**4.2. Ontology layer**

The ontology base contains the lightweight cube citation ontology base, the domain ontology base, and the user requirement ontology base. It simplifies the entity level, builds a convenient, simple citation ontology, organizes, stores, and queries data in a machine-readable mode. For example, the terms "dc: title," "fabio: hasTranslatedTitle," "bibo: pageStart," and "bibo: pageEnd," respectively, define the title of the journal, the English title, the start page, and other related attributes. These describe the citation in detail and realize the knowledge association of the citation information.

The domain ontology base contains the domain ontology, which includes class, property, and instance of domain ontology, as well as the ontological semantics of citation resources. Song and Zhang [37] agree that ontology can represent the complex semantic relations in the content of the information resources; it has a solid concept of hierarchical structure that supports logical reasoning. It is helpful for us to organize and retrieve information.

User requirements ontology conducts user need surveys for users and obtains user preferences directly. It analyzes users' search behavior, retrieves content, analyzes the users' preferences, obtains the user database, and builds the users' need ontology.
