Wikipedia 10K Redux by Reagle from Starling archive. Bugs abound!!!

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SetTheory

Sets are one of the base concepts of mathematics.  A set is, more or less, just a collection of objects, called its elements.  Standard notation uses braces around the list of elements, as in:

   {red, green, blue}
   {x : x is a primary color}

If every x in some set A is contained in some set B, then A is said to be a SubSet of B.  Every set has at least two subsets: itself, called the improper subset, and the empty set {}.  The union of some sets is the set of all elements contained in at least one, and the intersection is the set of all elements contained in them all.  These are denoted

   A1 u A2 u A3 ...
   A1 n A2 n A3 ...

respectively.  If you don't mind jumping ahead a bit, the subsets of a given set form a BooleanAlgebra under these operations.  The set of all subsets of X is called its PowerSet and is denoted 2^X.