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# Preorder

[awr-der]

In mathematics, especially in order theory, preorders are binary relations that satisfy certain conditions. For example, all partial orders and equivalence relations are preorders. The name quasiorder is also common for preorders. Other notations are pre-order, quasi-order, and quasi order. Many order theoretical definitions for partially ordered sets can be generalized to preorders, but the extra effort of generalization is rarely needed.

## Formal definition

Consider some set P and a binary relation $lesssim$ on P. Then $lesssim$ is a preorder, or quasiorder, if it is reflexive and transitive, i.e., for all a, b and c in P, we have that:

a $lesssim$ a (reflexivity)
if a $lesssim$ b and b $lesssim$ c then a $lesssim$ c (transitivity)

A set that is equipped with a preorder is called a preordered set.

If a preorder is also antisymmetric, that is, a $lesssim$ b and b $lesssim$ a implies a = b, then it is a partial order.

On the other hand, if it is symmetric, that is, if a $lesssim$ b implies b $lesssim$ a, then it is an equivalence relation.

A preorder which is preserved in all contexts is called a precongruence. A precongruence which is also symmetric (i.e. is an equivalence relation) is a congruence relation.

## Constructions

Every binary relation R on a set S can be extended to a preorder on S by taking the transitive closure and reflexive closure, R+=. The transitive closure indicates path connection in R: x R+ y if and only if there is an R- path from x to y.

Given a preorder $lesssim$ on S one may define an equivalence relation ~ on S such that a ~ b if and only if a $lesssim$ b and b $lesssim$ a. (The resulting relation is reflexive since a preorder is reflexive, transitive by applying transitivity of the preorder twice, and symmetric by definition.)

Using this relation, it is possible to construct a partial order on the quotient set of the equivalence, S / ~, the set of all equivalence classes of ~. Note that if the preorder is R+=, S / ~ is the set of R- cycle equivalence classes: x ∈ [y] if and only if x = y or x is in an R-cycle with y. In any case, on S / ~ we can define [x] ≤ [y] if and only if x $lesssim$ y. By the construction of ~ , this definition is independent of the chosen representatives and the corresponding relation is indeed well-defined. It is readily verified that this yields a partially ordered set.

Conversely, from a partial order on a partition of a set S one can construct a preorder on S. There is a 1-to-1 correspondence between preorders and pairs (partition, partial order).

For a preorder "$lesssim$", a relation "<" can be defined as a < b if and only if (a $lesssim$ b and not b $lesssim$ a), or equivalently, using the equivalence relation introduced above, (a $lesssim$ b and not a ~ b). It is a strict partial order; every strict partial order can be the result of such a construction. If the preorder is anti-symmetric, hence a partial order "≤", the equivalence is equality, so the relation "<" can also be defined as a < b if and only if (ab and ab).

(Alternatively, for a preorder "$lesssim$", a relation "<" can be defined as a < b if and only if (a $lesssim$ b and ab). The result is the reflexive reduction of the preorder. However, if the preorder is not anti-symmetric the result is not transitive, and if it is, as we have seen, it is the same as before.)

Conversely we have a $lesssim$ b if and only if a < b or a ~ b. This is the reason for using the notation "$lesssim$"; "≤" can be confusing for a preorder that is not anti-symmetric, it may suggest that ab implies that a < b or a = b.

Note that with this construction multiple preorders "$lesssim$" can give the same relation "<", so without more information, such as the equivalence relation, "$lesssim$" cannot be reconstructed from "<". Possible preorders include the following:

• Define ab as a < b or a = b (i.e., take the reflexive closure of the relation). This gives the partial order associated with the strict partial order "<" through reflexive closure; in this case the equivalence is equality, so we don't need the notations $lesssim$ and ~.
• Define a $lesssim$ b as "not b < a" (i.e., take the inverse complement of the relation), which corresponds to defining a ~ b as "neither a < b nor b < a"; these relations $lesssim$ and ~ are in general not transitive; however, if they are, ~ is an equivalence; in that case "<" is a strict weak order. The resulting preorder is total.

Example of a :

## Number of preorders

As explained above, there is a 1-to-1 correspondence between preorders and pairs (partition, partial order). Thus the number of preorders is the sum of the number of partial orders on every partition. For example:

• for n=3:
• 1 partition of 3, giving 1 preorder
• 3 partitions of 2+1, giving 3 × 3 = 9 preorders
• 1 partition of 1+1+1, giving 19 preorders

i.e. together 29 preorders.

• for n=4:
• 1 partition of 4, giving 1 preorder
• 7 partitions with two classes (4 of 3+1 and 3 of 2+2), giving 7 × 3 = 21 preorders
• 6 partitions of 2+1+1, giving 6 × 19 = 114 preorders
• 1 partition of 1+1+1+1, giving 219 preorders

i.e. together 355 preorders.

## Interval

For a $lesssim$ b, the interval [a,b] is the set of points x satisfying a $lesssim$ x and x $lesssim$ b, also written a $lesssim$ x $lesssim$ b. It contains at least the points a and b. One may choose to extend the definition to all pairs (a,b). The extra intervals are all empty.

Using the corresponding strict relation "<", one can also define the interval (a,b) as the set of points x satisfying a < x and x < b, also written a < x < b. An open interval may be empty even if a < b.

Also [a,b) and (a,b] can be defined similarly.