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PlanetMath
PlanetMath is a free, collaborative, online mathematics encyclopedia. The emphasis is on peer review, rigor, openness, pedagogy, realtime 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 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 as Wikipedia: the GNU Free Documentation License. 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 textual content and mathematical formulas are 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 highquality output. The user can explicitly create links to other articles, and the system also automatically turns certain words into links to the defining articles. For more details on the automatic linking, see the collaboration on PlanetMath automatic reference linking. For more details on controlling the linking of an article, see the collaboration on controlling linking The topic area of every article is classified by the Mathematics Subject Classification of the American Mathematical Society. Users may attach addenda, errata, and discussions to articles.
The most common method of public communication within PlanetMath is posts. Users can add posts in the forums as well as attach posts to articles, corrections, collaborations, requests for new articles, and other posts. A system for private messaging among users is also in place.
Users who are new to PlanetMath are highly encouraged to read the following collaborations:
The software running PlanetMath is written in Perl and runs on Linux and the Apache Web server. It is known as $No\"{o}sphere$ and has been released under the free BSD License.
Most of the very most basic topics are covered, though PlanetMath is striving to improve coverage of elementary and intermediate topics. Due to the increasing popularity of the package PSTricks, more members of PlanetMath are able to incorporate graphics into their articles. This has enabled PlanetMath to cover many elementary and intermediate topics in geometry that were once lacking. There are several methods of creating graphics on PlanetMath. For more details on creating graphics on PlanetMath, see the collaboration on graphics and PlanetMath.
PlanetMath also has entries on highly advanced and specialized topics. PlanetMath has entries on the integers 42 and 666. The following toplevel Mathematics Subject Classification categories have only one or two topic entries at PlanetMath:

74XX, mechanics of deformable solids;

76XX, fluid mechanics;

85XX, astronomy and astrophysics.
The Wikipedia:WikiProject Mathematics/PlanetMath Exchange project assists in content exchange between PlanetMath and Wikipedia.
This entry was adapted from the Wikipedia article PlanetMath as of February 24, 2007.
Mathematics Subject Classification
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Recent Activity
new image: informationtheoreticdistributedmeasurementdds.png by rspuzio
new image: informationtheoreticdistributedmeasurement4.2 by rspuzio
new image: informationtheoreticdistributedmeasurement4.1 by rspuzio
new image: informationtheoreticdistributedmeasurement3.2 by rspuzio
new image: informationtheoreticdistributedmeasurement3.1 by rspuzio
new image: informationtheoreticdistributedmeasurement2.1 by rspuzio
Apr 19
new collection: On the InformationTheoretic Structure of Distributed Measurements by rspuzio
Apr 15
new question: Prove a formula is part of the Gentzen System by LadyAnne
Mar 30
new question: A problem about Euler's totient function by mbhatia
new problem: Problem: Show that phi(a^n1), (where phi is the Euler totient function), is divisible by n for any natural number n and any natural number a >1. by mbhatia