Preorder

In mathematics, especially in order theory, a preorder or quasiorder is a binary relation that is reflexive and transitive. The name is meant to suggest that preorders are almost partial orders, but not quite, as they are not necessarily antisymmetric.
A natural example of a preorder is the divides relation "x divides y" between integers. This relation is reflexive as every integer divides itself. It is also transitive. But it is not antisymmetric, because e.g. divides and divides, but is not equal to. It is to this preorder that "least" refers in the phrase "least common multiple".
Preorders are closely related to equivalence relations and partial orders. Both of these are special cases of a preorder: an antisymmetric preorder is a partial order, and a symmetric preorder is an equivalence relation. Moreover, a preorder on a set can equivalently be defined as an equivalence relation on, together with a partial order on the set of equivalence class, cf. picture. Like partial orders and equivalence relations, preorders are never asymmetric.
A preorder can be visualized as a directed graph, with elements of the set corresponding to vertices, and the order relation between pairs of elements corresponding to the directed edges between vertices. The converse is not true: most directed graphs are neither reflexive nor transitive. A preorder that is antisymmetric no longer has cycles; it is a partial order, and corresponds to a directed acyclic graph. A preorder that is symmetric is an equivalence relation; it can be thought of as having lost the direction markers on the edges of the graph. In general, a preorder's corresponding directed graph may have many disconnected components.
A preorder is often denoted or.

Definition

A binary relation on a set is called a ' or ' if it is reflexive and transitive; that is, if it satisfies:

Reflexivity: for all and
Transitivity: if for all

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

Preorders as partial orders on partitions

Given a preorder on one may define an equivalence relation on by
The resulting relation is reflexive since the preorder is reflexive; transitive by applying the transitivity of twice; and symmetric by definition.
Using this relation, it is possible to construct a partial order on the quotient set of the equivalence,
by defining if
That this is well-defined, meaning that it does not depend on the particular choice of representatives and, follows from the definition of.
Conversely, from any partial order on a partition of a set it is possible to construct a preorder on itself. There is a one-to-one correspondence between preorders and pairs.
Then is a preorder on : every sentence can be proven from itself, and if can be proven from, and from, then can also be proven from .
The corresponding equivalence relation is usually denoted, and defined as and ; in this case and are called "logically equivalent". The equivalence class of a sentence is the set of all sentences that are logically equivalent to ; formally:.
The preordered set is a directed set: given two sentences, their logical conjunction, pronounced "both and ", is a common upper bound of them, since is a consequence of, and so is. The partially ordered set is hence also a directed set.
See Lindenbaum–Tarski algebra for a related example.

Relationship to strict partial orders

If reflexivity is replaced with irreflexivity then we get the definition of a strict partial order on. For this reason, the term is sometimes used for a strict partial order. That is, this is a binary relation on that satisfies:

Irreflexivity or anti-reflexivity: for all that is, is for all and
Transitivity: if for all

Strict partial order induced by a preorder

Any preorder gives rise to a strict partial order defined by if and only if and not.
Using the equivalence relation introduced above, if and only if
and so the following holds
The relation is a strict partial order and strict partial order can be constructed this way.
the preorder is antisymmetric then the equivalence is equality and so in this case, the definition of can be restated as:
But importantly, this new condition is used as the general definition of the relation because if the preorder is not antisymmetric then the resulting relation would not be transitive.
This is the reason for using the symbol "" instead of the "less than or equal to" symbol "", which might cause confusion for a preorder that is not antisymmetric since it might misleadingly suggest that implies

Preorders induced by a strict partial order

Using the construction above, multiple non-strict preorders can produce the same strict preorder so without more information about how was constructed, it might not be possible to reconstruct the original non-strict preorder from Possible preorders that induce the given strict preorder include the following:

Define as . This gives the partial order associated with the strict partial order "" through reflexive closure; in this case the equivalence is equality so the symbols and are not needed.
Define as "", which corresponds to defining as "neither "; these relations and are in general not transitive; however, if they are then is an equivalence; in that case "" is a strict weak order. The resulting preorder is connected ; that is, a total preorder.

If then
The converse holds if and only if whenever then or

Examples

Graph theory

The reachability relationship in any directed graph gives rise to a preorder, where in the preorder if and only if there is a path from x to y in the directed graph. Conversely, every preorder is the reachability relationship of a directed graph. However, many different graphs may have the same reachability preorder as each other. In the same way, reachability of directed acyclic graphs, directed graphs with no cycles, gives rise to partially ordered sets.
The graph-minor relation is also a preorder.
Computer science

In computer science, one can find examples of the following preorders.

Asymptotic order causes a preorder over functions. The corresponding equivalence relation is called asymptotic equivalence.
Polynomial-time, many-one and Turing reductions are preorders on complexity classes.
Subtyping relations are usually preorders.
Simulation preorders are preorders.
Reduction relations in abstract rewriting systems.
The encompassment preorder on the set of terms, defined by if a subterm of t is a substitution instance of s.
Theta-subsumption, which is when the literals in a disjunctive first-order formula are contained by another, after applying a substitution to the former.
Category theory
A category with at most one morphism from any object x to any other object y is a preorder. Such categories are called thin. Here the objects correspond to the elements of and there is one morphism for objects which are related, zero otherwise. In this sense, categories "generalize" preorders by allowing more than one relation between objects: each morphism is a distinct preorder relation.
Alternately, a preordered set can be understood as an enriched category, enriched over the category
Other

Further examples:

Every finite topological space gives rise to a preorder on its points by defining if and only if x belongs to every neighborhood of y. Every finite preorder can be formed as the specialization preorder of a topological space in this way. That is, there is a one-to-one correspondence between finite topologies and finite preorders. However, the relation between infinite topological spaces and their specialization preorders is not one-to-one.
A net is a directed preorder, that is, each pair of elements has an upper bound. The definition of convergence via nets is important in topology, where preorders cannot be replaced by partially ordered sets without losing important features.
The relation defined by if where f is a function into some preorder.
The relation defined by if there exists some injection from x to y. Injection may be replaced by surjection, or any type of structure-preserving function, such as ring homomorphism, or permutation.
The embedding relation for countable total orderings.

Example of a total preorder:

Preference, according to common models.
Constructions

Every binary relation on a set can be extended to a preorder on by taking the transitive closure and reflexive closure, The transitive closure indicates path connection in if and only if there is an -path from to
Left residual preorder induced by a binary relation
Given a binary relation the complemented composition forms a preorder called the left residual, where denotes the converse relation of and denotes the complement relation of while denotes relation composition.

Related definitions

If a preorder is also antisymmetric, that is, and implies then it is a partial order.
On the other hand, if it is symmetric, that is, if implies then it is an equivalence relation.
A preorder is total if or for all
A preordered class is a class equipped with a preorder. Every set is a class and so every preordered set is a preordered class.

Uses

Preorders play a pivotal role in several situations:

Every preorder can be given a topology, the Alexandrov topology; and indeed, every preorder on a set is in one-to-one correspondence with an Alexandrov topology on that set.
Preorders may be used to define interior algebras.
Preorders provide the Kripke semantics for certain types of modal logic.
Preorders are used in forcing in set theory to prove consistency and independence results.