PlanetMath is a free, collaborative, online mathematics encyclopedia. The emphasis is on peer review, rigour, openness, pedagogy, real-time content, interlinked content, and community-drivenness. PlanetMath is intended to be a comprehensive online encyclopedia of mathematics. The project is located at the Digital Library Research Lab at Virginia Tech.
PlanetMath was started when the popular free online math encyclopedia MathWorld was taken offline by a court injunction as a result of the CRC Press lawsuit against the company Wolfram Research and its author and employee 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 subject classification system of the AMS.
Users may attach addenda, errata and discussions to articles. A system for private messaging among users is also in place.
The peer-review-centric nature of PlanetMath has given the content a different, more rigorous and academic flavour than MathWorld's.