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A partially ordered set (poset for short) is an ordered pair = (,) consisting of a set (called the ground set of ) and a partial order on . When the meaning is clear from context and there is no ambiguity about the partial order, the set X {\displaystyle X} itself is sometimes called a poset.
The set of all linearly independent subsets of a vector space V, ordered by inclusion. The set of all partial choice functions on a collection of non-empty sets, ordered by restriction. The set of all prime ideals of a ring, ordered by inclusion. The specialization order of any sober space is a dcpo.
In mathematics, and more specifically in order theory, several different types of ordered set have been studied. They include: Cyclic orders, orderings in which triples of elements are either clockwise or counterclockwise; Lattices, partial orders in which each pair of elements has a greatest lower bound and a least upper bound.
A set with a partial order on it is called a partially ordered set, poset, or just ordered set if the intended meaning is clear. By checking these properties, one immediately sees that the well-known orders on natural numbers , integers , rational numbers and reals are all orders in the above sense.
In the mathematical area of order theory, every partially ordered set P gives rise to a dual (or opposite) partially ordered set which is often denoted by P op or P d.This dual order P op is defined to be the same set, but with the inverse order, i.e. x ≤ y holds in P op if and only if y ≤ x holds in P.
It is a total ordering if both and are totally ordered. However the product order of two total orders is not in general total; for example, the pairs ( 0 , 1 ) {\displaystyle (0,1)} and ( 1 , 0 ) {\displaystyle (1,0)} are incomparable in the product order of the ordering 0 < 1 {\displaystyle 0<1} with itself.
In mathematics, especially order theory, the interval order for a collection of intervals on the real line is the partial order corresponding to their left-to-right precedence relation—one interval, I 1, being considered less than another, I 2, if I 1 is completely to the left of I 2.
Consider a partially ordered set (X, ≤). As a first simple example, let 1 = {*} be a specified one-element set with the only possible partial ordering. There is an obvious mapping j: X → 1 with j(x) = * for all x in X. X has a least element if and only if the function j has a lower adjoint j *: 1 → X.