nLab pseudo-order

Idea

A pseudo-order or strict total order or strict linear order is the irreflexive version of a total order. This is sometimes called linear order, but linear order is also used to refer to the non-strict total order.

A pseudo-ordered set or strictly totally ordered set is a set equipped with a pseudo-order.

In classical mathematics, the distinction between strict total orders and total orders is merely a terminological technicality, which is not always observed; more precisely, there is a natural bijection between the set of total orders on a given set $S$ and the set of strict total orders on $S$, and one distinguishes them by the notation $\lt$ (for the strict total orders) and $\leq$ (for the total order). In constructive mathematics, however, they are irreducibly different.

Definitions

A pseudo-order or strict total order on a set $S$ is a (binary) relation $\lt$ with the following properties:

• irreflexivity: $x \nless x$;
• asymmetry: if $x \lt y$, then $y \nless x$;
• transitivity: if $x \lt y \lt z$, then $x \lt z$;
• weak linearity: if $x \lt z$, then $x \lt y$ or $y \lt z$;
• connectedness: if $x \nless y$ and $y \nless x$, then $x = y$.

In classical mathematics, one may see these versions of asymmetry and connectedness:

• $x \nless y$ or $y \nless x$;
• $x \lt y$ or $y \lt x$ or $x = y$.

Using excluded middle, these are equivalent to asymmetry and connectedness as given above, but they need not hold for all pseudo-orders in constructive mathematics.

Actually, these axioms are overkill; to begin with, irreflexivity is simply a special case of asymmetry and so can be dropped. Additionally, one can either drop transitivity or drop asymmetry (which then requires keeping irreflexivity); they will still follow from the other axioms. Dropping transitivity shows manifestly the duality (see below) between pseudo-orders and total orders (even in constructive mathematics), and dropping asymmetry shows that a pseudo-order is a weakly linear strict preorder. Keeping transitivity and irreflexivity (thus allowing one to drop asymmetry) shows manifestly that a pseudo-order is a special kind of strict preorder.

In classical mathematics, there are even more options. Now weak linearity can be dropped, as it follows from transitivity and connectedness. Also, one often combines irreflexivity, asymmetry, and connectedness into a single axiom:

• trichotomy: exactly one of $x \lt y$, $y \lt x$, $x = y$ is true.

Thus the most common definition uses only trichotomy and transitivity.

One can also interpret connectedness not as an axiom but as a definition of equality, getting a pseudo-order on a quotient set of $S$. In other words, if $\lt$ is an asymmetric and weakly linear relation on $S$, and we define $x \equiv y$ to mean that neither $x \lt y$ nor $y \lt x$, then $\equiv$ is an equivalence relation and $\lt$ induces a pseudo-order on $S/{\equiv}$.

Examples

Classically, any total order defines an example of a pseudo-order (as explained below), and this also holds constructively in discrete mathematics. Since most pseudo-orders in these cases are usually described in terms of their total orders, the focus here is on constructive analysis. (The first item, however, is an exception.)

• A lexicographic order, even in a classical or discrete context, is more easily described as a pseudo-order than as a total order.
• The big example in analysis is the field of real numbers. Both the Dedekind reals and the Cauchy reals (even if weak countable choice fails so these are not equivalent) have a pseudo-order $\lt$ that extends the (decidable) pseudo-order on the rational numbers. Since the corresponding partial order $\leq$ cannot be proved total (and in some classically invalid versions of constructive mathematics can even be proved not total), $\lt$ is more directly useful than $\leq$ in constructive analysis. In any case, $\lt$ is more fundamental, since $\leq$ can be defined in terms of $\lt$ but not the other way around. (The Mac Neille real numbers have both $\lt$ and $\leq$; however, in this case, neither is necessarily a pseudo-order or a total order, nor can they necessarily be defined in terms of one another.)
• Baire space and Cantor space, being representable as subspaces of the real line, of course are strictly totally ordered. It's also interesting to see them as coming from the (decidable) pseudo-orders on $\mathbf{N}$ and $\mathbf{2}$, which they are $\mathbf{N}$-fold products of.

Properties

Relation to total orders

Using excluded middle, one can move between strict linear orders and total orders using negation; that is, the negation of a strict linear order is a total order and vice versa. Actually one usually swaps the order too, as follows:

• $x \lt y$ iff $y \nleq x$;
• $x \leq y$ iff $y \nless x$.

To prove this, it's enough to see that the properties of a strict linear order are dual to the properties of a total order, as follows:

strict linear order		total order
irreflexivity		reflexivity
asymmetry		totality
transitivity		weak linearity
weak linearity		transitivity
connectedness		antisymmetry

Constructively, these are still the default definitions to use; that is, if one is given a strict linear order or a total order and wants to interpret the other symbol, then one does so using these definitions. However, the result will not necessarily be a total order or a strict linear order. To be specific, if one starts with a strict linear order $\lt$ and defines $\leq$ as above, then totality does not follow; and if one starts with a total order $\leq$ and defines $\lt$ as above, then weak linearity does not follow. Nevertheless, at least $\leq$ will be a partial order, and least $\lt$ will be a strict preorder. Furthermore, the duality between the axioms is still there, even though negation no longer mediates between them; although weak linearity need not hold for a total order constructively, the duality is preserved if one defines strict linear orders without using transitivity.

One often sees $x \lt y$ defined as $x \le y$ but $x \ne y$; this is equivalent, but doesn't show the de Morgan duality explicitly. Similarly, one often sees $x \leq y$ defined as $x \lt y$ or $x = y$; this is not even equivalent constructively, although it is classically.

Keep in mind, however, that the only use of excluded middle in the classical theory is the assumption that $x = y$, $x \lt y$, and $x \leq y$ are always either true or false. There is therefore a perfect correspondence between decidable strict linear orders and decidable total orders on sets with decidable equality.

See also

• total order
• strict weak order
• strict preorder
• trichotomous relation

 References

• Wikipedia, https://en.wikipedia.org/wiki/Pseudo-order

• Heyting, Arend (1966). Intuitionism: an introduction (2nd ed.). Amsterdam: North-Holland Pub. Co. p. 106. ISBN:978-0-444-53406-4