MathWorld: free online math encyclopedia

MathWorld is a free, extensive, and reliable online encyclopedia of mathematics. As it says on the website, it is supported by Wolfram Research, and it has been “created, developed, and nurtured by Eric Weisstein with contributions from the world’s mathematical community.”

Have a question about Bessel functions? Need some background on Hilbert’s Problems? Looking for the Neyman-Pearson Lemma? Give MathWorld a try! You can search or browse for information, and entries include references to books and journal articles so you can follow up for more information.