Planet Math

 http://planetmath.org
 http://planetmath.org/?op=sitedoc (Site Docs)
 http://planetmath.org/?op=getobj&from=collab&id=36 (New User Guide)

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PlanetMath is a virtual community which aims to help make mathematical knowledge more accessible. PlanetMath's content is created collaboratively: the main feature is the mathematics encyclopedia with entries written and reviewed by members. The entries are contributed under the terms of the GNU Free Documentation License (FDL) in order to preserve the rights of both the authors and readers in a sensible way.

PlanetMath entries are written in LaTeX, the lingua franca of the worldwide mathematics community. All of the entries are automatically cross-referenced with each other, and the entire corpus is kept updated in real-time.