We want to formalize assertions like “the problem of counting the number of hands in bridge
is essentially the same as the problem of counting tickets in the lottery”. The usual tool
in mathematics to do so is the notion of a set. Any collection of things, called elements,
is a set. The deck of cards is a set, whose elements are the cards. The participants of the
party form a set, whose elements are Alice, Bob, Carl, Diane, Eve, Frank and George (let
us denote this set by P). Every lottery ticket contains a set of 5 numbers.
For mathematics, various sets of numbers are important: the set of real numbers, denoted by R; the set of rational numbers, denoted by Q; the set of integers, denote by Z; the
set of non-negative integers, denoted by Z+; the set of positive integers, denoted by N. The
empty set, the set with no elements is another important (although not very interesting)
set; it is denoted by ∅.