A set is clopen if it is simultaneously open and closed in a topological space.

Example: Members of the base B = { |x,r):x, r in R, x < r, r is rational } for the Sorgenfrey line are clopen with respect to the topology generated by B.
Example: Given a set X, X and Ø are always clopen in X. If a subset U of X is clopen, its complement, X/U, is also clopen.

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