Academic and scientific paper search API. Search research papers and scholarly literature across arXiv, PubMed, OpenAlex, Crossref, and Semantic Scholar in one call, deduplicated into a ranked result set — title, authors, year, venue, abstract, citation count, DOI, and direct PDF link. Query by keyword or restrict to a single source (arxiv search, pubmed search, crossref, openalex, semantic scholar); filter by year range; sort by relevance, recency, or citations. Keyless.