BibTeX is a citation system frequently used in some areas of computer science, mathematics, and physics. It was created for use with the LaTeX markup system for mathematical notation, but is usable with other tools as well. Software for using LaTeX and BibTeX is widely available, much of it under open source or similar licensing.
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Zotero is a tool for collecting, managing, and citing sources. You can install it on your computer and also access your personal database of sources online.
