math links site

http://www.cut-the-knot.org/index.shtml - TONS of math links, though they aim a little high... okay, a lot high. This is one reason why so many people struggle with math. From the "divisibility criteria" page:

* Divisibility by 3. Let s+(A) = an + an-1 + ... + a0. Then A is divisible by 3 iff s+(A) is.
* Divisibility by 9. With the same function s+, A is divisible by 9 iff s+(A) is.
* Divisibility by 11. Let s±(A) = a0 - a1 + ... (-1)nan. Then A is divisible by 11 iff s±(A) is.

If you don't know about divisibility, you're not likely to understand that terminology...