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Upper set
Subset of a preorder that contains all larger elements / From Wikipedia, the free encyclopedia
In mathematics, an upper set (also called an upward closed set, an upset, or an isotone set in X)[1] of a partially ordered set is a subset
with the following property: if s is in S and if x in X is larger than s (that is, if
), then x is in S. In other words, this means that any x element of X that is
to some element of S is necessarily also an element of S.
The term lower set (also called a downward closed set, down set, decreasing set, initial segment, or semi-ideal) is defined similarly as being a subset S of X with the property that any element x of X that is
to some element of S is necessarily also an element of S.
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