total order

A *total order* on a set is a way of ordering its elements to say that some elements precede others, with the understanding that any two elements can be compared one way or the other.

Given a set $S$, a **total order** on $S$ is a (binary) relation $\leq$ with the following properties:

- reflexivity: for any element $x$ of $S$, $x \leq x$;
- transitivity: whenever $x \leq y \leq z$, then $x \leq z$;
- antisymmetry: whenever $x \leq y \leq x$, then $x = y$;
- totality: for any $x$ and $y$ in $S$, $x \leq y$ or $y \leq x$.

A **toset** is a set equipped with a total order.

The category of finite nonempty totally ordered sets and order-preserving maps is called $\Delta$, the simplex category.

The category of *all* finite totally ordered sets and order-preserving maps is called $\Delta_a$, the **augmented** simplex category.

A linear order is much like a total order, except that it is based on an irreflexive relation $\lt$.

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

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

One often sees $x \lt y$ defined as $x \le y$ but $x \ne y$; this is equivalent, but doesn't show the 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.

In classical mathematics, the distinction between total orders and linear 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 linear orders on $S$, and one distinguishes them by their notation. In constructive mathematics, however, they are irreducibly different.

For more, including why linear orders are more often useful in constructive mathematics, see linear order.

Revised on September 4, 2014 00:34:07
by Mike Shulman
(107.201.31.217)