment. Users can integrate the ESCO classifier into their applications and services. In addition,
support of local API and APIs for web services provides applications information from ESCO classifier
in real time.
      </p>
      <p>These examples highlight that it is not enough to simply locate ontologies related to the subject
area of the designed IAS (in this case, education or scientific research). It is essential to manually
analyze the content of each ontology to identify the classes that can serve as a basis for the CIO
structure and their relationships with other CIOs in the system.</p>
      <p>In addition to specialized academic and educational ontologies, information about the structure
and relations between the IOs of the system at the top level can be acquired from general-purpose
ontologies such as Schema.org, DBpedia, and Wikidata.</p>
      <p>Schema.org Dictionary (https://schema.org/docs/schemas.html) supports schemas for structured
data. It currently contains more over 2000 types and properties that cover concepts, actions and
relations between them and, and this set of structural elements can be expanded according to task
needs. Many sites and applications (such as Google and Microsoft) use this dictionary to mark up web
pages.</p>
      <p>DBpedia ontology (http://mappings.dbpedia.org/server/ontology/ classes/) is a cross-domain
ontology manually constructed from the most frequently used information blocks of Wikipedia. It is
currently one of the largest general-purpose ontologies: it contains hierarchically organized set
classes described by almost 3 thousand properties. DBpedia is the central hub of a cloud of Linked
open data, and it can be used as a source of entity relations if information about them is not found in
more specific ontologies.</p>
      <p>
        Wikidata [
        <xref ref-type="bibr" rid="ref23">23</xref>
        ] is an open knowledge base used as a central data management platform for
Wikipedia and other Wikimedia projects. The Wikidata repository consists mostly of elements and
assertions about those elements. Elements that have unique identifiers, labels and descriptions are
used to represent a variety of entities, including concepts and objects. Instructions used to write data
about elements consist of at least one “property-value” pair to connect elements to each other’s and
result in a Linked data structure. Elements are divided into classes and class instances that are
connected by a relation of belonging. Classes can be hierarchically related to each other by the
taxonomic relation "subclass".
      </p>
      <p>WikiProject ontology (https://www.wikidata.org/wiki/ Wikidata:WikiProject_ Ontology) consists
of several top-level classes and properties that aim to support broad semantics of interaction between
other well-known ontologies such as DOLCE, BFO, SUMO, Lemon, RDA, etc., and to integrate the
main branches of the wiki data core concept tree. Such ontology can be used as a knowledge source
to reconcile different term systems of related domains for search.
10.Implementation of CIOs for ActiveBook
The development of the ActiveBook system, considered as an example of an IAS requiring an external
CIO repository, is currently in the prototype phase. The following steps have been completed:
 the main subjects and objects of the system are defined and classified, their structure is
formalized and model of interaction between them is built;</p>
      <p> the authorities of subject groups (users) regarding the ability to edit and view information
are defined;</p>
      <p> the initial set of basic properties for CIOs is determined, and ontologies that allow for their
clarification and improvement are selected;
 the software environment based on MediaWiki and Semantic MediaWiki is analyzed for
implementation;</p>
      <p> examples of pages corresponded to main IOs of IAS are created, and their content is
semantically marked according to selected ontology;</p>
      <p> examples of requests for content integration that allow automated generation of information
on pages are created.</p>
      <p>The next step involves supplementing of real-world LOs with their metadata, and their processing
can caused refinement and expansion of CIO models. Information about additional properties of CIO
elements can be acquired from LO descriptions, from metadata structure of external libraries and
from external ontologies that describe similar IOs or LOs. Other aspect of further system development
is a creation of user-friendly interface for performing individual semantic queries by parameters
defines by CIO properties. From the point of view of this study, the creation of ActiveBook prototype
demonstrates the need for CIO repository that facilitates the search for such objects in relevant
ontologies, allows importing information about the structure of these objects, and supports informing
about changes in external ontologies used to build the knowledge base of the system.
11.Representation and search of CIO in the Semantic</p>
    </sec>
    <sec id="sec-11">
      <title>Environment</title>
    </sec>
    <sec id="sec-12">
      <title>MediaWiki</title>
      <p>The formalization of the CIO structure in ActiveBook IAS is based on semantic properties of the
Semantic MediaWiki (SMW) framework. SMW enables the creation of templates, with parameters
representing semantic properties of wiki pages where the templates are used. It names links between
pages or page elements by terms that connect them with certain domain concepts by semantic
properties of the page. Semantic properties and their types are the main tools for entering semantics
into Wiki resources. Such semantic marking of the text provides a much more expressive
representation of information in comparison to the traditional categories used by Wikipedia.
Information becomes available not only for reading, but also for automated machine processing.</p>
      <p>SMW allows the automatic integration of information from multiple pages, the generation of
complex semantic queries, and the visualization of results. This technology supports the construction
of ontological knowledge bases that formally represent the semantics of CIOs and perform logical
inference. Therefore, the development of analytical portal based on this technological platform meets
the requirements for creating an open directory of learning resources.</p>
      <p>SMW plug-in supports the SMW-QL query language that provides capabilities for semantic search
into content of wiki resources. This query language allows filtering pages by specified criteria and
displaying only the selected parts of information instead of the entire content of a wiki page. This
possibility can be used to integrate dynamically relevant information and to represent it in
userfriendly and more understandable forms: as diagrams, geographical maps, tables, etc. If the pages
with information that satisfy query requirements are changed then the query results are updated
automatically, ensuring data consistency.</p>
      <p>Semantic queries can be embedded into the text of Wiki templates represented by Wiki pages with
unique names situated in the special namespace. Templates can use parser functions, special names
and a simple scripting language. Templates provide representation of information blocks replicated
on many Wiki pages, often with customizable elements. Parameters of Wiki templates can be
incorporated dynamically into the page.</p>
      <p>SMW can use in templates the values of the Wiki page semantic properties as parameters: if some
template used by Wiki page receives some values of parameters then these values can be retrieved as
values of the page semantic properties. If Wiki pages represent some IO (or CIO) it is ad visible to
develop the template that formalize structure of these objects, unified it and help in its dissemination.
Fig. 3 shows a fragment of the "e-Textbook" prototype knowledge base, demonstrating the
representation of basic CIOs.</p>
      <p>The "Textbook" template (it can also describe some other types of LOs) defines the connections of
class individuals with other IOs defined by semantic relations with use of properties “Author”,
“Publisher”, etc. Information about other CIO instances is entered directly into the system using this
template, unlike other CIOs, where some data may be generated automatically via semantic queries.</p>
      <p>Although this CIO structure may seem redundant because some parameters duplicate each other
in different forms, it enhances automated information integration from various sources. For instance,
some sources have information about the code of the specialty for which the textbook is used, and
other ones – about the name of this specialty, and the transformation of one information into another
is quite simple, but in practice it is more appropriate to enter all available information automatically,
and to perform further transformations under the control of a human expert.</p>
      <sec id="sec-12-1">
        <title>Page instance “Textbook”</title>
      </sec>
      <sec id="sec-12-2">
        <title>Page instance “Author”</title>
      </sec>
      <sec id="sec-12-3">
        <title>Page instance “Publisher”</title>
        <p>Template “Textbook”
{{Textbook
|Author=
|Name=
|Publisher=
|Year of publication=
|Type=
|Terms of access=
|Format of the electronic version=
|Volume=
|Difficulty level=
|Language=
|Specialty=
|Competence=
|Link=
}}</p>
      </sec>
      <sec id="sec-12-4">
        <title>Template “Publisher”</title>
        <p>Some parameters in this CIO template can be considered as object properties from an ontological
perspective (such as "Author", "Publisher", "Specialty"), which reference other IOs, and templates of
these IOs make it possible to define their structure. In the terms of wiki technology, such properties
are of the "Reference" type, and markup tags (or appropriate template parameters) define the
semantics of these links. Other template parameters from the point of view of ontological analysis
can be considered as data properties (such as "Year of publication" of “Integer” type and "Language"
of “Text” type) (Fig.4).</p>
        <p>Search tools in semantic wiki resources can take into account the application specifics and use
CIO structure and characteristics. These search tools can be categorized into the following groups:
 Search by Keywords: This type of search focuses on specific terms found in the titles of the
wiki pages that correspond to CIO instances such as "Textbook", "Author", "Specialty", "Competence".
For example, a user might search for textbooks by the initial letters of the title or the author's name;
 Search by Domain Topics and IO Type: In this search, users can filter by categories and
subcategories of CIOs (e.g., "Learning Courses", "Monographs"). This type of search (Fig.5) simplifies
navigation within the resource by grouping similar CIOs under specific domains and topics, making
it easier to explore related educational materials;</p>
      </sec>
      <sec id="sec-12-5">
        <title>Semantic search form</title>
      </sec>
      <sec id="sec-12-6">
        <title>Semantic search results</title>
        <p> Search According to the IO Semantics: This search method combines the requirements for
category and semantic property values (such as the range or value of certain properties). For example,
users can search for textbooks that are categorized under a specific domain and also match certain
property values, such as particular language or year of publishing.</p>
        <p>In the ActiveBook system, semantic search goes beyond simple keyword searches. One advanced
search method involves retrieving semantically similar objects, where the matching is done based on
a set of properties, taking into account domain knowledge about the proximity of these properties'
values. For instance, a search could identify educational materials for a particular competency, even
if that competency is not explicitly specified in the materials. This could be possible because the
competency is a subclass or superclass of the given one, and can be identified using taxonomy of
competencies.</p>
        <p>
          However, SMW does not inherently support this kind of advanced semantic search. This type of
functionality could be integrated into the system through the development of separate API
modules and specialized services. These services would enable more sophisticated queries that are
capable of understanding and matching semantically similar concepts within the system. But practice
shows a lot of various relevance problems of domain and top-level ontologies [
          <xref ref-type="bibr" rid="ref26">26</xref>
          ] ontological models:
these models do not reflect automatically all recent changes in the structure of IAS knowledge base.
        </p>
        <p>As a result, it would be more effective to provide users with links to the relevant elements of the
CIO repository that accurately correspond to the current state of the IAS knowledge base. This
approach ensures that the search results are consistently aligned with the most up-to-date data and
knowledge.
12.Conclusions
The development process of the ActiveBook IAS highlights the need for a repository of complex
information objects that facilitates the search for instances of various ontological classes linked by
specified types of semantic relations. An analysis of existing ontology repositories reveals that this
functionality is lacking, and this fact significantly complicates the development of intelligent
applications that rely on external knowledge sources and require the ability to track changes in those
sources.</p>
        <p>This paper outlines the essential requirements for such repository of complex information objects,
examines the technologies that can be used to populate it, and provides examples of its practical
applications. The proposed method and services can be utilized in artificial intelligence applications
for various domains.</p>
      </sec>
    </sec>
  </body>
  <back>
    <ref-list>
      <ref id="ref1">
        <mixed-citation>
          [1]
          <string-name>
            <given-names>J.</given-names>
            <surname>Rogushina</surname>
          </string-name>
          ,
          <string-name>
            <given-names>A.</given-names>
            <surname>Gladun</surname>
          </string-name>
          ,
          <article-title>Use of Ontological Knowledge for Multi-Criteria Comparison of Complex Information Objects</article-title>
          , UkrProg-2022
          <source>: Proc. of the 13th International Conference of Programming, CEUR</source>
          Vol-
          <volume>3501</volume>
          , (
          <year>2022</year>
          ):
          <fpage>222</fpage>
          -
          <lpage>231</lpage>
          . URL: https://ceur-ws.
          <source>org/</source>
          Vol-
          <volume>3501</volume>
          /s21.pdf.
        </mixed-citation>
      </ref>
      <ref id="ref2">
        <mixed-citation>
          [2]
          <string-name>
            <given-names>N.</given-names>
            <surname>Guarino</surname>
          </string-name>
          ,
          <source>Formal Ontology and Information Systems. Formal Ontology in Information Systems, in: Proc. of FOIS'98</source>
          ,
          <string-name>
            <surname>by</surname>
            <given-names>N.</given-names>
          </string-name>
          <string-name>
            <surname>Guarino</surname>
          </string-name>
          (ed.).
          <source>Trento. Italy</source>
          , Amsterdam, IOS-Press.
          <year>1998</year>
          . pp.
          <fpage>3</fpage>
          -
          <lpage>15</lpage>
          . URL: https://www.researchgate.net/publication/ 272169039_Formal_Ontologies_and_Information_Systems
        </mixed-citation>
      </ref>
      <ref id="ref3">
        <mixed-citation>
          [3]
          <string-name>
            <given-names>J.</given-names>
            <surname>Rogushina</surname>
          </string-name>
          ,
          <string-name>
            <surname>I. Grishanova</surname>
          </string-name>
          ,
          <article-title>Semantic Information Resources with a Complex Structure: Knowledge Representation, Scaling and Search Problems</article-title>
          , UkrProg-2022
          <source>: Proc. of the 13th International Conference of Programming, CEUR</source>
          Vol-
          <volume>3501</volume>
          , (
          <year>2022</year>
          ):
          <fpage>158</fpage>
          -
          <lpage>171</lpage>
          . URL:
          <article-title>ceur-ws</article-title>
          .org/Vol3501/s15.pdf
        </mixed-citation>
      </ref>
      <ref id="ref4">
        <mixed-citation>
          [4]
          <string-name>
            <given-names>J.</given-names>
            <surname>Rogushina</surname>
          </string-name>
          ,
          <string-name>
            <given-names>S.</given-names>
            <surname>Priyma</surname>
          </string-name>
          ,
          <article-title>Use of competence ontological model for matching of qualifications</article-title>
          ,
          <source>Chemistry: Bulgarian Journal of Science Education</source>
          , Vol
          <volume>26</volume>
          ,
          <source>Number</source>
          <volume>2</volume>
          (
          <year>2017</year>
          ):
          <fpage>216</fpage>
          -
          <lpage>228</lpage>
          . URL: http://elar.tsatu.edu.ua/bitstream/123456789/3181/1/2.pdf
        </mixed-citation>
      </ref>
      <ref id="ref5">
        <mixed-citation>
          [5]
          <string-name>
            <given-names>J.</given-names>
            <surname>Rogushina</surname>
          </string-name>
          ,
          <string-name>
            <given-names>A.</given-names>
            <surname>Gladun</surname>
          </string-name>
          ,
          <string-name>
            <given-names>R.</given-names>
            <surname>Valencia-Garcia</surname>
          </string-name>
          ,
          <article-title>Reuse of Ontological Knowledge in Open Science: Models, Sources</article-title>
          , Repositories, Communications, Communications in Computer and Information Science (CCIS),
          <string-name>
            <surname>V.</surname>
          </string-name>
          <year>1873</year>
          , (
          <year>2023</year>
          ):
          <fpage>157</fpage>
          -
          <lpage>172</lpage>
          . Cham: Springer Nature Switzerland.
        </mixed-citation>
      </ref>
      <ref id="ref6">
        <mixed-citation>
          <article-title>[6] FAIR data</article-title>
          . URL: https://en.wikipedia.org/wiki/FAIR_data.
        </mixed-citation>
      </ref>
      <ref id="ref7">
        <mixed-citation>
          [7]
          <string-name>
            <given-names>J.</given-names>
            <surname>Rogushina</surname>
          </string-name>
          ,
          <article-title>Ontological Approach in the Smart Data Paradigm as a Basis for Open Data Semantic Markup</article-title>
          ,
          <string-name>
            <surname>COLINS</surname>
          </string-name>
          , CEUR Vol-
          <volume>3403</volume>
          , (
          <year>2023</year>
          ):
          <fpage>12</fpage>
          -
          <lpage>27</lpage>
          . URL:
          <article-title>ceur-ws</article-title>
          .
          <source>org/</source>
          Vol-
          <volume>3403</volume>
          /paper2.pdf.
        </mixed-citation>
      </ref>
      <ref id="ref8">
        <mixed-citation>
          [8]
          <string-name>
            <given-names>N.</given-names>
            <surname>Bassiliades</surname>
          </string-name>
          ,
          <article-title>EvdoGraph: A Knowledge Graph for the EVDOXUS Textbook Management Service for Greek Universities</article-title>
          ,
          <source>in: Proc. Of 15th International Conference on Knowledge Engineering and Ontology Development KEOD</source>
          ,
          <year>2023</year>
          , Rome, Italy. URL: https://intelligence.csd.auth.gr/wpcontent/uploads/2023/08/EvdoGraph-CR.pdf
        </mixed-citation>
      </ref>
      <ref id="ref9">
        <mixed-citation>
          [9]
          <string-name>
            <given-names>C.</given-names>
            <surname>Bizer</surname>
          </string-name>
          ,
          <string-name>
            <given-names>T.</given-names>
            <surname>Heath</surname>
          </string-name>
          ,
          <string-name>
            <given-names>T.</given-names>
            <surname>Berners-Lee</surname>
          </string-name>
          ,
          <article-title>Linked data: The story so far, in: Semantic services, interoperability and web applications: emerging concepts</article-title>
          ,
          <source>IGI global</source>
          , (
          <year>2011</year>
          ):
          <fpage>205</fpage>
          -
          <lpage>227</lpage>
          .
        </mixed-citation>
      </ref>
      <ref id="ref10">
        <mixed-citation>
          [10]
          <string-name>
            <given-names>M.</given-names>
            <surname>Wylot</surname>
          </string-name>
          ,
          <string-name>
            <given-names>M.</given-names>
            <surname>Hauswirth</surname>
          </string-name>
          ,
          <string-name>
            <given-names>P.</given-names>
            <surname>Cudré-Mauroux</surname>
          </string-name>
          ,
          <string-name>
            <given-names>S.</given-names>
            <surname>Sakr</surname>
          </string-name>
          ,
          <article-title>RDF data storage and query processing schemes: A survey</article-title>
          .
          <source>ACM Computing Surveys (CSUR)</source>
          ,
          <year>2018</year>
          ,
          <volume>51</volume>
          (
          <issue>4</issue>
          ):
          <fpage>1</fpage>
          -
          <lpage>36</lpage>
          .
        </mixed-citation>
      </ref>
      <ref id="ref11">
        <mixed-citation>
          [11]
          <string-name>
            <given-names>G.</given-names>
            <surname>Antoniou</surname>
          </string-name>
          , Web ontology language: Owl / G. Antoniou, Van Harmelen F. In: Handbook on ontologies. Springer Berlin Heidelberg,
          <year>2004</year>
          , pp.
          <fpage>67</fpage>
          -
          <lpage>92</lpage>
          .
        </mixed-citation>
      </ref>
      <ref id="ref12">
        <mixed-citation>
          [12]
          <string-name>
            <given-names>A.</given-names>
            <surname>Hogan</surname>
          </string-name>
          , E. Blomqvist,
          <string-name>
            <given-names>M.</given-names>
            <surname>Cochez</surname>
          </string-name>
          ,
          <string-name>
            <surname>C.</surname>
          </string-name>
          <article-title>D'amato</article-title>
          , G. Melo,
          <string-name>
            <given-names>C.</given-names>
            <surname>Gutierrez</surname>
          </string-name>
          ,
          <string-name>
            <given-names>A.</given-names>
            <surname>Zimmermann</surname>
          </string-name>
          ,
          <article-title>Knowledge graphs</article-title>
          .
          <source>ACM Computing Surveys</source>
          ,
          <year>2022</year>
          ,
          <volume>54</volume>
          (
          <issue>4</issue>
          ):
          <fpage>1</fpage>
          -
          <lpage>37</lpage>
          . doi:
          <volume>10</volume>
          .1145/3447772.
        </mixed-citation>
      </ref>
      <ref id="ref13">
        <mixed-citation>
          [13]
          <string-name>
            <given-names>L.</given-names>
            <surname>Yu</surname>
          </string-name>
          ,
          <string-name>
            <given-names>L.</given-names>
            <surname>Yu</surname>
          </string-name>
          ,
          <article-title>Linked open data. A Developer's Guide to the Semantic Web</article-title>
          ,
          <year>2011</year>
          , pp.
          <fpage>409</fpage>
          -
          <lpage>466</lpage>
          .
        </mixed-citation>
      </ref>
      <ref id="ref14">
        <mixed-citation>
          [14]
          <string-name>
            <given-names>C.</given-names>
            <surname>Binding</surname>
          </string-name>
          ,
          <string-name>
            <given-names>D.</given-names>
            <surname>Tudhope</surname>
          </string-name>
          ,
          <string-name>
            <surname>D.</surname>
          </string-name>
          <article-title>Improving interoperability using vocabulary linked data</article-title>
          .
          <source>In: International Journal on Digital Libraries</source>
          ,
          <volume>17</volume>
          ,
          <year>2016</year>
          , pp.
          <fpage>5</fpage>
          -
          <lpage>21</lpage>
          .
        </mixed-citation>
      </ref>
      <ref id="ref15">
        <mixed-citation>
          [15]
          <string-name>
            <given-names>K.</given-names>
            <surname>Stancin</surname>
          </string-name>
          ,
          <string-name>
            <given-names>P.</given-names>
            <surname>Poscic</surname>
          </string-name>
          ,
          <string-name>
            <given-names>D.</given-names>
            <surname>Jaksic</surname>
          </string-name>
          ,
          <article-title>Ontologies in education - state of the art</article-title>
          .
          <source>Education and Information Technologies</source>
          ,
          <year>2020</year>
          ,
          <volume>25</volume>
          (
          <issue>6</issue>
          ):
          <fpage>5301</fpage>
          -
          <lpage>5320</lpage>
          . doi:
          <volume>10</volume>
          .1007/s10639-020-10226-z.
        </mixed-citation>
      </ref>
      <ref id="ref16">
        <mixed-citation>
          [16]
          <string-name>
            <given-names>M.</given-names>
            <surname>Färber</surname>
          </string-name>
          ,
          <article-title>The Microsoft Academic Knowledge Graph: A Linked Data Source with 8 Billion Triples of Scholarly Data</article-title>
          .
          <source>ISWC</source>
          <year>2019</year>
          , LNCS.
          <volume>11779</volume>
          ,
          <year>2019</year>
          , pp.
          <fpage>113</fpage>
          -
          <lpage>129</lpage>
          . Springer. doi:
          <volume>10</volume>
          .1007/978- 3-
          <fpage>030</fpage>
          -30796-
          <issue>7</issue>
          _
          <fpage>8</fpage>
          .
        </mixed-citation>
      </ref>
      <ref id="ref17">
        <mixed-citation>
          [17]
          <string-name>
            <given-names>M. Y.</given-names>
            <surname>Jaradeh</surname>
          </string-name>
          ,
          <string-name>
            <given-names>A.</given-names>
            <surname>Oelen</surname>
          </string-name>
          ,
          <string-name>
            <given-names>K. E.</given-names>
            <surname>Farfar</surname>
          </string-name>
          ,
          <string-name>
            <given-names>M.</given-names>
            <surname>Prinz</surname>
          </string-name>
          ,
          <string-name>
            <surname>J. D'Souza</surname>
            ,
            <given-names>G.</given-names>
          </string-name>
          <string-name>
            <surname>Kismihók</surname>
            ,
            <given-names>S.</given-names>
          </string-name>
          <string-name>
            <surname>Auer</surname>
          </string-name>
          ,
          <article-title>Open research knowledge graph: Next generation infrastructure for semantic scholarly knowledge</article-title>
          .
          <source>KCAP</source>
          <year>2019</year>
          , ACM, (
          <year>2019</year>
          ):
          <fpage>243</fpage>
          -
          <lpage>246</lpage>
          . doi:
          <volume>10</volume>
          .1145/3360901.3364435.
        </mixed-citation>
      </ref>
      <ref id="ref18">
        <mixed-citation>
          [18]
          <string-name>
            <given-names>B.</given-names>
            <surname>Abu-Salih</surname>
          </string-name>
          ,
          <article-title>Domain-specific knowledge graphs: A survey</article-title>
          .
          <source>Journal of Network and Computer Applications</source>
          ,
          <volume>185</volume>
          ,
          <year>2021</year>
          . doi:
          <volume>10</volume>
          .1016/j.jnca.
          <year>2021</year>
          .
          <volume>103076</volume>
          .
        </mixed-citation>
      </ref>
      <ref id="ref19">
        <mixed-citation>
          [19]
          <string-name>
            <surname>P.-Y. Vandenbussche</surname>
            ,
            <given-names>G. A.</given-names>
          </string-name>
          <string-name>
            <surname>Atemezing</surname>
            ,
            <given-names>M.</given-names>
          </string-name>
          <string-name>
            <surname>Poveda-Villalón</surname>
            ,
            <given-names>B.</given-names>
          </string-name>
          <string-name>
            <surname>Vatant</surname>
          </string-name>
          ,
          <article-title>Linked Open Vocabularies (LOV): A gateway to reusable semantic vocabularies on the Web</article-title>
          .
          <source>Semantic Web</source>
          ,
          <volume>8</volume>
          (
          <issue>3</issue>
          ), (
          <year>2017</year>
          ):
          <fpage>437</fpage>
          -
          <lpage>452</lpage>
          . doi:
          <volume>10</volume>
          .3233/SW-160213.
        </mixed-citation>
      </ref>
      <ref id="ref20">
        <mixed-citation>
          [20]
          <string-name>
            <given-names>J.</given-names>
            <surname>Corson-Rikert</surname>
          </string-name>
          ,
          <string-name>
            <given-names>S.</given-names>
            <surname>Mitchell</surname>
          </string-name>
          ,
          <string-name>
            <given-names>B.</given-names>
            <surname>Lowe</surname>
          </string-name>
          ,
          <string-name>
            <given-names>N.</given-names>
            <surname>Rejack</surname>
          </string-name>
          ,
          <string-name>
            <given-names>Y.</given-names>
            <surname>Ding</surname>
          </string-name>
          ,
          <string-name>
            <surname>C.</surname>
          </string-name>
          <article-title>Guo The VIVO Ontology</article-title>
          .
          <source>In: VIVO, Synthesis Lectures on Data, Semantics, and Knowledge</source>
          ,
          <year>2012</year>
          , pp.
          <fpage>15</fpage>
          -
          <lpage>33</lpage>
          . doi:
          <volume>10</volume>
          .1007/978-3-
          <fpage>031</fpage>
          - 79435-
          <issue>3</issue>
          _
          <fpage>2</fpage>
          .
        </mixed-citation>
      </ref>
      <ref id="ref21">
        <mixed-citation>
          [21]
          <string-name>
            <given-names>G.</given-names>
            <surname>Demartini</surname>
          </string-name>
          ,
          <string-name>
            <surname>I. Enchev</surname>
          </string-name>
          ,
          <string-name>
            <given-names>J.</given-names>
            <surname>Gapany</surname>
          </string-name>
          ,
          <string-name>
            <given-names>P.</given-names>
            <surname>Cudré-Mauroux</surname>
          </string-name>
          ,
          <article-title>The Bowlogna ontology: Fostering open curricula and agile knowledge bases for Europe's higher education landscape</article-title>
          .
          <source>Semantic Web</source>
          ,
          <volume>4</volume>
          (
          <issue>1</issue>
          ), (
          <year>2013</year>
          ):
          <fpage>53</fpage>
          -
          <lpage>63</lpage>
          . doi:
          <volume>10</volume>
          .3233/SW-2012-0064.
        </mixed-citation>
      </ref>
      <ref id="ref22">
        <mixed-citation>
          [22]
          <string-name>
            <surname>ESCO</surname>
          </string-name>
          <article-title>(the European Multilingual Classifier of Skills, Competences, Qualifications and Occupations</article-title>
          . URL: https://ec.europa.eu/esco/portal/home.
        </mixed-citation>
      </ref>
      <ref id="ref23">
        <mixed-citation>
          [23]
          <string-name>
            <given-names>D.</given-names>
            <surname>Vrandečić</surname>
          </string-name>
          ,
          <string-name>
            <given-names>K.</given-names>
            <surname>Krötzsch</surname>
          </string-name>
          ,
          <article-title>Wikidata: a free collaborative knowledgebase</article-title>
          .
          <source>Communications ACM</source>
          ,
          <volume>10</volume>
          , (
          <year>2014</year>
          )
          <fpage>78</fpage>
          -
          <lpage>85</lpage>
          . doi:
          <volume>10</volume>
          .1145/2629489.
        </mixed-citation>
      </ref>
      <ref id="ref24">
        <mixed-citation>
          [24]
          <string-name>
            <given-names>J. H.</given-names>
            <surname>Gennari</surname>
          </string-name>
          ,
          <string-name>
            <given-names>M. A.</given-names>
            <surname>Musen</surname>
          </string-name>
          ,
          <string-name>
            <given-names>R. W.</given-names>
            <surname>Fergerson</surname>
          </string-name>
          ,
          <string-name>
            <given-names>W. E.</given-names>
            <surname>Grosso</surname>
          </string-name>
          ,
          <string-name>
            <given-names>M.</given-names>
            <surname>Crubézy</surname>
          </string-name>
          ,
          <string-name>
            <given-names>H.</given-names>
            <surname>Eriksson</surname>
          </string-name>
          ,
          <string-name>
            <given-names>S. W.</given-names>
            <surname>Tu</surname>
          </string-name>
          ,
          <article-title>The evolution of Protégé: an environment for knowledge-based systems development</article-title>
          .
          <source>International Journal of Human-computer studies</source>
          ,
          <volume>58</volume>
          (
          <issue>1</issue>
          ), (
          <year>2003</year>
          ):
          <fpage>89</fpage>
          -
          <lpage>123</lpage>
          .
        </mixed-citation>
      </ref>
      <ref id="ref25">
        <mixed-citation>
          [25]
          <string-name>
            <given-names>J.</given-names>
            <surname>Rogushina</surname>
          </string-name>
          ,
          <article-title>Semantic Wiki resources and their use for the construction of personalized ontologies</article-title>
          . UkrProg-2016
          <source>: Proc. of the 10th International Conference of Programming, CEUR Vol1631</source>
          , (
          <year>2016</year>
          ): pp.
          <fpage>188</fpage>
          -
          <lpage>195</lpage>
          .
        </mixed-citation>
      </ref>
      <ref id="ref26">
        <mixed-citation>
          [26]
          <string-name>
            <given-names>C.</given-names>
            <surname>Partridge</surname>
          </string-name>
          ,
          <string-name>
            <given-names>A.</given-names>
            <surname>Mitchell</surname>
          </string-name>
          ,
          <string-name>
            <given-names>A.</given-names>
            <surname>Cook</surname>
          </string-name>
          ,
          <string-name>
            <given-names>J.</given-names>
            <surname>Sullivan</surname>
          </string-name>
          ,
          <string-name>
            <given-names>M.</given-names>
            <surname>West</surname>
          </string-name>
          ,
          <article-title>A Survey of Top-Level Ontologies-to inform the ontological choices for a Foundation Data Model</article-title>
          .
          <year>2020</year>
          . URL: https://api.repository.cam.ac.uk/server/api/core/bitstreams/829fe47b-d4fa-
          <fpage>4430</fpage>
          -a639- 3f0904e1684b/content.
        </mixed-citation>
      </ref>
    </ref-list>
  </back>
</article>