Type of site
|Created by||Nathan Egge, Aaron Krowne|
|Registration||GitHub account required to edit sources via pull request|
PlanetMath is a free, collaborative, mathematics online encyclopedia. The emphasis is on rigour, openness, pedagogy, real-time content, interlinked content, and also community of about 24,000 people with various maths interests. Intended to be comprehensive, the project is currently hosted by the University of Waterloo. The site is owned by a US-based nonprofit corporation, "PlanetMath.org, Ltd". 
PlanetMath was started when the popular free online mathematics encyclopedia MathWorld was temporarily 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.
As of 15 February 2013 [update], the encyclopedia hosted about 9,289 entries and over 16,258 concepts (a concept may be for example a specific notion defined within a more general entry).  An overview of the current PlanetMath contents is also available.  About 300 Wikipedia entries incorporate text from PlanetMath articles; they are listed in Category:Wikipedia articles incorporating text from PlanetMath.
An all-inclusive PlanetMath Free Encyclopedia book of 2,300 pages is available for the encyclopedia contents up to 2006 as a free download PDF file. 
PlanetMath implements a specific content creation system called authority model. 
An author who starts a new article becomes its owner, that is the only person authorized to edit that article. Other users may add corrections and discuss improvements but the resulting modifications of the article, if any, are always made by the owner. However, if there are long lasting unresolved corrections, the ownership can be removed. More precisely, after 2 weeks the system starts to remind the owner by mail; at 6 weeks any user can "adopt" the article; at 8 weeks the ownership of the entry is completely removed (and such an entry is called "orphaned").
To make the development more smooth, the owner may also choose to grant editing rights to other individuals or groups.
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).
The site is supervised by the Content Committee. Its basic mission is to maintain the integrity and quality of the mathematical content and organization of PlanetMath.  As defined in its Charter, the tasks of the Committee include:
- Developing/maintaining the standards for PlanetMath content
- Improving individual PlanetMath entries in its Encyclopedia, Book, Paper, and Exposition)
- Developing topic areas
- Developing/improving site and user documentation
- Managing the PlanetMath Request list and Unproved Theorems list
- Improving categorization and other meta-attributes of entries.
- Developing software recommendations for improved content authoring and editorial functions.
PlanetMath content is licensed under the copyleft Creative Commons Attribution/Share-Alike License. All content is written in LaTeX, a typesetting system popular among mathematicians because of its support of the technical needs of mathematical typesetting and its high-quality output.
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. As of March 13, 2013 PlanetMath has retired Noösphere and runs now on a software called Planetary, which itself was implemented with Drupal.
Encyclopedic content and bibliographic materials related to physics, mathematics and mathematical physics are developed by PlanetPhysics. The site, launched in 2005, uses similar software (Noosphere), but a significantly different moderation model with emphasis on current research in physics and peer review.   Additionally, a PlanetComputing project is envisaged that would also include Computational Physics and AI together with logical, categorical, ontological and mathematical foundations of computers and automata.[ citation needed]
- Self-descriptions of PM "Archived copy". Archived from the original on 2016-08-26. Retrieved 2009-05-15.CS1 maint: archived copy as title ( link), "Archived copy". Archived from the original on 2016-06-11. Retrieved 2008-05-02.CS1 maint: archived copy as title ( link) "Archived copy". Archived from the original on 2008-05-09. Retrieved 2009-05-19.CS1 maint: archived copy as title ( link)
- "PlanetMath Encyclopedia". Archived from the original on 2013-02-15.
- "Overview of the content of PlanetMath". Planetmath.org. 2013-03-21. Archived from the original on 2012-08-04. Retrieved 2013-09-06.
- PlanetMath "Free Encyclopedia" book Archived June 20, 2016, at the Wayback Machine based on 2004-2005 content of PlanetMath
- Noosphere's Archived March 4, 2016, at the Wayback Machine authority model
- Username *. "ContentCommittee Charter". Planetmath.org. Archived from the original on 2010-11-26. Retrieved 2013-09-06.
- PlanetPhysics.org Archived February 25, 2009, at the Wayback Machine
- Free Book downloads in PDF format, Original Papers (~10 pages) and Expositions (>30 pages)[ dead link]
- Official website
- "PlanetPhysics". Archived from the original on February 25, 2009.
- Klemm, Aaron E. (Feb 16, 2005). "Motivation and value of free resources: Wikipedia and Planetmath show the way". Free Software magazine.
- "Motivation and value of free resources" (PDF). Free Software Magazine (1): 52–56. Feb 2005.
- "Web Projects: Grassroots Math Guide". Science. Netwatch. AAAS. 308 (5729): 1721. 17 June 2005. doi: 10.1126/science.308.5729.1721e.
- Lange, Christoph (2008). "SWiM – A Semantic Wiki for Mathematical Knowledge Management". The Semantic Web: Research and Applications. European Semantic Web Conference (ESWC). 5021. pp. 832–837. arXiv: 1003.5196. doi: 10.1007/978-3-540-68234-9_68. ISBN 978-3-540-68233-2.
- Milson, Robert; Krowne, Aaron (2005). "Adapting CBPP platforms for instructional use". arXiv: cs/0507028.
- Andrew, Alex M. (2008). "Archives, mathematics encyclopaedia, dancing robots, ASC". Kybernetes. 37 (9/10): 1466–1468. doi: 10.1108/03684920810907805.