Hierarchical BM25: Lexical Search at Billion-Document Scale

Collection of arXiv information-retrieval and recommendation research papers. Topics include billion-scale hierarchical BM25, semantic evidence for lexical query expansion, LLM-assisted cultural heritage, multimodal text-video retrieval, graph-grounded recommendation, robustness evaluation of GraphRAG and vector RAG, generative recommender optimization, reranker training, and real-time ads retrieval. The content is academic and does not describe a specific vulnerability, exploit, malware, or security incident.