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The minimum excluded value. The mex of a set S of nonnegative integers is the least nonnegative integer not in the set.

A point which is a member of the set closure of a given set S and the set closure of its complement set. If A is a subset of R^n, then a point x in R^n is a boundary point of ...

A definition of a set by mentioning a defining property.

A set in a Polish space is a Borel set iff it is both analytic and coanalytic. For subsets of w, a set is delta_1^1 iff it is "hyperarithmetic."

The set theory symbol (aleph) for the cardinal number of a well-orderable infinite set.

A partially ordered set P is a circle order if it is isomorphic to a set of disks ordered by containment.

A generalization of the concept of set unions and intersections.

Fatou dust is a Fatou set for a point outside the underlying set. Such a set has zero area and has an infinite number of disconnected components.

For every partition of all the points on a line into two nonempty sets such that no point of either lies between two points of the other, there is a point of one set which ...

A relation R on a set S is reflexive provided that xRx for every x in S.