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Interval order

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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, I1, being considered less than another, I2, if I1 is completely to the left of I2. More formally, a countable poset is an interval order if and only if there exists a bijection from to a set of real intervals, so , such that for any we have in exactly when .

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A partial order on the set {a, b, c, d, e, f} illustrated by its Hasse diagram (left) and a collection of intervals that represents it (right).
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The poset (black Hasse diagram) cannot be part of an interval order: if a is completely right of b, and d overlaps with both a and b, and c is completely right of d, then c must be completely right of b (light gray edge).

Such posets may be equivalently characterized as those with no induced subposet isomorphic to the pair of two-element chains, in other words as the -free posets .[1] Fully written out, this means that for any two pairs of elements and one must have or .

The subclass of interval orders obtained by restricting the intervals to those of unit length, so they all have the form , is precisely the semiorders.

The complement of the comparability graph of an interval order (, ≤) is the interval graph .

Interval orders should not be confused with the interval-containment orders, which are the inclusion orders on intervals on the real line (equivalently, the orders of dimension ≤ 2).

Interval orders' practical applications include modelling evolution of species and archeological histories of pottery styles.[2][example needed]

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Interval orders and dimension

Unsolved problem in mathematics
What is the complexity of determining the order dimension of an interval order?

An important parameter of partial orders is order dimension: the dimension of a partial order is the least number of linear orders whose intersection is . For interval orders, dimension can be arbitrarily large. And while the problem of determining the dimension of general partial orders is known to be NP-hard, determining the dimension of an interval order remains a problem of unknown computational complexity.[3]

A related parameter is interval dimension, which is defined analogously, but in terms of interval orders instead of linear orders. Thus, the interval dimension of a partially ordered set is the least integer for which there exist interval orders on with exactly when and . The interval dimension of an order is never greater than its order dimension.[4]

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Combinatorics

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In addition to being isomorphic to -free posets, unlabeled interval orders on are also in bijection with a subset of fixed-point-free involutions on ordered sets with cardinality .[5] These are the involutions with no so-called left- or right-neighbor nestings where, for any involution on , a left nesting is an such that and a right nesting is an such that .

Such involutions, according to semi-length, have ordinary generating function[6]

The coefficient of in the expansion of gives the number of unlabeled interval orders of size . The sequence of these numbers (sequence A022493 in the OEIS) begins

1, 2, 5, 15, 53, 217, 1014, 5335, 31240, 201608, 1422074, 10886503, 89903100, 796713190, 7541889195, 75955177642, …
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Notes

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