Metamath Proof Explorer - Constructs mathematics from scratch, starting from ZFC set theory axioms. Over 23,000 proofs.   Theorem list   Recent proofs (this mirror)
Intuitionistic Logic Explorer - Derives mathematics from a constructive point of view, starting from axioms of intuitionistic logic.
New Foundations Explorer - Constructs mathematics from scratch, starting from Quine's NF set theory axioms.
Higher-Order Logic Explorer - Starts with HOL (also called simple type theory) and derives equivalents to ZFC axioms, connecting the two approaches.
Other Metamath-Related Topics - user-contributed proof verifiers, Metamath 100 list, open problems, other downloads, and miscellany. Filip Cernatescu's Milpgame and practice problems, and also his XPuzzle Android app.
 Older pages:
Hilbert Space Explorer - Extends ZFC set theory into Hilbert space, which is the foundation for quantum mechanics. Includes over 1,000 complete formal proofs.
Quantum Logic Explorer - Starts from the orthomodular lattice properties proved in the Hilbert Space Explorer and takes you into quantum logic with around 1,000 proofs.
Metamath Solitaire - A Java applet that demonstrates simple proofs. Built-in axiom systems include ZFC; modal, intuitionistic, and quantum logics; and Tarski's plane geometry.
GIF and PNG Images for Math Symbols - A copyright-free collection of over 1,000 bit-mapped images for math symbols.
Metamath Music Page - Strictly for fun. You can listen to what mathematical proofs "sound" like!

Mini FAQ
Q: What is Metamath?
A: Metamath is a tiny language that can express theorems in abstract mathematics, accompanied by proofs that can be verified by a computer program. This site has a collection of web pages generated from those proofs and lets you see mathematics developed in complete detail from first principles, with absolute rigor. Hopefully it will amuse you, amaze you, and possibly enlighten you in its own special way.

Q: How can I ask questions or discuss Metamath-related topics?
A: The Metamath Google Group [retrieved 22-Sep-2020] mailing list is being used for discussion about Metamath. If you have questions, that is a good place to ask them. (The AsteroidMeta [retrieved 22-Sep-2020] wiki was used for many older Metamath discussions, but is no longer available. Archived discussions such as this one can be found on archive.org.)

Q: Where do I start?
A: Read Sections 1, 2, and 3 of the Metamath Proof Explorer. Then look at a few proofs in Section 4 to make sure you understand how they work.
Knowledge of mathematics can be helpful, although it isn't strictly necessary to be able to mechanically follow the proofs on this site. If you want to start acquiring a higher-level understanding, a nice independent introduction to logic is Hirst and Hirst's A Primer for Logic and Proof [retrieved 27-Sep-2017] (PDF, 0.5MB); compare its axioms to ours. Wikipedia has an overview of set theory [retrieved 4-Aug-2016]. The video series "Introduction to Higher Mathematics" by Bill Shillito [retrieved 27-Sep-2017] may also be helpful.
You can experiment with simple proofs in the Metamath Solitaire applet. To actually create real metamath proofs, you'll want to download a tool. A common tool is mmj2. David A. Wheeler produced an introductory video, "Introduction to Metamath & mmj2" [retrieved 4-Aug-2016].

Q: Will Metamath help me learn abstract mathematics?
A: Yes, but probably not by itself. In order to follow a proof in an advanced math textbook, you may need to know prerequisites that could take years to learn. Some people find this frustrating. In contrast, Metamath uses a single, simple substitution rule that allows you to follow any proof mechanically. You can actually jump in anywhere and be convinced that the symbol string you see in a proof step is a consequence of the symbol strings in the earlier steps that it references, even if you don't understand what the symbols mean. But this is quite different from understanding the meaning of the math that results. Metamath alone probably will not give you an intuitive feel for abstract math, in the same way it can be hard to grasp a large computer program just by reading its source code, even though you may understand each individual instruction. However, the Bibliographic Cross-Reference lets you compare informal proofs in math textbooks and see all the implicit missing details "left to the reader."

Q: Who is the intended audience for Metamath?
A: Metamath is not for everyone, of course. A person with no interest in math may find it boring or, optimistically, might find a spark of inspiration. Professional mathematicians may view it as a curiosity more than a tool - they need to do things at a high level to work efficiently. On the other hand, Metamath can appeal to those who enjoy picking things apart to see how they work. Others may like the absolute rigor that Metamath offers. Someone new to logic and set theory, who is still developing the mathematical maturity needed to follow informal textbook proofs, may find some reassurance in Metamath's step-by-step breakdown. And anyone who appreciates the austere elegance of formal mathematics for its own sake might enjoy just casually browsing through the proofs for their aesthetic appeal.

Q: I already have an abstract mathematics background. How can I grasp the key ideas in a Metamath proof more quickly?
A: On the web page with the proof, look at the little colored numbers in the Ref column. The steps with the largest numbers are usually the ones you want to look at first. The steps with smaller numbers are typically logic "glue" to tie them together. The colors follow roughly the rainbow colors as the statement number increases, so that the largest numbers tend to stand out from the others. With a little practice, this feature, together with the gray indentation levels showing the tree structure, should help you figure out the "important" steps so that you could write down an informal version of the proof if you wanted to.
(By the way, it's best not to use the colored numbers to reference theorems in an archived discussion, since they change when new theorems are inserted at an earlier point in the database.)

Q: What does the Metamath language look like?
A: The precise technical specification of the language is given in Section 4.1 (p. 112) of the Metamath book and is about 4 pages long. A simple example is given in Section 2.2.2 (p. 40). Compare this source screenshot with the generated web page. But you don't have to know or even look at the language if you just want to follow the proofs on these web pages.
The metamath program and mmj2 are the main tools for working with the Metamath language. As an indication of the language's simplicity, Raph Levien independently wrote the remarkably small mmverify proof verifier in Python. He writes, "I find the whole thing a bit magical. Those 300 lines of code, plus a couple dozen axioms, effectively give you the building blocks for all of mathematics." Bob Solovay wrote a nicely commented presentation of Peano arithmetic in the Metamath language, peano.mm, that is worth reading as a stand-alone file.

Q: What other programs have been written for the Metamath language?
A: Over a dozen proof verifiers for the Metamath language have been written and are listed at Known Metamath proof verifiers. Also, several proof languages have been based on Metamath, and the software and other documentation for these can be found under Metamath-related programs.

Q: How confident can I be in the proofs?
A: You can be extremely confident that the proofs follow from their axioms. All reasoning is done directly in the proof itself rather than by algorithms embedded in the verification program. Computer verification programs never get tired and rigorously check every step. There is the risk that a verifier has a programming bug, but this is countered by the Metamath language's small size (this simplicity reduces the likelihood of such bugs) and by using multiple independently-implemented verifiers (since it is unlikely that all verifiers will have the same kind of bug). For example, the Metamath Proof Explorer is routinely checked by 4 independent verifiers: metamath (a C verifier by Norm Megill), mmj2 (a Java verifier by Mel O'Cat and Mario Carneiro), smetamath-rs (a high-speed Rust verifier by Stefan O'Rear), and checkmm (a C++ verifier by Eric Schmidt). In addition, the databases are public and can easily be inspected; the hypertext links in generated proofs make it especially easy to move from one theorem to the next. Metamath enables an extremely rigorous form of peer review.

Q: Why is it called "Metamath"?
A: It means "metavariable math." See A Note on the Axioms. Metamath shouldn't be confused with metamathematics (occasionally abbreviated metamath, metamaths, or meta math), which is a specialized branch of mathematics that studies mathematics itself, leading to results such as Gödel's incompleteness theorem. An expert in the latter is called a metamathematician, so to avoid confusion one might use "metamathician" for someone knowledgeable about Metamath.

Q: Are there other sites that formalize math from its foundations?
A: Another project that aims to rigorously formalize and verify math is Mizar [retrieved 4-Aug-2016]. It is intended to appeal to professional mathematicians and requires a certain mathematical maturity to be able to follow its proofs. It tries to mimic mathematical proofs they way they are normally published, whereas Metamath shows you every little detail.
Some other well-known interactive theorem provers are HOL Light [retrieved 4-Aug-2016], Isabelle [retrieved 4-Aug-2016], and Coq [retrieved 4-Aug-2016]. There are a few languages based on or derived from Metamath, e.g., Raph Levien has developed a related language called Ghilbert [retrieved 4-Aug-2016] that strives to improve upon Metamath by guaranteeing the soundness of definitions and providing features useful for collaborative work. Freek Wiedijk wrote an interesting collection of notes [retrieved 4-Aug-2016] comparing several mathematical proof languages. His book, The Seventeen Provers of the World [retrieved 4-Aug-2016] (PDF, 0.6MB), compares the proofs that the square root of 2 is irrational in 17 proof languages, including Metamath (theorem sqrt2irr). The Metamath 100 page shows metamath's progress in Formalizing 100 Theorems (a challenge set of theorems for math formalization systems).
Unlike most other systems, Metamath attempts to use the minimum possible framework needed to express mathematics and its proofs. Other systems do not consider that aspect necessarily important, and their underlying computer programs can be large and complex in order to perform mathematical reasoning at a higher level. Metamath's proofs are often quite long compared to those of other systems, but they are completely transparent with nothing hidden from the user. All reasoning is done directly in the proof itself rather than by algorithms embedded in the verification program. Metamath is unique in this sense, offering an alternative approach for those attracted to its philosophy of simplicity.

Q: How can I contribute to Metamath?
A: We'd be delighted to get your contributions! The Metamath community has a large set of inter-related projects, so you first need to determine which specific project you want to contribute to. Here are some common cases:

1. If you're contributing to "set.mm" (the set of proofs which starts from ZFC set theory axioms and shown in the "Metamath Proof Explorer"), the recommended approach is to use its GitHub repository at https://github.com/metamath/set.mm (at least as a starting point). For detailed instructions on using GitHub for this project, read Getting started with contributing and CONTRIBUTING.md. As an alternative to submitting GitHub pull requests (if you don't want to go through that learning curve in the beginning), you can email patch files (differences) to Norm Megill or Mario Carneiro or even post to the Metamath mailing list.
2. If you want to patch the mmj2 program (the editor/GUI proof assistant written in Java by Mel O'Cat and enhanced by Mario Carneiro), email Mario Carneiro and/or get yourself added to https://github.com/digama0/mmj2.
3. If you want to patch the metamath.exe program (the original tool implementation written in C), send your patch as a "unified diff" ("diff -u") via email to Norm Megill.
4. If you want to modify a web page, send email to Norm Megill.
When in doubt, ask or post your proposal to the metamath mailing list, and/or privately email Norm Megill and Mario Carneiro.
• metamath.pdf (1.3 MB)
• Description: The book Metamath: A Computer Language for Mathematical Proofs (248 pp.), written by Norman Megill with extensive revisions by David A. Wheeler, provides an in-depth understanding of the Metamath language and program. It is also called the Metamath book. The first part of the book includes an easy-to-read informal discussion of abstract mathematics and computers, with references to other proof verifiers and automated theorem provers.
• A hardcover version of the Metamath book (ISBN 978-0-3597-02237) is also available if you prefer a printed copy. This was released in 2019 and is labeled second edition.
• A large print and narrow width version of the book, suitable for reading on small devices such as smartphones, is metamath-narrow.pdf. This version updates the Kindle version provided by John D. Baker in 2011.
• You can also view the Metamath book errata.
• The LaTeX source file for the book is metamath.tex; the comment at the beginning explains how to compile it. The source is maintained on GitHub at https://github.com/metamath/metamath-book [retrieved 6-Feb-2019], which also provides an archive of older editions.
• The following BibTeX citation is suggested for the printed version.

@Book{metamath,
author = {Norman D. Megill},
author = {David A. Wheeler},
title = {Metamath: A Computer Language for Mathematical Proofs},
year = {2019},
publisher = {Lulu Press},
}

• metamath.tar.bz2 (14 MB) or metamath.tar.gz (17 MB) or metamath.zip (17 MB)
• Description: The metamath program (version 0.198 7-Aug-2021), which is an ASCII-based ANSI C program with a command-line interface. It was used (along with mmj2 below) to build and verify the proofs in the Metamath Proof Explorer, and it generated its web pages. The *.mm ASCII databases (set.mm and others) are also included in this download.
• Instructions: 1. Extract all files, which will be contained in a directory called "metamath". 2. For Windows, double-click on "metamath.exe" and type "read set.mm". For Linux/MacOSX/Unix, compile with the command "gcc *.c -o metamath" inside the "metamath" directory, then type "./metamath set.mm" to run. 3. For all systems, once in the program, use the "help" command to guide you. Consult the Metamath book (above) for an in-depth understanding.
• To uninstall: Just delete the "metamath" directory. Nothing else on your system was touched by the installation.
• Notes:
• Quicker installation for Windows users who just want the main (set.mm) database: 1. Download the Metamath program metamath.exe (0.5MB) 2. Download the set.mm database (25MB) into the same folder. 3. Double-click on "metamath.exe" and type "read set.mm". 4. To uninstall, just delete these two files. Nothing else is touched on your system.
• On MacOSX, select the Terminal application from Applications/Utilities to get to the command line. On recent versions of MacOSX, you need to install gcc separately. Typing "whereis gcc" will return "/usr/bin/gcc" if it is installed. The XCode package is typically used to install it, but it can also be installed without XCode[retrieved 4-Aug-2016].
• On Linux/MacOSX/Unix, the Metamath program will be more pleasant to use if you run it inside of rlwrap [retrieved 30-Mar-2019], which provides up-arrow command history and other command-line editing features. After you install rlwrap per its instructions (see rlwrap installation [retrieved 30-Mar-2019] for macOS), invoke the Metamath program with "rlwrap ./metamath set.mm". (Thanks to Marnix Klooster for bringing rlwrap to my attention. The Windows version of the Metamath program, by the way, was compiled with lcc, which has some similar features built-in.)
• mmj2.zip (7.2 MB) (latest version, 2.4.1 26-Jan-2016, maintained by Mario Carneiro)
mmj2-orig.zip (Mel O'Cat's last official version, 11-Oct-2011)
https://github.com/digama0/mmj2 (development repository)
• Description: Mel O'Cat and Mario Carneiro's mmj2 GUI Proof Assistant for the Metamath language. Includes thorough file validation and proof verification, syntactic parsing of Metamath formulas and many other features.
• Instructions: Download mmj2jar.zip, unzip and read the enclosed documentation. David A. Wheeler produced two introductory videos "Introduction to Metamath & mmj2" [retrieved 1-Aug-2016] and "Creating Functions in Metamath" [retrieved 1-Aug-2016]. Some documentation is also available at the (now archived) Asteroid Meta wiki mmj2 [retrieved 24-May-2016].
• Quick startup for Windows:
2. Copy the mmj2\mmj2jar directory to C:
3. Edit C:\mmj2jar\RunParms.txt (with e.g. Notepad).
3a.   The first line will read "LoadFile,set.mm"; change it if necessary to point to your set.mm file.
3b.   Add the following 2 lines immediately above the last line that reads "RunProofAsstGUI" (to improve automation in the proof assistant):
ProofAsstDeriveAutocomplete, yes
ProofAsstUseAutotransformations, yes,no,yes
3c.   Add the following 2 lines to the end of the file (to ensure set.mm definitions are sound):
RunMacro,definitionCheck,ax-*,df-bi,df-clab,df-cleq,df-clel
*done
4. Edit C:\mmj2jar\mmj2.bat.   Change "-Xmx256M" to "-Xmx512M" (to increase heap space for current set.mm size). Change "C:\metamath" to a directory that exists (to store .mmp worksheets).
5. Start -> All Programs -> Accessories -> Command Prompt
6. Type: java then ENTER. If the response is "'java' is not recognized...", you need to install the Java runtime system from java.com [retrieved 11-May-2016], then exit and reenter the Command Prompt.
7. Type:
CD C:\mmj2jar
mmj2.bat
• Notes:
• The eimm export-import program links the mmj2 and Metamath proof assistants without exiting from either program, giving you the features of both during proof development.
• The mmj2 directory listing also has the source code, older releases, and MD5 checksums.
• mpeuni.tar.bz2 (70 MB) or mpeuni.tar.gz (140 MB) or mpeuni.zip (180 MB)
• Description: The complete set of Metamath Proof Explorer web pages. Includes the Hilbert Space Explorer and the Metamath Music Page. (Does not include the GIF version of the pages.)
• Instructions: Extract all files (around 35,000) into a directory called "mpeuni". The home page is the file "mmset.html". You will need about 3.5 GB of free space.
• qleuni.tar.bz2 (1 MB) or qleuni.tar.gz (2 MB) or qleuni.zip (4 MB)
• Description: The complete set of Quantum Logic Explorer web pages.
• Instructions: Extract all files (around 1,000) into a directory called "qleuni". The home page is the file "mmql.html".
• mmsolitaire.tar.bz2 (0.2 MB) or mmsolitaire.tar.gz (0.2 MB) or mmsolitaire.zip (0.3 MB)
• Description: The Metamath Solitaire web page, compiled Java applet, and applet source code.
• Instructions: Extract all files into a directory called "mmsolitaire". Use the page "mms.html" to run the applet.
• symbols.tar.bz2 (0.2 MB) or symbols.tar.gz (0.3 MB) or symbols.zip (0.8 MB)
• Description: A collection of over 1,000 mathematical symbols in the form of transparent GIFs that you can use on your own web pages.
• Instructions: Extract all files into a directory called "symbols". The home page is the file "symbols.html".
• mmverify.py (version of 27-Jan-2013)   (previous version)
• Description: Raph Levien's independently-written Python proof verifier for the Metamath language.
• Instructions: See the comments at the top of the program listing.
• eimm.zip (0.1 MB)
• Description: An experimental proof export-import program (version 0.08 23-Mar-2021) that translates incomplete proofs in progress between the Metamath program's CLI Proof Assistant and Mel O'Cat's mmj2 GUI Proof Assistant, without exiting from either proof assistant, giving you the features of both assistants during proof development.
• Instructions: Extract all files into a directory called "eimm". See the readme.txt file for detailed instructions. A pre-compiled Windows binary is provided; gcc is required to compile for Linux/MacOSX/Unix.

Metamath program's Proof Assistant (MM-PA> prompt)
|            ^
|            |
submit eimmexp.cmd /s   submit eimmimp.cmd /s
|            |
v            |
[*.mmp proof worksheet file]
|            ^
|            |
File/Open    File/Save
|            |
v            |
mmj2 GUI Proof Assistant

• Status: There are no known bugs. The development of this prototype is believed to be complete. The only change in the future might be to incorporate the import-export algorithms natively as Metamath program commands, for convenience. Suggestions for other possible features are, of course, welcome.
• finiteaxiom.pdf (0.2 MB)
• Description: Preprint of the article "A Finitely Axiomatized Formalization of Predicate Calculus with Equality," which provides the theoretical basis for Metamath and is referenced on the Metamath Proof Explorer pages. [This PDF file was generated from the LaTeX source file finiteaxiom.tex (0.1 MB).] The correspondence between the axioms in this paper and the ones in the set.mm database is described in Appendix 8 of the Metamath Proof Explorer Home Page. See technical note 1 for some additional notes.
• weakd.pdf (0.2 MB)
• Description: The article "Weaker D-Complete Logics," which is referenced in the Metamath Solitaire applet.
• Quantum logic papers
• Description: Several papers on quantum logic, orthomodular lattices, and Hilbert space can be downloaded from here.
• quantum-logic.tar.bz2 (0.05 MB) or quantum-logic.tar.gz (0.1 MB) or quantum-logic.zip (0.1 MB)
• Description: Several programs (lattice.c, latticeg.c, beran.c, bercomb.c) referenced in the papers "Algorithms for Greechie Diagrams" and "Orthomodular Lattices and a Quantum Algebra."
• Instructions: Extract all files into a directory called "quantum-logic". See the README.TXT file therein for instructions on compiling and using the programs. You will need a C compiler such as gcc.
• Note:
• The above programs are frozen at the versions used for the papers and will reproduce the papers' results exactly. Each .c file is a stand-alone program. After compiling (under Linux/Cygwin/MacOSX/Unix) with "gcc program.c -o program", type "./program --help" for instructions.
• metamathsite.tar.bz2 (111 MB) or metamathsite.tar.gz (116 MB) or metamathsite.zip (117 MB)
• Description: A mirror of the entire Metamath web site including all the downloads listed above (that aren't external links). This can be useful if you have a slow connection or want to browse the site off-line. A script builds the site from source files and requires a Linux/MacOSX/Unix operating system (or the free Cygwin [retrieved 4-Aug-2016] for Windows). About 9 GB of disk space will be needed.
• Instructions: Extract all files into a directory called "metamathsite". Go to that directory then type "./install.sh". This may take several hours to run. The home page (this page) will be "index.html".
• In Cygwin, to go to a directory, type "cd c:/tmp/metamathsite" if your directory (folder) is C:\tmp\metamathsite.
• On MacOSX, select the Terminal application from Applications/Utilities to get to the command line.
• To uninstall: Just delete the "metamathsite" directory. Nothing else on your system was touched by the installation.
• Notes:
• Another way to install your local copy is with rsync (on Linux/MacOSX/Unix or Cygwin). The download will be compressed to about 2GB and automatically expanded to about 3.5GB. Create and go to the metamathsite directory, then type (including the last period):
rsync -vrltS -z --delete --delete-after rsync://rsync.metamath.org/metamath .
Rerunning this same command periodically will also keep your copy updated, downloading only the files that changed. Note that you need twice the disk space during rsync, i.e. 7GB.
• A third way to install your local copy is with wget (see the Download and Extraction Help below). The full uncompressed 3.5GB site will be downloaded, so it will take a long time, depending on your connection speed. Create and go to the metamathsite directory, then type:
wget -nH --mirror "http://us.metamath.org/index.html"
• If you would like to set up a mirror site for public access, read the instructions in mirror.txt.

• Go to http://www.filewatcher.com/m/wget-1.8.2b.zip.278487-0.html [retrieved 4-Aug-2016] (or another mirror site) and download wget-1.8.2b.zip (272kB).
• Extract the file called WGET.EXE into the folder you will be using for your downloads. The other files are not needed for a minimal installation.
• From the Start menu, choose Programs -> Accessories -> Command Prompt. If Command Prompt is missing, then from the Start menu, choose Run..., type CMD (or COMMAND in Windows 95/98), and click OK.
• In the DOS or command window, type
drive-letter:
and press Enter, where drive-letter (C, D, E,...) is the disk you will be using for your downloads. Then type
cd  folder
and press Enter, where folder is the folder (without the drive letter and colon) you will be using for your downloads.
• Type
wget "url"
(include the quotes around url) and press Enter, where url is the URL (internet address, which begins with "http://" or "ftp://") of the .tar.bz2 or other file you want to download. Most browsers can copy a URL from a web page display, for example by right-clicking on the link and selecting "Copy Shortcut" or "Copy Link Location", which you can then paste into the wget argument. To paste, right-click on the top of the command window and select Edit -> Paste.
• If you have trouble retrieving FTP files because you are behind a network firewall, try typing
wget --passive-ftp "url"
Extracting   To extract .tar.bz2 files in Linux/MacOSX/Unix, use the command "tar -xjf xxx.tar.bz2", where xxx corresponds to the file name. To preview what will be extracted, use the command "tar -tjf xxx.tar.bz2 | more"; press the space bar to show the next page and "q" to quit the preview. (On MacOSX, select the Terminal application from Applications/Utilities to get to the command line.)

To extract .tar.gz files in Linux/MacOSX/Unix, use "tar -xzf xxx.tar.gz". To preview them, use "tar -tzf xxx.tar.gz | more".

To extract .zip files in Linux/MacOSX/Unix, use "unzip xxx.zip". To preview them, use "unzip -l xxx.zip | more".

To extract .tar.gz and .zip files in Windows, you can use WinZip, WinAce, or WinRAR, among others. Of these, I have been told that only WinRAR can extract .tar.bz2 files. Recent Windows versions will open .zip files automatically. If you have the free Cygwin [retrieved 4-Aug-2016] installed, you can use the Unix commands above for .tar.bz2, .tar.gz, and .zip files.

Text files  The ASCII (text) files in the downloads are in Unix format, which uses a bare line-feed character at the end of each line. This may cause them to display improperly in some Windows text editors such as Notepad, which requires a carriage-return/line-feed combination. The better text editors don't have this problem, but if you need to convert the format, a free program that has been recommended for Windows is ToX [retrieved 4-Aug-2016]. (For Linux/MacOSX/Unix, use the command: sed -e 's/\$/\r/' unixfile > windowsfile. To go back, use the command: tr -d '\015' < windowsfile > unixfile.)

On Windows, the "write source" command in the Metamath program will automatically convert .mm database text files from Unix format to Windows format (and vice-versa on Linux/MacOSX/Unix).

Note: Some of the links in the section below are obsolete. Let me know if you have current links. --NM 16-Feb-2013
 Reviews The Assayer open-content book reviews (Jan. 8, 2004) University of Waterloo Archimedes' Sandbox Reviews (Oct. 28, 2002) Multimedia Education Resource for Learning and Online Teaching (Jul. 21, 1997) Also: John Bethencourt, Principia Mathematica Revisited (Jan. 24, 2004) Also: American Scientist, Metamath (site of the week) review (Jul. 25, 2005) [retrieved 6-Jul-2016] Also: University at Albany Science Library, 2007 Top 30 Science Resources (Dec. 20, 2007)
Directories

Wikipedia

Drexel University's Math Forum Internet Mathematics Library (another mention)

Government of Australia Education Portal

Encyclopædia Britannica "approved iGuide site" (Oct. 11, 2006) (free set theory full text article)

Awards

The Golden House Sparrow Award: Site of the Day (Jul. 20, 2000) (check out their eclectic current page)

Scout Report for Science and Engineering Selection (Jul. 19, 2000)

Knot a Braid of Links "Cool math site of the week" (Jul. 7-13, 1998)

Rated by JARS (Apr. 26, 1998)