大規模言語モデルにおける構造的ハルシネーション：知識構造と引用の完全性に関するネットワークベースの評価

Large Language Models (LLMs) increasingly mediate access to scholarly information, yet their outputs are typically evaluated at the level of individual statements rather than knowledge structure. This paper introduces structural hallucination: systematic distortion of conceptual organization, relational architecture, and bibliographic grounding that remains invisible to sentence-level accuracy metrics. To detect such distortions, we develop a network-based hallucination stress test grounded in knowledge graph extraction, graph similarity analysis, centrality comparison, and citation integrity verification. The protocol is applied to three structured domains representing core forms of scholarly knowledge: Roget's Thesaurus (1911) as a lexical ontology, Wikidata philosophers as a biographical knowledge graph, and bibliographic citation records retrieved from the Dimensions.ai database. Across all domains, substantial structural divergence is observed. In the lexical benchmark, macro-averaged F1 scores fall below 0.05; in the biographical benchmark, hallucination rates exceed 93%; and in the bibliometric benchmark, citation omission reaches 91.9%. Network-level comparison in the Roget reconstruction further reveals node-set Jaccard similarity of 0.028 and fabrication rates above 94%. These findings show that structural fidelity cannot be inferred from local fluency alone. The proposed stress test provides a reproducible instrument for evaluating the structural integrity of LLM-generated knowledge representations within knowledge organization and information quality research.