| From: _Topology A First Course_, James R. Munkres,
Prentice-Hall, Inc., Englewood Cliffs, New Jersey.
(pg. 76) [The following is almost but not quite verbatim.]
Definition. A topology on a set X is a collection T of
subsets of X having the following properties:
1. The empty set and X are in T.
2. The union of the elements of any subcollection
of T is in T.
3. The intersection of the elements of any finite
subcollection of T is in T.
A topological space is an ordered pair (X,T) consisting
of a set X and a topology T on X. A subset U of X is
an open set of (X,T) if U belongs to the collection T.
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