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Hasse diagram of the natural numbers, partially ordered by "xy if x divides y". The numbers 4 and 6 are incomparable, since neither divides the other.

In mathematics, any two elements x and y of a set P that is partially ordered by a binary relation ≤ are comparable when either xy or yx. If it is not the case that x and y are comparable, then they are called incomparable.

A totally ordered set is exactly a partially ordered set in which every pair of elements is comparable.

It follows immediately from the definitions of comparability and incomparability that both relations are symmetric, that is x is comparable to y if and only if y is comparable to x, and likewise for incomparability.

Contents

NotationEdit

Comparability is denoted by the symbol  , and incomparability by the symbol  .[1] Thus, for any pair of elements x and y of a partially ordered set, exactly one of

  •   and
  •  

is true.

Comparability graphsEdit

The comparability graph of a partially ordered set P has as vertices the elements of P and has as edges precisely those pairs {x, y} of elements for which   .[2]

ClassificationEdit

When classifying mathematical objects (e.g., topological spaces), two criteria are said to be comparable when the objects that obey one criterion constitute a subset of the objects that obey the other, which is to say when they are comparable under the partial order ⊂. For example, the T1 and T2 criteria are comparable, while the T1 and sobriety criteria are not.

See alsoEdit

ReferencesEdit

"PlanetMath: partial order". Retrieved 6 April 2010.

  1. ^ Trotter, William T. (1992), Combinatorics and Partially Ordered Sets:Dimension Theory, Johns Hopkins Univ. Press, p. 3
  2. ^ Gilmore, P. C.; Hoffman, A. J. (1964), "A characterization of comparability graphs and of interval graphs", Canadian Journal of Mathematics, 16: 539–548, doi:10.4153/CJM-1964-055-5.