OCLC WorldCat is an online computer library containing high quality, global
Google Scholar is a freely accessible web search engine that indexes the full text or metadata of scholarly literature across an array of publishing formats and disciplines.
Semantic Scholar is a project developed at the Allen Institute for Artificial Intelligence. Publicly released in November 2015, it is designed to be an AI-backed search engine for scientific journal articles. Semantic Scholar is designed to highlight the most important and influential papers, and to identify the connections between them.