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What are really helpful math resources out there on the web?

Please don't only post a link but a short description of what it does and why it is helpful.

Please only one resource per answer and let the votes decide which are the best!

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I edited your question a bit, my main goal was to remove the reference to WolframAlpha; and make the text easier to read. But feel free to revert if you like! (You can see the edit history and revert by clicking on "edited ... ago") –  Ilya Nikokoshev Nov 4 '09 at 21:23
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The answers below are great. Here's an idea I don't see that may be interesting to think about starting: An online example repository. This would be a place where one could upload and search various "first nontrivial example" notes. I think such a thing would really move mathematics along. –  Jon Bannon Apr 20 '12 at 13:38

66 Answers 66

An excellent catalogue of mathematical information available on the web is Keith Mathhews'

http://www.numbertheory.org/ntw/gateways.html

and if you are interested in Number Theory, see

http://www.numbertheory.org/ntw/

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http://functions.wolfram.com/

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We have that already - see above –  vonjd Oct 27 '10 at 6:27

For people who are interested in prime factorization : www.mersenneforum.org

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Jahrbuch Database

http://www.emis.de/MATH/JFM/

A sort of Mathscinet and Zentralblatt for the period 1868-1942. Most of the reviews are in German. It is interesting to read the reviews written by mathematicians like Frobenius, Hilbert, Minkowski, Hasse, E. Noether, Artin, Mittag-Leffler, Landau, Van der Waerden, ...

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NPTEL provides E-learning through online Web and Video courses in Mathematics organized by Indian Institute of Technology. http://nptel.iitm.ac.in/

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www.proofwiki.org

It is a wikipedia, for proofs.

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