 ## partial order

(Informally, "order", "ordering") A binary relation R that is a pre-order (i.e. it is reflexive (x R x) and transitive (x R y R z => x R z)) and antisymmetric (x R y R x => x = y).

The order is partial, rather than total, because there may exist elements x and y for which neither x R y nor y R x.

In domain theory, if D is a set of values including the undefined value (bottom) then we can define a partial ordering relation <= on D by

``` x <= y  if  x = bottom or x = y.

```
The constructed set D x D contains the very undefined element, (bottom, bottom) and the not so undefined elements, (x, bottom) and (bottom, x). The partial ordering on D x D is then

``` (x1,y1) <= (x2,y2)  if  x1 <= x2 and y1 <= y2.

```
The partial ordering on D -> D is defined by

``` f <= g  if  f(x) <= g(x)  for all x in D.

```
(No f x is more defined than g x.)

A lattice is a partial ordering where all finite subsets have a least upper bound and a greatest lower bound.

("<=" is written in LaTeX as \sqsubseteq).

Last updated: 1995-02-03

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