# PlanetMath

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**PlanetMath** is a free, collaborative, online mathematics encyclopedia. The emphasis is on peer review, rigour, openness, pedagogy, real-time content, interlinked content, and community. Intended to be comprehensive, the project is located at the Digital Library Research Lab at Virginia Tech.

PlanetMath was started when the popular free online mathematics encyclopedia MathWorld was taken offline for 12 months by a court injunction as a result of the CRC Press lawsuit against the Wolfram Research company and its employee (and MathWorld's author) Eric Weisstein.

PlanetMath uses the same copyleft GFDL license used by Wikipedia. An author who starts a new article becomes the owner of that article; he or she may then choose to grant editing rights to other individuals or groups. All content is written in LaTeX, a typesetting system that requires some learning but is popular among mathematicians because of its support of the technical needs of mathematical typesetting and its high-quality output. The user can explicitly create links to other articles, and the system also automatically turns certain words into links to the defining articles. The topic area of every article is classified by the Mathematics Subject Classification (MSC) of the American Mathematical Society (AMS).

Users may attach addenda, errata and discussions to articles. A system for private messaging among users is also in place.

The software running PlanetMath is written in Perl and runs on Linux and the web server Apache. It is known as Noösphere and has been released under the free BSD License.

The Wikipedia:WikiProject Mathematics/PlanetMath Exchange project assists in content exchange between PlanetMath and Wikipedia.