Description of Sets

A set is a collection of things.
We call the "things" ... the elements of the set.
Two sets are equal if they have the same elements.

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We use curly braces to denote sets: {3} is a set of one element, which is the number three.

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The symbol means "is an element of", while
the symbol means "is not an element of".
Thus we write
3{3} and 4{3}.

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Capital letters are frequently used to stand for sets,
and small letters are frequently used to stand for the elements of a set.
Thus we write xA .

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Particular sets are often indicated using the following notation:
A = { x : statement about x },
which is read as follows:
A is the set of all elements, x, such that the statement about x is true.