Sage is a free open-source mathematics software system licensed under the GPL. It combines the power of many existing open-source packages into a common Python-based interface.
Mission: Creating a viable free open source alternative to Magma, Maple, Mathematica and Matlab.

People who use ropes, fishing lines, yarn, etc., in their work or in their recreation (such as sailors, rock climbers, fishermen, weavers, farmers, search-and-rescue workers, and so on) are familiar with various knots that they use frequently. But for the average person who only uses knots occasionally, it is helpful to learn just a few knots which are easy to remember and easy to tie, yet are strong and secure and safe for a wide variety of purposes. By learning a few good knots and practicing them now and then, you will be well prepared for those times when you need to tie secure knots in rope or string.

A cognitive bias is a pattern of deviation in judgment that occurs in particular situations (see also cognitive distortion and the lists of thinking-related topics). Implicit in the concept of a "pattern of deviation" is a standard of comparison; this may be the judgment of people outside those particular situations, or may be a set of independently verifiable facts. The existence of some of these cognitive biases has been verified empirically in the field of psychology, others are widespread beliefs, and may themselves be a consequence of cognitive bias.

Ever since its creation in 1992, one of the most popular pastimes on the Usenet newsgroup alt.fan.pratchett has been discussing the many jokes, parodies, allusions and references with which Terry Pratchett seasons his work. Since, as Terry once put it, "alt.fan.pratchett as an entity has the attention span of a butterfly on cocaine", it quickly became clear that it would be a good idea to distil some of these discussions into something with a little more persistence and staying power than individual Usenet articles

This will be a textbook for a "logic literacy" course. It was designed for beginning graduate students in philosophy, but it is also suitable for advanced undergraduate courses. The goal is to introduce students to the logic they need to know in order to read contemporary philosophy journal articles. It emphasizes breadth rather than depth. For example, it discusses modal logic and counterfactuals, but does not contain completeness proofs for predicate logic.

Arcana publicata vilescunt;
et gratiam prophana amittunt.

Ergo: ne Margaritas obijce porcis,
seu Asino substerne rosas.

BK-Trees, or Burkhard-Keller Trees are a tree-based data structure engineered for quickly finding near-matches to a string, for example, as used by a spelling checker, or when doing a 'fuzzy' search for a term. The aim is to return, for example, "seek" and "peek" if I search for "aeek". What makes BK-Trees so cool is that they take a problem which has no obvious solution besides brute-force search, and present a simple and elegant method for speeding up searches substantially.