Although it is presented as a collection of lecture notes on numerical analysis, this is essentially a full-fledged online book on the topic. Written by a professor of mathematics at Pennsylvania State University, the 125-page document spans three chapters and covers various forms of differential and difference equations, methods for solving them, and linear algebra. Scattered throughout the text...

How much can you really learn from one number a day? Quite a bit actually, and this fun feature from the Mathematical Association of America (MAA) may prove to be rather addictive. Posted daily since September 2008, the MAA posts a number (for example, the number 11,185,272) and then offers a selection of that number's properties. Visitors to the site can click on some of the external links, look...